Chapter 20. The Z File System (ZFS)
The Z File System, or ZFS, is an advanced file system designed to overcome many of the major problems found in previous designs.
Originally developed at Sun™, ongoing open source ZFS development has moved to the OpenZFS Project.
ZFS has three major design goals:
Data integrity: All data includes a checksum of the data. When data is written, the checksum is calculated and written along with it. When that data is later read back, the checksum is calculated again. If the checksums do not match, a data error has been detected. ZFS will attempt to automatically correct errors when data redundancy is available.
Pooled storage: physical storage devices are added to a pool, and storage space is allocated from that shared pool. Space is available to all file systems, and can be increased by adding new storage devices to the pool.
Performance: multiple caching mechanisms provide increased performance. ARC is an advanced memory-based read cache. A second level of disk-based read cache can be added with L2ARC, and disk-based synchronous write cache is available with ZIL.
A complete list of features and terminology is shown in ZFS Features and Terminology.
20.1. What Makes ZFS Different
ZFS is significantly different from any previous file system because it is more than just a file system. Combining the traditionally separate roles of volume manager and file system provides ZFS with unique advantages. The file system is now aware of the underlying structure of the disks. Traditional file systems could only be created on a single disk at a time. If there were two disks then two separate file systems would have to be created. In a traditional hardware RAID configuration, this problem was avoided by presenting the operating system with a single logical disk made up of the space provided by a number of physical disks, on top of which the operating system placed a file system. Even in the case of software RAID solutions like those provided by GEOM, the UFS file system living on top of the RAID transform believed that it was dealing with a single device. ZFS’s combination of the volume manager and the file system solves this and allows the creation of many file systems all sharing a pool of available storage. One of the biggest advantages to ZFS’s awareness of the physical layout of the disks is that existing file systems can be grown automatically when additional disks are added to the pool. This new space is then made available to all of the file systems. ZFS also has a number of different properties that can be applied to each file system, giving many advantages to creating a number of different file systems and datasets rather than a single monolithic file system.
20.2. Quick Start Guide
There is a startup mechanism that allows FreeBSD to mount ZFS pools during system initialization. To enable it, add this line to /etc/rc.conf:
Then start the service:
# service zfs start
The examples in this section assume three SCSI disks with the device names da0, da1, and da2. Users of SATA hardware should instead use ada device names.
20.2.1. Single Disk Pool
To create a simple, non-redundant pool using a single disk device:
# zpool create example /dev/da0
To view the new pool, review the output of
# df Filesystem 1K-blocks Used Avail Capacity Mounted on /dev/ad0s1a 2026030 235230 1628718 13% / devfs 1 1 0 100% /dev /dev/ad0s1d 54098308 1032846 48737598 2% /usr example 17547136 0 17547136 0% /example
This output shows that the
example pool has been created and mounted. It is now accessible as a file system. Files can be created on it and users can browse it:
# cd /example # ls # touch testfile # ls -al total 4 drwxr-xr-x 2 root wheel 3 Aug 29 23:15 . drwxr-xr-x 21 root wheel 512 Aug 29 23:12 .. -rw-r--r-- 1 root wheel 0 Aug 29 23:15 testfile
However, this pool is not taking advantage of any ZFS features. To create a dataset on this pool with compression enabled:
# zfs create example/compressed # zfs set compression=gzip example/compressed
example/compressed dataset is now a ZFS compressed file system. Try copying some large files to /example/compressed.
Compression can be disabled with:
# zfs set compression=off example/compressed
To unmount a file system, use
zfs umount and then verify with
# zfs umount example/compressed # df Filesystem 1K-blocks Used Avail Capacity Mounted on /dev/ad0s1a 2026030 235232 1628716 13% / devfs 1 1 0 100% /dev /dev/ad0s1d 54098308 1032864 48737580 2% /usr example 17547008 0 17547008 0% /example
To re-mount the file system to make it accessible again, use
zfs mount and verify with
# zfs mount example/compressed # df Filesystem 1K-blocks Used Avail Capacity Mounted on /dev/ad0s1a 2026030 235234 1628714 13% / devfs 1 1 0 100% /dev /dev/ad0s1d 54098308 1032864 48737580 2% /usr example 17547008 0 17547008 0% /example example/compressed 17547008 0 17547008 0% /example/compressed
The pool and file system may also be observed by viewing the output from
# mount /dev/ad0s1a on / (ufs, local) devfs on /dev (devfs, local) /dev/ad0s1d on /usr (ufs, local, soft-updates) example on /example (zfs, local) example/compressed on /example/compressed (zfs, local)
After creation, ZFS datasets can be used like any file systems. However, many other features are available which can be set on a per-dataset basis. In the example below, a new file system called
data is created. Important files will be stored here, so it is configured to keep two copies of each data block:
# zfs create example/data # zfs set copies=2 example/data
It is now possible to see the data and space utilization by issuing
# df Filesystem 1K-blocks Used Avail Capacity Mounted on /dev/ad0s1a 2026030 235234 1628714 13% / devfs 1 1 0 100% /dev /dev/ad0s1d 54098308 1032864 48737580 2% /usr example 17547008 0 17547008 0% /example example/compressed 17547008 0 17547008 0% /example/compressed example/data 17547008 0 17547008 0% /example/data
Notice that each file system on the pool has the same amount of available space. This is the reason for using
df in these examples, to show that the file systems use only the amount of space they need and all draw from the same pool. ZFS eliminates concepts such as volumes and partitions, and allows multiple file systems to occupy the same pool.
To destroy the file systems and then destroy the pool as it is no longer needed:
# zfs destroy example/compressed # zfs destroy example/data # zpool destroy example
Disks fail. One method of avoiding data loss from disk failure is to implement RAID. ZFS supports this feature in its pool design. RAID-Z pools require three or more disks but provide more usable space than mirrored pools.
This example creates a RAID-Z pool, specifying the disks to add to the pool:
# zpool create storage raidz da0 da1 da2
Sun™ recommends that the number of devices used in a RAID-Z configuration be between three and nine. For environments requiring a single pool consisting of 10 disks or more, consider breaking it up into smaller RAID-Z groups. If only two disks are available and redundancy is a requirement, consider using a ZFS mirror. Refer to zpool(8) for more details.
The previous example created the
storage zpool. This example makes a new file system called
home in that pool:
# zfs create storage/home
Compression and keeping extra copies of directories and files can be enabled:
# zfs set copies=2 storage/home # zfs set compression=gzip storage/home
To make this the new home directory for users, copy the user data to this directory and create the appropriate symbolic links:
# cp -rp /home/* /storage/home # rm -rf /home /usr/home # ln -s /storage/home /home # ln -s /storage/home /usr/home
Users data is now stored on the freshly-created /storage/home. Test by adding a new user and logging in as that user.
Try creating a file system snapshot which can be rolled back later:
# zfs snapshot storage/home@08-30-08
Snapshots can only be made of a full file system, not a single directory or file.
@ character is a delimiter between the file system name or the volume name. If an important directory has been accidentally deleted, the file system can be backed up, then rolled back to an earlier snapshot when the directory still existed:
# zfs rollback storage/home@08-30-08
To list all available snapshots, run
ls in the file system’s .zfs/snapshot directory. For example, to see the previously taken snapshot:
# ls /storage/home/.zfs/snapshot
It is possible to write a script to perform regular snapshots on user data. However, over time, snapshots can consume a great deal of disk space. The previous snapshot can be removed using the command:
# zfs destroy storage/home@08-30-08
After testing, /storage/home can be made the real /home using this command:
# zfs set mountpoint=/home storage/home
mount to confirm that the system now treats the file system as the real /home:
# mount /dev/ad0s1a on / (ufs, local) devfs on /dev (devfs, local) /dev/ad0s1d on /usr (ufs, local, soft-updates) storage on /storage (zfs, local) storage/home on /home (zfs, local) # df Filesystem 1K-blocks Used Avail Capacity Mounted on /dev/ad0s1a 2026030 235240 1628708 13% / devfs 1 1 0 100% /dev /dev/ad0s1d 54098308 1032826 48737618 2% /usr storage 26320512 0 26320512 0% /storage storage/home 26320512 0 26320512 0% /home
This completes the RAID-Z configuration. Daily status updates about the file systems created can be generated as part of the nightly periodic(8) runs. Add this line to /etc/periodic.conf:
20.2.3. Recovering RAID-Z
Every software RAID has a method of monitoring its
state. The status of RAID-Z devices may be viewed with this command:
# zpool status -x
If all pools are Online and everything is normal, the message shows:
all pools are healthy
If there is an issue, perhaps a disk is in the Offline state, the pool state will look similar to:
pool: storage state: DEGRADED status: One or more devices has been taken offline by the administrator. Sufficient replicas exist for the pool to continue functioning in a degraded state. action: Online the device using 'zpool online' or replace the device with 'zpool replace'. scrub: none requested config: NAME STATE READ WRITE CKSUM storage DEGRADED 0 0 0 raidz1 DEGRADED 0 0 0 da0 ONLINE 0 0 0 da1 OFFLINE 0 0 0 da2 ONLINE 0 0 0 errors: No known data errors
This indicates that the device was previously taken offline by the administrator with this command:
# zpool offline storage da1
Now the system can be powered down to replace da1. When the system is back online, the failed disk can replaced in the pool:
# zpool replace storage da1
From here, the status may be checked again, this time without
-x so that all pools are shown:
# zpool status storage pool: storage state: ONLINE scrub: resilver completed with 0 errors on Sat Aug 30 19:44:11 2008 config: NAME STATE READ WRITE CKSUM storage ONLINE 0 0 0 raidz1 ONLINE 0 0 0 da0 ONLINE 0 0 0 da1 ONLINE 0 0 0 da2 ONLINE 0 0 0 errors: No known data errors
In this example, everything is normal.
20.2.4. Data Verification
ZFS uses checksums to verify the integrity of stored data. These are enabled automatically upon creation of file systems.
Checksums can be disabled, but it is not recommended! Checksums take very little storage space and provide data integrity. Many ZFS features will not work properly with checksums disabled. There is no noticeable performance gain from disabling these checksums.
Checksum verification is known as scrubbing. Verify the data integrity of the
storage pool with this command:
# zpool scrub storage
The duration of a scrub depends on the amount of data stored. Larger amounts of data will take proportionally longer to verify. Scrubs are very I/O intensive, and only one scrub is allowed to run at a time. After the scrub completes, the status can be viewed with
# zpool status storage pool: storage state: ONLINE scrub: scrub completed with 0 errors on Sat Jan 26 19:57:37 2013 config: NAME STATE READ WRITE CKSUM storage ONLINE 0 0 0 raidz1 ONLINE 0 0 0 da0 ONLINE 0 0 0 da1 ONLINE 0 0 0 da2 ONLINE 0 0 0 errors: No known data errors
The completion date of the last scrub operation is displayed to help track when another scrub is required. Routine scrubs help protect data from silent corruption and ensure the integrity of the pool.
ZFS administration is divided between two main utilities. The
zpool utility controls the operation of the pool and deals with adding, removing, replacing, and managing disks. The
zfs utility deals with creating, destroying, and managing datasets, both file systems and volumes.
20.3.1. Creating and Destroying Storage Pools
Creating a ZFS storage pool (zpool) involves making a number of decisions that are relatively permanent because the structure of the pool cannot be changed after the pool has been created. The most important decision is what types of vdevs into which to group the physical disks. See the list of vdev types for details about the possible options. After the pool has been created, most vdev types do not allow additional disks to be added to the vdev. The exceptions are mirrors, which allow additional disks to be added to the vdev, and stripes, which can be upgraded to mirrors by attaching an additional disk to the vdev. Although additional vdevs can be added to expand a pool, the layout of the pool cannot be changed after pool creation. Instead, the data must be backed up and the pool destroyed and recreated.
Create a simple mirror pool:
# zpool create mypool mirror /dev/ada1 /dev/ada2 # zpool status pool: mypool state: ONLINE scan: none requested config: NAME STATE READ WRITE CKSUM mypool ONLINE 0 0 0 mirror-0 ONLINE 0 0 0 ada1 ONLINE 0 0 0 ada2 ONLINE 0 0 0 errors: No known data errors
Multiple vdevs can be created at once. Specify multiple groups of disks separated by the vdev type keyword,
mirror in this example:
# zpool create mypool mirror /dev/ada1 /dev/ada2 mirror /dev/ada3 /dev/ada4 # zpool status pool: mypool state: ONLINE scan: none requested config: NAME STATE READ WRITE CKSUM mypool ONLINE 0 0 0 mirror-0 ONLINE 0 0 0 ada1 ONLINE 0 0 0 ada2 ONLINE 0 0 0 mirror-1 ONLINE 0 0 0 ada3 ONLINE 0 0 0 ada4 ONLINE 0 0 0 errors: No known data errors
Pools can also be constructed using partitions rather than whole disks. Putting ZFS in a separate partition allows the same disk to have other partitions for other purposes. In particular, partitions with bootcode and file systems needed for booting can be added. This allows booting from disks that are also members of a pool. There is no performance penalty on FreeBSD when using a partition rather than a whole disk. Using partitions also allows the administrator to under-provision the disks, using less than the full capacity. If a future replacement disk of the same nominal size as the original actually has a slightly smaller capacity, the smaller partition will still fit, and the replacement disk can still be used.
Create a RAID-Z2 pool using partitions:
# zpool create mypool raidz2 /dev/ada0p3 /dev/ada1p3 /dev/ada2p3 /dev/ada3p3 /dev/ada4p3 /dev/ada5p3 # zpool status pool: mypool state: ONLINE scan: none requested config: NAME STATE READ WRITE CKSUM mypool ONLINE 0 0 0 raidz2-0 ONLINE 0 0 0 ada0p3 ONLINE 0 0 0 ada1p3 ONLINE 0 0 0 ada2p3 ONLINE 0 0 0 ada3p3 ONLINE 0 0 0 ada4p3 ONLINE 0 0 0 ada5p3 ONLINE 0 0 0 errors: No known data errors
A pool that is no longer needed can be destroyed so that the disks can be reused. Destroying a pool involves first unmounting all of the datasets in that pool. If the datasets are in use, the unmount operation will fail and the pool will not be destroyed. The destruction of the pool can be forced with
-f, but this can cause undefined behavior in applications which had open files on those datasets.
20.3.2. Adding and Removing Devices
There are two cases for adding disks to a zpool: attaching a disk to an existing vdev with
zpool attach, or adding vdevs to the pool with
zpool add. Only some vdev types allow disks to be added to the vdev after creation.
A pool created with a single disk lacks redundancy. Corruption can be detected but not repaired, because there is no other copy of the data. The copies property may be able to recover from a small failure such as a bad sector, but does not provide the same level of protection as mirroring or RAID-Z. Starting with a pool consisting of a single disk vdev,
zpool attach can be used to add an additional disk to the vdev, creating a mirror.
zpool attach can also be used to add additional disks to a mirror group, increasing redundancy and read performance. If the disks being used for the pool are partitioned, replicate the layout of the first disk on to the second.
gpart backup and
gpart restore can be used to make this process easier.
Upgrade the single disk (stripe) vdev ada0p3 to a mirror by attaching ada1p3:
# zpool status pool: mypool state: ONLINE scan: none requested config: NAME STATE READ WRITE CKSUM mypool ONLINE 0 0 0 ada0p3 ONLINE 0 0 0 errors: No known data errors # zpool attach mypool ada0p3 ada1p3 Make sure to wait until resilver is done before rebooting. If you boot from pool 'mypool', you may need to update boot code on newly attached disk 'ada1p3'. Assuming you use GPT partitioning and 'da0' is your new boot disk you may use the following command: gpart bootcode -b /boot/pmbr -p /boot/gptzfsboot -i 1 da0 # gpart bootcode -b /boot/pmbr -p /boot/gptzfsboot -i 1 ada1 bootcode written to ada1 # zpool status pool: mypool state: ONLINE status: One or more devices is currently being resilvered. The pool will continue to function, possibly in a degraded state. action: Wait for the resilver to complete. scan: resilver in progress since Fri May 30 08:19:19 2014 527M scanned out of 781M at 47.9M/s, 0h0m to go 527M resilvered, 67.53% done config: NAME STATE READ WRITE CKSUM mypool ONLINE 0 0 0 mirror-0 ONLINE 0 0 0 ada0p3 ONLINE 0 0 0 ada1p3 ONLINE 0 0 0 (resilvering) errors: No known data errors # zpool status pool: mypool state: ONLINE scan: resilvered 781M in 0h0m with 0 errors on Fri May 30 08:15:58 2014 config: NAME STATE READ WRITE CKSUM mypool ONLINE 0 0 0 mirror-0 ONLINE 0 0 0 ada0p3 ONLINE 0 0 0 ada1p3 ONLINE 0 0 0 errors: No known data errors
When adding disks to the existing vdev is not an option, as for RAID-Z, an alternative method is to add another vdev to the pool. Additional vdevs provide higher performance, distributing writes across the vdevs. Each vdev is responsible for providing its own redundancy. It is possible, but discouraged, to mix vdev types, like
RAID-Z. Adding a non-redundant vdev to a pool containing mirror or RAID-Z vdevs risks the data on the entire pool. Writes are distributed, so the failure of the non-redundant disk will result in the loss of a fraction of every block that has been written to the pool.
Data is striped across each of the vdevs. For example, with two mirror vdevs, this is effectively a RAID 10 that stripes writes across two sets of mirrors. Space is allocated so that each vdev reaches 100% full at the same time. There is a performance penalty if the vdevs have different amounts of free space, as a disproportionate amount of the data is written to the less full vdev.
When attaching additional devices to a boot pool, remember to update the bootcode.
Attach a second mirror group (ada2p3 and ada3p3) to the existing mirror:
# zpool status pool: mypool state: ONLINE scan: resilvered 781M in 0h0m with 0 errors on Fri May 30 08:19:35 2014 config: NAME STATE READ WRITE CKSUM mypool ONLINE 0 0 0 mirror-0 ONLINE 0 0 0 ada0p3 ONLINE 0 0 0 ada1p3 ONLINE 0 0 0 errors: No known data errors # zpool add mypool mirror ada2p3 ada3p3 # gpart bootcode -b /boot/pmbr -p /boot/gptzfsboot -i 1 ada2 bootcode written to ada2 # gpart bootcode -b /boot/pmbr -p /boot/gptzfsboot -i 1 ada3 bootcode written to ada3 # zpool status pool: mypool state: ONLINE scan: scrub repaired 0 in 0h0m with 0 errors on Fri May 30 08:29:51 2014 config: NAME STATE READ WRITE CKSUM mypool ONLINE 0 0 0 mirror-0 ONLINE 0 0 0 ada0p3 ONLINE 0 0 0 ada1p3 ONLINE 0 0 0 mirror-1 ONLINE 0 0 0 ada2p3 ONLINE 0 0 0 ada3p3 ONLINE 0 0 0 errors: No known data errors
Currently, vdevs cannot be removed from a pool, and disks can only be removed from a mirror if there is enough remaining redundancy. If only one disk in a mirror group remains, it ceases to be a mirror and reverts to being a stripe, risking the entire pool if that remaining disk fails.
Remove a disk from a three-way mirror group:
# zpool status pool: mypool state: ONLINE scan: scrub repaired 0 in 0h0m with 0 errors on Fri May 30 08:29:51 2014 config: NAME STATE READ WRITE CKSUM mypool ONLINE 0 0 0 mirror-0 ONLINE 0 0 0 ada0p3 ONLINE 0 0 0 ada1p3 ONLINE 0 0 0 ada2p3 ONLINE 0 0 0 errors: No known data errors # zpool detach mypool ada2p3 # zpool status pool: mypool state: ONLINE scan: scrub repaired 0 in 0h0m with 0 errors on Fri May 30 08:29:51 2014 config: NAME STATE READ WRITE CKSUM mypool ONLINE 0 0 0 mirror-0 ONLINE 0 0 0 ada0p3 ONLINE 0 0 0 ada1p3 ONLINE 0 0 0 errors: No known data errors
20.3.3. Checking the Status of a Pool
Pool status is important. If a drive goes offline or a read, write, or checksum error is detected, the corresponding error count increases. The
status output shows the configuration and status of each device in the pool and the status of the entire pool. Actions that need to be taken and details about the last
scrub are also shown.
# zpool status pool: mypool state: ONLINE scan: scrub repaired 0 in 2h25m with 0 errors on Sat Sep 14 04:25:50 2013 config: NAME STATE READ WRITE CKSUM mypool ONLINE 0 0 0 raidz2-0 ONLINE 0 0 0 ada0p3 ONLINE 0 0 0 ada1p3 ONLINE 0 0 0 ada2p3 ONLINE 0 0 0 ada3p3 ONLINE 0 0 0 ada4p3 ONLINE 0 0 0 ada5p3 ONLINE 0 0 0 errors: No known data errors
20.3.4. Clearing Errors
When an error is detected, the read, write, or checksum counts are incremented. The error message can be cleared and the counts reset with
zpool clear mypool. Clearing the error state can be important for automated scripts that alert the administrator when the pool encounters an error. Further errors may not be reported if the old errors are not cleared.
20.3.5. Replacing a Functioning Device
There are a number of situations where it may be desirable to replace one disk with a different disk. When replacing a working disk, the process keeps the old disk online during the replacement. The pool never enters a degraded state, reducing the risk of data loss.
zpool replace copies all of the data from the old disk to the new one. After the operation completes, the old disk is disconnected from the vdev. If the new disk is larger than the old disk, it may be possible to grow the zpool, using the new space. See Growing a Pool.
Replace a functioning device in the pool:
# zpool status pool: mypool state: ONLINE scan: none requested config: NAME STATE READ WRITE CKSUM mypool ONLINE 0 0 0 mirror-0 ONLINE 0 0 0 ada0p3 ONLINE 0 0 0 ada1p3 ONLINE 0 0 0 errors: No known data errors # zpool replace mypool ada1p3 ada2p3 Make sure to wait until resilver is done before rebooting. If you boot from pool 'zroot', you may need to update boot code on newly attached disk 'ada2p3'. Assuming you use GPT partitioning and 'da0' is your new boot disk you may use the following command: gpart bootcode -b /boot/pmbr -p /boot/gptzfsboot -i 1 da0 # gpart bootcode -b /boot/pmbr -p /boot/gptzfsboot -i 1 ada2 # zpool status pool: mypool state: ONLINE status: One or more devices is currently being resilvered. The pool will continue to function, possibly in a degraded state. action: Wait for the resilver to complete. scan: resilver in progress since Mon Jun 2 14:21:35 2014 604M scanned out of 781M at 46.5M/s, 0h0m to go 604M resilvered, 77.39% done config: NAME STATE READ WRITE CKSUM mypool ONLINE 0 0 0 mirror-0 ONLINE 0 0 0 ada0p3 ONLINE 0 0 0 replacing-1 ONLINE 0 0 0 ada1p3 ONLINE 0 0 0 ada2p3 ONLINE 0 0 0 (resilvering) errors: No known data errors # zpool status pool: mypool state: ONLINE scan: resilvered 781M in 0h0m with 0 errors on Mon Jun 2 14:21:52 2014 config: NAME STATE READ WRITE CKSUM mypool ONLINE 0 0 0 mirror-0 ONLINE 0 0 0 ada0p3 ONLINE 0 0 0 ada2p3 ONLINE 0 0 0 errors: No known data errors
20.3.6. Dealing with Failed Devices
When a disk in a pool fails, the vdev to which the disk belongs enters the degraded state. All of the data is still available, but performance may be reduced because missing data must be calculated from the available redundancy. To restore the vdev to a fully functional state, the failed physical device must be replaced. ZFS is then instructed to begin the resilver operation. Data that was on the failed device is recalculated from available redundancy and written to the replacement device. After completion, the vdev returns to online status.
If the vdev does not have any redundancy, or if multiple devices have failed and there is not enough redundancy to compensate, the pool enters the faulted state. If a sufficient number of devices cannot be reconnected to the pool, the pool becomes inoperative and data must be restored from backups.
When replacing a failed disk, the name of the failed disk is replaced with the GUID of the device. A new device name parameter for
zpool replace is not required if the replacement device has the same device name.
Replace a failed disk using
# zpool status pool: mypool state: DEGRADED status: One or more devices could not be opened. Sufficient replicas exist for the pool to continue functioning in a degraded state. action: Attach the missing device and online it using 'zpool online'. see: http://illumos.org/msg/ZFS-8000-2Q scan: none requested config: NAME STATE READ WRITE CKSUM mypool DEGRADED 0 0 0 mirror-0 DEGRADED 0 0 0 ada0p3 ONLINE 0 0 0 316502962686821739 UNAVAIL 0 0 0 was /dev/ada1p3 errors: No known data errors # zpool replace mypool 316502962686821739 ada2p3 # zpool status pool: mypool state: DEGRADED status: One or more devices is currently being resilvered. The pool will continue to function, possibly in a degraded state. action: Wait for the resilver to complete. scan: resilver in progress since Mon Jun 2 14:52:21 2014 641M scanned out of 781M at 49.3M/s, 0h0m to go 640M resilvered, 82.04% done config: NAME STATE READ WRITE CKSUM mypool DEGRADED 0 0 0 mirror-0 DEGRADED 0 0 0 ada0p3 ONLINE 0 0 0 replacing-1 UNAVAIL 0 0 0 15732067398082357289 UNAVAIL 0 0 0 was /dev/ada1p3/old ada2p3 ONLINE 0 0 0 (resilvering) errors: No known data errors # zpool status pool: mypool state: ONLINE scan: resilvered 781M in 0h0m with 0 errors on Mon Jun 2 14:52:38 2014 config: NAME STATE READ WRITE CKSUM mypool ONLINE 0 0 0 mirror-0 ONLINE 0 0 0 ada0p3 ONLINE 0 0 0 ada2p3 ONLINE 0 0 0 errors: No known data errors
20.3.7. Scrubbing a Pool
It is recommended that pools be scrubbed regularly, ideally at least once every month. The
scrub operation is very disk-intensive and will reduce performance while running. Avoid high-demand periods when scheduling
scrub or use
vfs.zfs.scrub_delay to adjust the relative priority of the
scrub to prevent it interfering with other workloads.
# zpool scrub mypool # zpool status pool: mypool state: ONLINE scan: scrub in progress since Wed Feb 19 20:52:54 2014 116G scanned out of 8.60T at 649M/s, 3h48m to go 0 repaired, 1.32% done config: NAME STATE READ WRITE CKSUM mypool ONLINE 0 0 0 raidz2-0 ONLINE 0 0 0 ada0p3 ONLINE 0 0 0 ada1p3 ONLINE 0 0 0 ada2p3 ONLINE 0 0 0 ada3p3 ONLINE 0 0 0 ada4p3 ONLINE 0 0 0 ada5p3 ONLINE 0 0 0 errors: No known data errors
In the event that a scrub operation needs to be cancelled, issue
zpool scrub -s mypool.
The checksums stored with data blocks enable the file system to self-heal. This feature will automatically repair data whose checksum does not match the one recorded on another device that is part of the storage pool. For example, a mirror with two disks where one drive is starting to malfunction and cannot properly store the data any more. This is even worse when the data has not been accessed for a long time, as with long term archive storage. Traditional file systems need to run algorithms that check and repair the data like fsck(8). These commands take time, and in severe cases, an administrator has to manually decide which repair operation must be performed. When ZFS detects a data block with a checksum that does not match, it tries to read the data from the mirror disk. If that disk can provide the correct data, it will not only give that data to the application requesting it, but also correct the wrong data on the disk that had the bad checksum. This happens without any interaction from a system administrator during normal pool operation.
The next example demonstrates this self-healing behavior. A mirrored pool of disks /dev/ada0 and /dev/ada1 is created.
# zpool create healer mirror /dev/ada0 /dev/ada1 # zpool status healer pool: healer state: ONLINE scan: none requested config: NAME STATE READ WRITE CKSUM healer ONLINE 0 0 0 mirror-0 ONLINE 0 0 0 ada0 ONLINE 0 0 0 ada1 ONLINE 0 0 0 errors: No known data errors # zpool list NAME SIZE ALLOC FREE CKPOINT EXPANDSZ FRAG CAP DEDUP HEALTH ALTROOT healer 960M 92.5K 960M - - 0% 0% 1.00x ONLINE -
Some important data that have to be protected from data errors using the self-healing feature are copied to the pool. A checksum of the pool is created for later comparison.
# cp /some/important/data /healer # zfs list NAME SIZE ALLOC FREE CAP DEDUP HEALTH ALTROOT healer 960M 67.7M 892M 7% 1.00x ONLINE - # sha1 /healer > checksum.txt # cat checksum.txt SHA1 (/healer) = 2753eff56d77d9a536ece6694bf0a82740344d1f
Data corruption is simulated by writing random data to the beginning of one of the disks in the mirror. To prevent ZFS from healing the data as soon as it is detected, the pool is exported before the corruption and imported again afterwards.
This is a dangerous operation that can destroy vital data. It is shown here for demonstrational purposes only and should not be attempted during normal operation of a storage pool. Nor should this intentional corruption example be run on any disk with a different file system on it. Do not use any other disk device names other than the ones that are part of the pool. Make certain that proper backups of the pool are created before running the command!
# zpool export healer # dd if=/dev/random of=/dev/ada1 bs=1m count=200 200+0 records in 200+0 records out 209715200 bytes transferred in 62.992162 secs (3329227 bytes/sec) # zpool import healer
The pool status shows that one device has experienced an error. Note that applications reading data from the pool did not receive any incorrect data. ZFS provided data from the ada0 device with the correct checksums. The device with the wrong checksum can be found easily as the
CKSUM column contains a nonzero value.
# zpool status healer pool: healer state: ONLINE status: One or more devices has experienced an unrecoverable error. An attempt was made to correct the error. Applications are unaffected. action: Determine if the device needs to be replaced, and clear the errors using 'zpool clear' or replace the device with 'zpool replace'. see: http://illumos.org/msg/ZFS-8000-4J scan: none requested config: NAME STATE READ WRITE CKSUM healer ONLINE 0 0 0 mirror-0 ONLINE 0 0 0 ada0 ONLINE 0 0 0 ada1 ONLINE 0 0 1 errors: No known data errors
The error was detected and handled by using the redundancy present in the unaffected ada0 mirror disk. A checksum comparison with the original one will reveal whether the pool is consistent again.
# sha1 /healer >> checksum.txt # cat checksum.txt SHA1 (/healer) = 2753eff56d77d9a536ece6694bf0a82740344d1f SHA1 (/healer) = 2753eff56d77d9a536ece6694bf0a82740344d1f
The two checksums that were generated before and after the intentional tampering with the pool data still match. This shows how ZFS is capable of detecting and correcting any errors automatically when the checksums differ. Note that this is only possible when there is enough redundancy present in the pool. A pool consisting of a single device has no self-healing capabilities. That is also the reason why checksums are so important in ZFS and should not be disabled for any reason. No fsck(8) or similar file system consistency check program is required to detect and correct this and the pool was still available during the time there was a problem. A scrub operation is now required to overwrite the corrupted data on ada1.
# zpool scrub healer # zpool status healer pool: healer state: ONLINE status: One or more devices has experienced an unrecoverable error. An attempt was made to correct the error. Applications are unaffected. action: Determine if the device needs to be replaced, and clear the errors using 'zpool clear' or replace the device with 'zpool replace'. see: http://illumos.org/msg/ZFS-8000-4J scan: scrub in progress since Mon Dec 10 12:23:30 2012 10.4M scanned out of 67.0M at 267K/s, 0h3m to go 9.63M repaired, 15.56% done config: NAME STATE READ WRITE CKSUM healer ONLINE 0 0 0 mirror-0 ONLINE 0 0 0 ada0 ONLINE 0 0 0 ada1 ONLINE 0 0 627 (repairing) errors: No known data errors
The scrub operation reads data from ada0 and rewrites any data with an incorrect checksum on ada1. This is indicated by the
(repairing) output from
zpool status. After the operation is complete, the pool status changes to:
# zpool status healer pool: healer state: ONLINE status: One or more devices has experienced an unrecoverable error. An attempt was made to correct the error. Applications are unaffected. action: Determine if the device needs to be replaced, and clear the errors using 'zpool clear' or replace the device with 'zpool replace'. see: http://illumos.org/msg/ZFS-8000-4J scan: scrub repaired 66.5M in 0h2m with 0 errors on Mon Dec 10 12:26:25 2012 config: NAME STATE READ WRITE CKSUM healer ONLINE 0 0 0 mirror-0 ONLINE 0 0 0 ada0 ONLINE 0 0 0 ada1 ONLINE 0 0 2.72K errors: No known data errors
After the scrub operation completes and all the data has been synchronized from ada0 to ada1, the error messages can be cleared from the pool status by running
# zpool clear healer # zpool status healer pool: healer state: ONLINE scan: scrub repaired 66.5M in 0h2m with 0 errors on Mon Dec 10 12:26:25 2012 config: NAME STATE READ WRITE CKSUM healer ONLINE 0 0 0 mirror-0 ONLINE 0 0 0 ada0 ONLINE 0 0 0 ada1 ONLINE 0 0 0 errors: No known data errors
The pool is now back to a fully working state and all the errors have been cleared.
20.3.9. Growing a Pool
The usable size of a redundant pool is limited by the capacity of the smallest device in each vdev. The smallest device can be replaced with a larger device. After completing a replace or resilver operation, the pool can grow to use the capacity of the new device. For example, consider a mirror of a 1 TB drive and a 2 TB drive. The usable space is 1 TB. When the 1 TB drive is replaced with another 2 TB drive, the resilvering process copies the existing data onto the new drive. As both of the devices now have 2 TB capacity, the mirror’s available space can be grown to 2 TB.
Expansion is triggered by using
zpool online -e on each device. After expansion of all devices, the additional space becomes available to the pool.
20.3.10. Importing and Exporting Pools
Pools are exported before moving them to another system. All datasets are unmounted, and each device is marked as exported but still locked so it cannot be used by other disk subsystems. This allows pools to be imported on other machines, other operating systems that support ZFS, and even different hardware architectures (with some caveats, see zpool(8)). When a dataset has open files,
zpool export -f can be used to force the export of a pool. Use this with caution. The datasets are forcibly unmounted, potentially resulting in unexpected behavior by the applications which had open files on those datasets.
Export a pool that is not in use:
# zpool export mypool
Importing a pool automatically mounts the datasets. This may not be the desired behavior, and can be prevented with
zpool import -N.
zpool import -o sets temporary properties for this import only.
zpool import altroot= allows importing a pool with a base mount point instead of the root of the file system. If the pool was last used on a different system and was not properly exported, an import might have to be forced with
zpool import -f.
zpool import -a imports all pools that do not appear to be in use by another system.
List all available pools for import:
# zpool import pool: mypool id: 9930174748043525076 state: ONLINE action: The pool can be imported using its name or numeric identifier. config: mypool ONLINE ada2p3 ONLINE
Import the pool with an alternative root directory:
# zpool import -o altroot=/mnt mypool # zfs list zfs list NAME USED AVAIL REFER MOUNTPOINT mypool 110K 47.0G 31K /mnt/mypool
20.3.11. Upgrading a Storage Pool
After upgrading FreeBSD, or if a pool has been imported from a system using an older version of ZFS, the pool can be manually upgraded to the latest version of ZFS to support newer features. Consider whether the pool may ever need to be imported on an older system before upgrading. Upgrading is a one-way process. Older pools can be upgraded, but pools with newer features cannot be downgraded.
Upgrade a v28 pool to support
# zpool status pool: mypool state: ONLINE status: The pool is formatted using a legacy on-disk format. The pool can still be used, but some features are unavailable. action: Upgrade the pool using 'zpool upgrade'. Once this is done, the pool will no longer be accessible on software that does not support feat flags. scan: none requested config: NAME STATE READ WRITE CKSUM mypool ONLINE 0 0 0 mirror-0 ONLINE 0 0 0 ada0 ONLINE 0 0 0 ada1 ONLINE 0 0 0 errors: No known data errors # zpool upgrade This system supports ZFS pool feature flags. The following pools are formatted with legacy version numbers and can be upgraded to use feature flags. After being upgraded, these pools will no longer be accessible by software that does not support feature flags. VER POOL --- ------------ 28 mypool Use 'zpool upgrade -v' for a list of available legacy versions. Every feature flags pool has all supported features enabled. # zpool upgrade mypool This system supports ZFS pool feature flags. Successfully upgraded 'mypool' from version 28 to feature flags. Enabled the following features on 'mypool': async_destroy empty_bpobj lz4_compress multi_vdev_crash_dump
The newer features of ZFS will not be available until
zpool upgrade has completed.
zpool upgrade -v can be used to see what new features will be provided by upgrading, as well as which features are already supported.
Upgrade a pool to support additional feature flags:
# zpool status pool: mypool state: ONLINE status: Some supported features are not enabled on the pool. The pool can still be used, but some features are unavailable. action: Enable all features using 'zpool upgrade'. Once this is done, the pool may no longer be accessible by software that does not support the features. See zpool-features(7) for details. scan: none requested config: NAME STATE READ WRITE CKSUM mypool ONLINE 0 0 0 mirror-0 ONLINE 0 0 0 ada0 ONLINE 0 0 0 ada1 ONLINE 0 0 0 errors: No known data errors # zpool upgrade This system supports ZFS pool feature flags. All pools are formatted using feature flags. Some supported features are not enabled on the following pools. Once a feature is enabled the pool may become incompatible with software that does not support the feature. See zpool-features(7) for details. POOL FEATURE --------------- zstore multi_vdev_crash_dump spacemap_histogram enabled_txg hole_birth extensible_dataset bookmarks filesystem_limits # zpool upgrade mypool This system supports ZFS pool feature flags. Enabled the following features on 'mypool': spacemap_histogram enabled_txg hole_birth extensible_dataset bookmarks filesystem_limits
The boot code on systems that boot from a pool must be updated to support the new pool version. Use
For legacy boot using GPT, use the following command:
For systems using EFI to boot, execute the following command:
Apply the bootcode to all bootable disks in the pool. See gpart(8) for more information.
20.3.12. Displaying Recorded Pool History
Commands that modify the pool are recorded. Recorded actions include the creation of datasets, changing properties, or replacement of a disk. This history is useful for reviewing how a pool was created and which user performed a specific action and when. History is not kept in a log file, but is part of the pool itself. The command to review this history is aptly named
# zpool history History for 'tank': 2013-02-26.23:02:35 zpool create tank mirror /dev/ada0 /dev/ada1 2013-02-27.18:50:58 zfs set atime=off tank 2013-02-27.18:51:09 zfs set checksum=fletcher4 tank 2013-02-27.18:51:18 zfs create tank/backup
The output shows
zfs commands that were executed on the pool along with a timestamp. Only commands that alter the pool in some way are recorded. Commands like
zfs list are not included. When no pool name is specified, the history of all pools is displayed.
zpool history can show even more information when the options
-l are provided.
-i displays user-initiated events as well as internally logged ZFS events.
# zpool history -i History for 'tank': 2013-02-26.23:02:35 [internal pool create txg:5] pool spa 28; zfs spa 28; zpl 5;uts 9.1-RELEASE 901000 amd64 2013-02-27.18:50:53 [internal property set txg:50] atime=0 dataset = 21 2013-02-27.18:50:58 zfs set atime=off tank 2013-02-27.18:51:04 [internal property set txg:53] checksum=7 dataset = 21 2013-02-27.18:51:09 zfs set checksum=fletcher4 tank 2013-02-27.18:51:13 [internal create txg:55] dataset = 39 2013-02-27.18:51:18 zfs create tank/backup
More details can be shown by adding
-l. History records are shown in a long format, including information like the name of the user who issued the command and the hostname on which the change was made.
# zpool history -l History for 'tank': 2013-02-26.23:02:35 zpool create tank mirror /dev/ada0 /dev/ada1 [user 0 (root) on :global] 2013-02-27.18:50:58 zfs set atime=off tank [user 0 (root) on myzfsbox:global] 2013-02-27.18:51:09 zfs set checksum=fletcher4 tank [user 0 (root) on myzfsbox:global] 2013-02-27.18:51:18 zfs create tank/backup [user 0 (root) on myzfsbox:global]
The output shows that the
root user created the mirrored pool with disks /dev/ada0 and /dev/ada1. The hostname
myzfsbox is also shown in the commands after the pool’s creation. The hostname display becomes important when the pool is exported from one system and imported on another. The commands that are issued on the other system can clearly be distinguished by the hostname that is recorded for each command.
Both options to
zpool history can be combined to give the most detailed information possible for any given pool. Pool history provides valuable information when tracking down the actions that were performed or when more detailed output is needed for debugging.
20.3.13. Performance Monitoring
A built-in monitoring system can display pool I/O statistics in real time. It shows the amount of free and used space on the pool, how many read and write operations are being performed per second, and how much I/O bandwidth is currently being utilized. By default, all pools in the system are monitored and displayed. A pool name can be provided to limit monitoring to just that pool. A basic example:
# zpool iostat capacity operations bandwidth pool alloc free read write read write ---------- ----- ----- ----- ----- ----- ----- data 288G 1.53T 2 11 11.3K 57.1K
To continuously monitor I/O activity, a number can be specified as the last parameter, indicating a interval in seconds to wait between updates. The next statistic line is printed after each interval. Press Ctrl+C to stop this continuous monitoring. Alternatively, give a second number on the command line after the interval to specify the total number of statistics to display.
Even more detailed I/O statistics can be displayed with
-v. Each device in the pool is shown with a statistics line. This is useful in seeing how many read and write operations are being performed on each device, and can help determine if any individual device is slowing down the pool. This example shows a mirrored pool with two devices:
# zpool iostat -v capacity operations bandwidth pool alloc free read write read write ----------------------- ----- ----- ----- ----- ----- ----- data 288G 1.53T 2 12 9.23K 61.5K mirror 288G 1.53T 2 12 9.23K 61.5K ada1 - - 0 4 5.61K 61.7K ada2 - - 1 4 5.04K 61.7K ----------------------- ----- ----- ----- ----- ----- -----
20.3.14. Splitting a Storage Pool
A pool consisting of one or more mirror vdevs can be split into two pools. Unless otherwise specified, the last member of each mirror is detached and used to create a new pool containing the same data. The operation should first be attempted with
-n. The details of the proposed operation are displayed without it actually being performed. This helps confirm that the operation will do what the user intends.
zfs utility is responsible for creating, destroying, and managing all ZFS datasets that exist within a pool. The pool is managed using
20.4.1. Creating and Destroying Datasets
Unlike traditional disks and volume managers, space in ZFS is not preallocated. With traditional file systems, after all of the space is partitioned and assigned, there is no way to add an additional file system without adding a new disk. With ZFS, new file systems can be created at any time. Each dataset has properties including features like compression, deduplication, caching, and quotas, as well as other useful properties like readonly, case sensitivity, network file sharing, and a mount point. Datasets can be nested inside each other, and child datasets will inherit properties from their parents. Each dataset can be administered, delegated, replicated, snapshotted, jailed, and destroyed as a unit. There are many advantages to creating a separate dataset for each different type or set of files. The only drawbacks to having an extremely large number of datasets is that some commands like
zfs list will be slower, and the mounting of hundreds or even thousands of datasets can slow the FreeBSD boot process.
Create a new dataset and enable LZ4 compression on it:
# zfs list NAME USED AVAIL REFER MOUNTPOINT mypool 781M 93.2G 144K none mypool/ROOT 777M 93.2G 144K none mypool/ROOT/default 777M 93.2G 777M / mypool/tmp 176K 93.2G 176K /tmp mypool/usr 616K 93.2G 144K /usr mypool/usr/home 184K 93.2G 184K /usr/home mypool/usr/ports 144K 93.2G 144K /usr/ports mypool/usr/src 144K 93.2G 144K /usr/src mypool/var 1.20M 93.2G 608K /var mypool/var/crash 148K 93.2G 148K /var/crash mypool/var/log 178K 93.2G 178K /var/log mypool/var/mail 144K 93.2G 144K /var/mail mypool/var/tmp 152K 93.2G 152K /var/tmp # zfs create -o compress=lz4 mypool/usr/mydataset # zfs list NAME USED AVAIL REFER MOUNTPOINT mypool 781M 93.2G 144K none mypool/ROOT 777M 93.2G 144K none mypool/ROOT/default 777M 93.2G 777M / mypool/tmp 176K 93.2G 176K /tmp mypool/usr 704K 93.2G 144K /usr mypool/usr/home 184K 93.2G 184K /usr/home mypool/usr/mydataset 87.5K 93.2G 87.5K /usr/mydataset mypool/usr/ports 144K 93.2G 144K /usr/ports mypool/usr/src 144K 93.2G 144K /usr/src mypool/var 1.20M 93.2G 610K /var mypool/var/crash 148K 93.2G 148K /var/crash mypool/var/log 178K 93.2G 178K /var/log mypool/var/mail 144K 93.2G 144K /var/mail mypool/var/tmp 152K 93.2G 152K /var/tmp
Destroying a dataset is much quicker than deleting all of the files that reside on the dataset, as it does not involve scanning all of the files and updating all of the corresponding metadata.
Destroy the previously-created dataset:
# zfs list NAME USED AVAIL REFER MOUNTPOINT mypool 880M 93.1G 144K none mypool/ROOT 777M 93.1G 144K none mypool/ROOT/default 777M 93.1G 777M / mypool/tmp 176K 93.1G 176K /tmp mypool/usr 101M 93.1G 144K /usr mypool/usr/home 184K 93.1G 184K /usr/home mypool/usr/mydataset 100M 93.1G 100M /usr/mydataset mypool/usr/ports 144K 93.1G 144K /usr/ports mypool/usr/src 144K 93.1G 144K /usr/src mypool/var 1.20M 93.1G 610K /var mypool/var/crash 148K 93.1G 148K /var/crash mypool/var/log 178K 93.1G 178K /var/log mypool/var/mail 144K 93.1G 144K /var/mail mypool/var/tmp 152K 93.1G 152K /var/tmp # zfs destroy mypool/usr/mydataset # zfs list NAME USED AVAIL REFER MOUNTPOINT mypool 781M 93.2G 144K none mypool/ROOT 777M 93.2G 144K none mypool/ROOT/default 777M 93.2G 777M / mypool/tmp 176K 93.2G 176K /tmp mypool/usr 616K 93.2G 144K /usr mypool/usr/home 184K 93.2G 184K /usr/home mypool/usr/ports 144K 93.2G 144K /usr/ports mypool/usr/src 144K 93.2G 144K /usr/src mypool/var 1.21M 93.2G 612K /var mypool/var/crash 148K 93.2G 148K /var/crash mypool/var/log 178K 93.2G 178K /var/log mypool/var/mail 144K 93.2G 144K /var/mail mypool/var/tmp 152K 93.2G 152K /var/tmp
In modern versions of ZFS,
zfs destroy is asynchronous, and the free space might take several minutes to appear in the pool. Use
zpool get freeing poolname to see the
freeing property, indicating how many datasets are having their blocks freed in the background. If there are child datasets, like snapshots or other datasets, then the parent cannot be destroyed. To destroy a dataset and all of its children, use
-r to recursively destroy the dataset and all of its children. Use
-n -v to list datasets and snapshots that would be destroyed by this operation, but do not actually destroy anything. Space that would be reclaimed by destruction of snapshots is also shown.
20.4.2. Creating and Destroying Volumes
A volume is a special type of dataset. Rather than being mounted as a file system, it is exposed as a block device under /dev/zvol/poolname/dataset. This allows the volume to be used for other file systems, to back the disks of a virtual machine, or to be exported using protocols like iSCSI or HAST.
A volume can be formatted with any file system, or used without a file system to store raw data. To the user, a volume appears to be a regular disk. Putting ordinary file systems on these zvols provides features that ordinary disks or file systems do not normally have. For example, using the compression property on a 250 MB volume allows creation of a compressed FAT file system.
# zfs create -V 250m -o compression=on tank/fat32 # zfs list tank NAME USED AVAIL REFER MOUNTPOINT tank 258M 670M 31K /tank # newfs_msdos -F32 /dev/zvol/tank/fat32 # mount -t msdosfs /dev/zvol/tank/fat32 /mnt # df -h /mnt | grep fat32 Filesystem Size Used Avail Capacity Mounted on /dev/zvol/tank/fat32 249M 24k 249M 0% /mnt # mount | grep fat32 /dev/zvol/tank/fat32 on /mnt (msdosfs, local)
Destroying a volume is much the same as destroying a regular file system dataset. The operation is nearly instantaneous, but it may take several minutes for the free space to be reclaimed in the background.
20.4.3. Renaming a Dataset
The name of a dataset can be changed with
zfs rename. The parent of a dataset can also be changed with this command. Renaming a dataset to be under a different parent dataset will change the value of those properties that are inherited from the parent dataset. When a dataset is renamed, it is unmounted and then remounted in the new location (which is inherited from the new parent dataset). This behavior can be prevented with
Rename a dataset and move it to be under a different parent dataset:
# zfs list NAME USED AVAIL REFER MOUNTPOINT mypool 780M 93.2G 144K none mypool/ROOT 777M 93.2G 144K none mypool/ROOT/default 777M 93.2G 777M / mypool/tmp 176K 93.2G 176K /tmp mypool/usr 704K 93.2G 144K /usr mypool/usr/home 184K 93.2G 184K /usr/home mypool/usr/mydataset 87.5K 93.2G 87.5K /usr/mydataset mypool/usr/ports 144K 93.2G 144K /usr/ports mypool/usr/src 144K 93.2G 144K /usr/src mypool/var 1.21M 93.2G 614K /var mypool/var/crash 148K 93.2G 148K /var/crash mypool/var/log 178K 93.2G 178K /var/log mypool/var/mail 144K 93.2G 144K /var/mail mypool/var/tmp 152K 93.2G 152K /var/tmp # zfs rename mypool/usr/mydataset mypool/var/newname # zfs list NAME USED AVAIL REFER MOUNTPOINT mypool 780M 93.2G 144K none mypool/ROOT 777M 93.2G 144K none mypool/ROOT/default 777M 93.2G 777M / mypool/tmp 176K 93.2G 176K /tmp mypool/usr 616K 93.2G 144K /usr mypool/usr/home 184K 93.2G 184K /usr/home mypool/usr/ports 144K 93.2G 144K /usr/ports mypool/usr/src 144K 93.2G 144K /usr/src mypool/var 1.29M 93.2G 614K /var mypool/var/crash 148K 93.2G 148K /var/crash mypool/var/log 178K 93.2G 178K /var/log mypool/var/mail 144K 93.2G 144K /var/mail mypool/var/newname 87.5K 93.2G 87.5K /var/newname mypool/var/tmp 152K 93.2G 152K /var/tmp
Snapshots can also be renamed like this. Due to the nature of snapshots, they cannot be renamed into a different parent dataset. To rename a recursive snapshot, specify
-r, and all snapshots with the same name in child datasets will also be renamed.
# zfs list -t snapshot NAME USED AVAIL REFER MOUNTPOINT mypool/var/newname@first_snapshot 0 - 87.5K - # zfs rename mypool/var/newname@first_snapshot new_snapshot_name # zfs list -t snapshot NAME USED AVAIL REFER MOUNTPOINT mypool/var/newname@new_snapshot_name 0 - 87.5K -
20.4.4. Setting Dataset Properties
Each ZFS dataset has a number of properties that control its behavior. Most properties are automatically inherited from the parent dataset, but can be overridden locally. Set a property on a dataset with
zfs set property=value dataset. Most properties have a limited set of valid values,
zfs get will display each possible property and valid values. Most properties can be reverted to their inherited values using
User-defined properties can also be set. They become part of the dataset configuration and can be used to provide additional information about the dataset or its contents. To distinguish these custom properties from the ones supplied as part of ZFS, a colon (
:) is used to create a custom namespace for the property.
# zfs set custom:costcenter=1234 tank # zfs get custom:costcenter tank NAME PROPERTY VALUE SOURCE tank custom:costcenter 1234 local
To remove a custom property, use
zfs inherit with
-r. If the custom property is not defined in any of the parent datasets, it will be removed completely (although the changes are still recorded in the pool’s history).
# zfs inherit -r custom:costcenter tank # zfs get custom:costcenter tank NAME PROPERTY VALUE SOURCE tank custom:costcenter - - # zfs get all tank | grep custom:costcenter #
220.127.116.11. Getting and Setting Share Properties
Two commonly used and useful dataset properties are the NFS and SMB share options. Setting these define if and how ZFS datasets may be shared on the network. At present, only setting sharing via NFS is supported on FreeBSD. To get the current status of a share, enter:
# zfs get sharenfs mypool/usr/home NAME PROPERTY VALUE SOURCE mypool/usr/home sharenfs on local # zfs get sharesmb mypool/usr/home NAME PROPERTY VALUE SOURCE mypool/usr/home sharesmb off local
To enable sharing of a dataset, enter:
# zfs set sharenfs=on mypool/usr/home
It is also possible to set additional options for sharing datasets through NFS, such as
-network. To set additional options to a dataset shared through NFS, enter:
# zfs set sharenfs="-alldirs,-maproot=root,-network=192.168.1.0/24" mypool/usr/home
20.4.5. Managing Snapshots
Snapshots are one of the most powerful features of ZFS. A snapshot provides a read-only, point-in-time copy of the dataset. With Copy-On-Write (COW), snapshots can be created quickly by preserving the older version of the data on disk. If no snapshots exist, space is reclaimed for future use when data is rewritten or deleted. Snapshots preserve disk space by recording only the differences between the current dataset and a previous version. Snapshots are allowed only on whole datasets, not on individual files or directories. When a snapshot is created from a dataset, everything contained in it is duplicated. This includes the file system properties, files, directories, permissions, and so on. Snapshots use no additional space when they are first created, only consuming space as the blocks they reference are changed. Recursive snapshots taken with
-r create a snapshot with the same name on the dataset and all of its children, providing a consistent moment-in-time snapshot of all of the file systems. This can be important when an application has files on multiple datasets that are related or dependent upon each other. Without snapshots, a backup would have copies of the files from different points in time.
Snapshots in ZFS provide a variety of features that even other file systems with snapshot functionality lack. A typical example of snapshot use is to have a quick way of backing up the current state of the file system when a risky action like a software installation or a system upgrade is performed. If the action fails, the snapshot can be rolled back and the system has the same state as when the snapshot was created. If the upgrade was successful, the snapshot can be deleted to free up space. Without snapshots, a failed upgrade often requires a restore from backup, which is tedious, time consuming, and may require downtime during which the system cannot be used. Snapshots can be rolled back quickly, even while the system is running in normal operation, with little or no downtime. The time savings are enormous with multi-terabyte storage systems and the time required to copy the data from backup. Snapshots are not a replacement for a complete backup of a pool, but can be used as a quick and easy way to store a copy of the dataset at a specific point in time.
18.104.22.168. Creating Snapshots
Snapshots are created with
zfs snapshot dataset@snapshotname. Adding
-r creates a snapshot recursively, with the same name on all child datasets.
Create a recursive snapshot of the entire pool:
# zfs list -t all NAME USED AVAIL REFER MOUNTPOINT mypool 780M 93.2G 144K none mypool/ROOT 777M 93.2G 144K none mypool/ROOT/default 777M 93.2G 777M / mypool/tmp 176K 93.2G 176K /tmp mypool/usr 616K 93.2G 144K /usr mypool/usr/home 184K 93.2G 184K /usr/home mypool/usr/ports 144K 93.2G 144K /usr/ports mypool/usr/src 144K 93.2G 144K /usr/src mypool/var 1.29M 93.2G 616K /var mypool/var/crash 148K 93.2G 148K /var/crash mypool/var/log 178K 93.2G 178K /var/log mypool/var/mail 144K 93.2G 144K /var/mail mypool/var/newname 87.5K 93.2G 87.5K /var/newname mypool/var/newname@new_snapshot_name 0 - 87.5K - mypool/var/tmp 152K 93.2G 152K /var/tmp # zfs snapshot -r mypool@my_recursive_snapshot # zfs list -t snapshot NAME USED AVAIL REFER MOUNTPOINT mypool@my_recursive_snapshot 0 - 144K - mypool/ROOT@my_recursive_snapshot 0 - 144K - mypool/ROOT/default@my_recursive_snapshot 0 - 777M - mypool/tmp@my_recursive_snapshot 0 - 176K - mypool/usr@my_recursive_snapshot 0 - 144K - mypool/usr/home@my_recursive_snapshot 0 - 184K - mypool/usr/ports@my_recursive_snapshot 0 - 144K - mypool/usr/src@my_recursive_snapshot 0 - 144K - mypool/var@my_recursive_snapshot 0 - 616K - mypool/var/crash@my_recursive_snapshot 0 - 148K - mypool/var/log@my_recursive_snapshot 0 - 178K - mypool/var/mail@my_recursive_snapshot 0 - 144K - mypool/var/newname@new_snapshot_name 0 - 87.5K - mypool/var/newname@my_recursive_snapshot 0 - 87.5K - mypool/var/tmp@my_recursive_snapshot 0 - 152K -
Snapshots are not shown by a normal
zfs list operation. To list snapshots,
-t snapshot is appended to
-t all displays both file systems and snapshots.
Snapshots are not mounted directly, so no path is shown in the
MOUNTPOINT column. There is no mention of available disk space in the
AVAIL column, as snapshots cannot be written to after they are created. Compare the snapshot to the original dataset from which it was created:
# zfs list -rt all mypool/usr/home NAME USED AVAIL REFER MOUNTPOINT mypool/usr/home 184K 93.2G 184K /usr/home mypool/usr/home@my_recursive_snapshot 0 - 184K -
Displaying both the dataset and the snapshot together reveals how snapshots work in COW fashion. They save only the changes (delta) that were made and not the complete file system contents all over again. This means that snapshots take little space when few changes are made. Space usage can be made even more apparent by copying a file to the dataset, then making a second snapshot:
# cp /etc/passwd /var/tmp # zfs snapshot mypool/var/tmp@after_cp # zfs list -rt all mypool/var/tmp NAME USED AVAIL REFER MOUNTPOINT mypool/var/tmp 206K 93.2G 118K /var/tmp mypool/var/tmp@my_recursive_snapshot 88K - 152K - mypool/var/tmp@after_cp 0 - 118K -
The second snapshot contains only the changes to the dataset after the copy operation. This yields enormous space savings. Notice that the size of the snapshot
mypool/var/tmp@my_recursive_snapshot also changed in the
USED column to indicate the changes between itself and the snapshot taken afterwards.
22.214.171.124. Comparing Snapshots
ZFS provides a built-in command to compare the differences in content between two snapshots. This is helpful when many snapshots were taken over time and the user wants to see how the file system has changed over time. For example,
zfs diff lets a user find the latest snapshot that still contains a file that was accidentally deleted. Doing this for the two snapshots that were created in the previous section yields this output:
# zfs list -rt all mypool/var/tmp NAME USED AVAIL REFER MOUNTPOINT mypool/var/tmp 206K 93.2G 118K /var/tmp mypool/var/tmp@my_recursive_snapshot 88K - 152K - mypool/var/tmp@after_cp 0 - 118K - # zfs diff mypool/var/tmp@my_recursive_snapshot M /var/tmp/ + /var/tmp/passwd
The command lists the changes between the specified snapshot (in this case
mypool/var/tmp@my_recursive_snapshot) and the live file system. The first column shows the type of change:
The path or file was added.
The path or file was deleted.
The path or file was modified.
The path or file was renamed.
Comparing the output with the table, it becomes clear that passwd was added after the snapshot
mypool/var/tmp@my_recursive_snapshot was created. This also resulted in a modification to the parent directory mounted at
Comparing two snapshots is helpful when using the ZFS replication feature to transfer a dataset to a different host for backup purposes.
Compare two snapshots by providing the full dataset name and snapshot name of both datasets:
# cp /var/tmp/passwd /var/tmp/passwd.copy # zfs snapshot mypool/var/tmp@diff_snapshot # zfs diff mypool/var/tmp@my_recursive_snapshot mypool/var/tmp@diff_snapshot M /var/tmp/ + /var/tmp/passwd + /var/tmp/passwd.copy # zfs diff mypool/var/tmp@my_recursive_snapshot mypool/var/tmp@after_cp M /var/tmp/ + /var/tmp/passwd
A backup administrator can compare two snapshots received from the sending host and determine the actual changes in the dataset. See the Replication section for more information.
126.96.36.199. Snapshot Rollback
When at least one snapshot is available, it can be rolled back to at any time. Most of the time this is the case when the current state of the dataset is no longer required and an older version is preferred. Scenarios such as local development tests have gone wrong, botched system updates hampering the system’s overall functionality, or the requirement to restore accidentally deleted files or directories are all too common occurrences. Luckily, rolling back a snapshot is just as easy as typing
zfs rollback snapshotname. Depending on how many changes are involved, the operation will finish in a certain amount of time. During that time, the dataset always remains in a consistent state, much like a database that conforms to ACID principles is performing a rollback. This is happening while the dataset is live and accessible without requiring a downtime. Once the snapshot has been rolled back, the dataset has the same state as it had when the snapshot was originally taken. All other data in that dataset that was not part of the snapshot is discarded. Taking a snapshot of the current state of the dataset before rolling back to a previous one is a good idea when some data is required later. This way, the user can roll back and forth between snapshots without losing data that is still valuable.
In the first example, a snapshot is rolled back because of a careless
rm operation that removes too much data than was intended.
# zfs list -rt all mypool/var/tmp NAME USED AVAIL REFER MOUNTPOINT mypool/var/tmp 262K 93.2G 120K /var/tmp mypool/var/tmp@my_recursive_snapshot 88K - 152K - mypool/var/tmp@after_cp 53.5K - 118K - mypool/var/tmp@diff_snapshot 0 - 120K - # ls /var/tmp passwd passwd.copy vi.recover # rm /var/tmp/passwd* # ls /var/tmp vi.recover
At this point, the user realized that too many files were deleted and wants them back. ZFS provides an easy way to get them back using rollbacks, but only when snapshots of important data are performed on a regular basis. To get the files back and start over from the last snapshot, issue the command:
# zfs rollback mypool/var/tmp@diff_snapshot # ls /var/tmp passwd passwd.copy vi.recover
The rollback operation restored the dataset to the state of the last snapshot. It is also possible to roll back to a snapshot that was taken much earlier and has other snapshots that were created after it. When trying to do this, ZFS will issue this warning:
# zfs list -rt snapshot mypool/var/tmp AME USED AVAIL REFER MOUNTPOINT mypool/var/tmp@my_recursive_snapshot 88K - 152K - mypool/var/tmp@after_cp 53.5K - 118K - mypool/var/tmp@diff_snapshot 0 - 120K - # zfs rollback mypool/var/tmp@my_recursive_snapshot cannot rollback to 'mypool/var/tmp@my_recursive_snapshot': more recent snapshots exist use '-r' to force deletion of the following snapshots: mypool/var/tmp@after_cp mypool/var/tmp@diff_snapshot
This warning means that snapshots exist between the current state of the dataset and the snapshot to which the user wants to roll back. To complete the rollback, these snapshots must be deleted. ZFS cannot track all the changes between different states of the dataset, because snapshots are read-only. ZFS will not delete the affected snapshots unless the user specifies
-r to indicate that this is the desired action. If that is the intention, and the consequences of losing all intermediate snapshots is understood, the command can be issued:
# zfs rollback -r mypool/var/tmp@my_recursive_snapshot # zfs list -rt snapshot mypool/var/tmp NAME USED AVAIL REFER MOUNTPOINT mypool/var/tmp@my_recursive_snapshot 8K - 152K - # ls /var/tmp vi.recover
The output from
zfs list -t snapshot confirms that the intermediate snapshots were removed as a result of
zfs rollback -r.
188.8.131.52. Restoring Individual Files from Snapshots
Snapshots are mounted in a hidden directory under the parent dataset: .zfs/snapshots/snapshotname. By default, these directories will not be displayed even when a standard
ls -a is issued. Although the directory is not displayed, it is there nevertheless and can be accessed like any normal directory. The property named
snapdir controls whether these hidden directories show up in a directory listing. Setting the property to
visible allows them to appear in the output of
ls and other commands that deal with directory contents.
# zfs get snapdir mypool/var/tmp NAME PROPERTY VALUE SOURCE mypool/var/tmp snapdir hidden default # ls -a /var/tmp . .. passwd vi.recover # zfs set snapdir=visible mypool/var/tmp # ls -a /var/tmp . .. .zfs passwd vi.recover
Individual files can easily be restored to a previous state by copying them from the snapshot back to the parent dataset. The directory structure below .zfs/snapshot has a directory named exactly like the snapshots taken earlier to make it easier to identify them. In the next example, it is assumed that a file is to be restored from the hidden .zfs directory by copying it from the snapshot that contained the latest version of the file:
# rm /var/tmp/passwd # ls -a /var/tmp . .. .zfs vi.recover # ls /var/tmp/.zfs/snapshot after_cp my_recursive_snapshot # ls /var/tmp/.zfs/snapshot/after_cp passwd vi.recover # cp /var/tmp/.zfs/snapshot/after_cp/passwd /var/tmp
ls .zfs/snapshot was issued, the
snapdir property might have been set to hidden, but it would still be possible to list the contents of that directory. It is up to the administrator to decide whether these directories will be displayed. It is possible to display these for certain datasets and prevent it for others. Copying files or directories from this hidden .zfs/snapshot is simple enough. Trying it the other way around results in this error:
# cp /etc/rc.conf /var/tmp/.zfs/snapshot/after_cp/ cp: /var/tmp/.zfs/snapshot/after_cp/rc.conf: Read-only file system
The error reminds the user that snapshots are read-only and cannot be changed after creation. Files cannot be copied into or removed from snapshot directories because that would change the state of the dataset they represent.
Snapshots consume space based on how much the parent file system has changed since the time of the snapshot. The
written property of a snapshot tracks how much space is being used by the snapshot.
Snapshots are destroyed and the space reclaimed with
zfs destroy dataset@snapshot. Adding
-r recursively removes all snapshots with the same name under the parent dataset. Adding
-n -v to the command displays a list of the snapshots that would be deleted and an estimate of how much space would be reclaimed without performing the actual destroy operation.
20.4.6. Managing Clones
A clone is a copy of a snapshot that is treated more like a regular dataset. Unlike a snapshot, a clone is not read only, is mounted, and can have its own properties. Once a clone has been created using
zfs clone, the snapshot it was created from cannot be destroyed. The child/parent relationship between the clone and the snapshot can be reversed using
zfs promote. After a clone has been promoted, the snapshot becomes a child of the clone, rather than of the original parent dataset. This will change how the space is accounted, but not actually change the amount of space consumed. The clone can be mounted at any point within the ZFS file system hierarchy, not just below the original location of the snapshot.
To demonstrate the clone feature, this example dataset is used:
# zfs list -rt all camino/home/joe NAME USED AVAIL REFER MOUNTPOINT camino/home/joe 108K 1.3G 87K /usr/home/joe camino/home/joe@plans 21K - 85.5K - camino/home/joe@backup 0K - 87K -
A typical use for clones is to experiment with a specific dataset while keeping the snapshot around to fall back to in case something goes wrong. Since snapshots cannot be changed, a read/write clone of a snapshot is created. After the desired result is achieved in the clone, the clone can be promoted to a dataset and the old file system removed. This is not strictly necessary, as the clone and dataset can coexist without problems.
# zfs clone camino/home/joe@backup camino/home/joenew # ls /usr/home/joe* /usr/home/joe: backup.txz plans.txt /usr/home/joenew: backup.txz plans.txt # df -h /usr/home Filesystem Size Used Avail Capacity Mounted on usr/home/joe 1.3G 31k 1.3G 0% /usr/home/joe usr/home/joenew 1.3G 31k 1.3G 0% /usr/home/joenew
After a clone is created it is an exact copy of the state the dataset was in when the snapshot was taken. The clone can now be changed independently from its originating dataset. The only connection between the two is the snapshot. ZFS records this connection in the property
origin. Once the dependency between the snapshot and the clone has been removed by promoting the clone using
zfs promote, the
origin of the clone is removed as it is now an independent dataset. This example demonstrates it:
# zfs get origin camino/home/joenew NAME PROPERTY VALUE SOURCE camino/home/joenew origin camino/home/joe@backup - # zfs promote camino/home/joenew # zfs get origin camino/home/joenew NAME PROPERTY VALUE SOURCE camino/home/joenew origin - -
After making some changes like copying loader.conf to the promoted clone, for example, the old directory becomes obsolete in this case. Instead, the promoted clone can replace it. This can be achieved by two consecutive commands:
zfs destroy on the old dataset and
zfs rename on the clone to name it like the old dataset (it could also get an entirely different name).
# cp /boot/defaults/loader.conf /usr/home/joenew # zfs destroy -f camino/home/joe # zfs rename camino/home/joenew camino/home/joe # ls /usr/home/joe backup.txz loader.conf plans.txt # df -h /usr/home Filesystem Size Used Avail Capacity Mounted on usr/home/joe 1.3G 128k 1.3G 0% /usr/home/joe
The cloned snapshot is now handled like an ordinary dataset. It contains all the data from the original snapshot plus the files that were added to it like loader.conf. Clones can be used in different scenarios to provide useful features to ZFS users. For example, jails could be provided as snapshots containing different sets of installed applications. Users can clone these snapshots and add their own applications as they see fit. Once they are satisfied with the changes, the clones can be promoted to full datasets and provided to end users to work with like they would with a real dataset. This saves time and administrative overhead when providing these jails.
Keeping data on a single pool in one location exposes it to risks like theft and natural or human disasters. Making regular backups of the entire pool is vital. ZFS provides a built-in serialization feature that can send a stream representation of the data to standard output. Using this technique, it is possible to not only store the data on another pool connected to the local system, but also to send it over a network to another system. Snapshots are the basis for this replication (see the section on ZFS snapshots). The commands used for replicating data are
zfs send and
These examples demonstrate ZFS replication with these two pools:
# zpool list NAME SIZE ALLOC FREE CKPOINT EXPANDSZ FRAG CAP DEDUP HEALTH ALTROOT backup 960M 77K 896M - - 0% 0% 1.00x ONLINE - mypool 984M 43.7M 940M - - 0% 4% 1.00x ONLINE -
The pool named mypool is the primary pool where data is written to and read from on a regular basis. A second pool, backup is used as a standby in case the primary pool becomes unavailable. Note that this fail-over is not done automatically by ZFS, but must be manually done by a system administrator when needed. A snapshot is used to provide a consistent version of the file system to be replicated. Once a snapshot of mypool has been created, it can be copied to the backup pool. Only snapshots can be replicated. Changes made since the most recent snapshot will not be included.
# zfs snapshot mypool@backup1 # zfs list -t snapshot NAME USED AVAIL REFER MOUNTPOINT mypool@backup1 0 - 43.6M -
Now that a snapshot exists,
zfs send can be used to create a stream representing the contents of the snapshot. This stream can be stored as a file or received by another pool. The stream is written to standard output, but must be redirected to a file or pipe or an error is produced:
# zfs send mypool@backup1 Error: Stream can not be written to a terminal. You must redirect standard output.
To back up a dataset with
zfs send, redirect to a file located on the mounted backup pool. Ensure that the pool has enough free space to accommodate the size of the snapshot being sent, which means all of the data contained in the snapshot, not just the changes from the previous snapshot.
# zfs send mypool@backup1 > /backup/backup1 # zpool list NAME SIZE ALLOC FREE CKPOINT EXPANDSZ FRAG CAP DEDUP HEALTH ALTROOT backup 960M 63.7M 896M - - 0% 6% 1.00x ONLINE - mypool 984M 43.7M 940M - - 0% 4% 1.00x ONLINE -
zfs send transferred all the data in the snapshot called backup1 to the pool named backup. Creating and sending these snapshots can be done automatically with a cron(8) job.
Instead of storing the backups as archive files, ZFS can receive them as a live file system, allowing the backed up data to be accessed directly. To get to the actual data contained in those streams,
zfs receive is used to transform the streams back into files and directories. The example below combines
zfs send and
zfs receive using a pipe to copy the data from one pool to another. The data can be used directly on the receiving pool after the transfer is complete. A dataset can only be replicated to an empty dataset.
# zfs snapshot mypool@replica1 # zfs send -v mypool@replica1 | zfs receive backup/mypool send from @ to mypool@replica1 estimated size is 50.1M total estimated size is 50.1M TIME SENT SNAPSHOT # zpool list NAME SIZE ALLOC FREE CKPOINT EXPANDSZ FRAG CAP DEDUP HEALTH ALTROOT backup 960M 63.7M 896M - - 0% 6% 1.00x ONLINE - mypool 984M 43.7M 940M - - 0% 4% 1.00x ONLINE -
184.108.40.206. Incremental Backups
zfs send can also determine the difference between two snapshots and send only the differences between the two. This saves disk space and transfer time. For example:
# zfs snapshot mypool@replica2 # zfs list -t snapshot NAME USED AVAIL REFER MOUNTPOINT mypool@replica1 5.72M - 43.6M - mypool@replica2 0 - 44.1M - # zpool list NAME SIZE ALLOC FREE CKPOINT EXPANDSZ FRAG CAP DEDUP HEALTH ALTROOT backup 960M 61.7M 898M - - 0% 6% 1.00x ONLINE - mypool 960M 50.2M 910M - - 0% 5% 1.00x ONLINE -
A second snapshot called replica2 was created. This second snapshot contains only the changes that were made to the file system between now and the previous snapshot, replica1. Using
zfs send -i and indicating the pair of snapshots generates an incremental replica stream containing only the data that has changed. This can only succeed if the initial snapshot already exists on the receiving side.
# zfs send -v -i mypool@replica1 mypool@replica2 | zfs receive /backup/mypool send from @replica1 to mypool@replica2 estimated size is 5.02M total estimated size is 5.02M TIME SENT SNAPSHOT # zpool list NAME SIZE ALLOC FREE CKPOINT EXPANDSZ FRAG CAP DEDUP HEALTH ALTROOT backup 960M 80.8M 879M - - 0% 8% 1.00x ONLINE - mypool 960M 50.2M 910M - - 0% 5% 1.00x ONLINE - # zfs list NAME USED AVAIL REFER MOUNTPOINT backup 55.4M 240G 152K /backup backup/mypool 55.3M 240G 55.2M /backup/mypool mypool 55.6M 11.6G 55.0M /mypool # zfs list -t snapshot NAME USED AVAIL REFER MOUNTPOINT backup/mypool@replica1 104K - 50.2M - backup/mypool@replica2 0 - 55.2M - mypool@replica1 29.9K - 50.0M - mypool@replica2 0 - 55.0M -
The incremental stream was successfully transferred. Only the data that had changed was replicated, rather than the entirety of replica1. Only the differences were sent, which took much less time to transfer and saved disk space by not copying the complete pool each time. This is useful when having to rely on slow networks or when costs per transferred byte must be considered.
A new file system, backup/mypool, is available with all of the files and data from the pool mypool. If
-P is specified, the properties of the dataset will be copied, including compression settings, quotas, and mount points. When
-R is specified, all child datasets of the indicated dataset will be copied, along with all of their properties. Sending and receiving can be automated so that regular backups are created on the second pool.
220.127.116.11. Sending Encrypted Backups over SSH
Sending streams over the network is a good way to keep a remote backup, but it does come with a drawback. Data sent over the network link is not encrypted, allowing anyone to intercept and transform the streams back into data without the knowledge of the sending user. This is undesirable, especially when sending the streams over the internet to a remote host. SSH can be used to securely encrypt data send over a network connection. Since ZFS only requires the stream to be redirected from standard output, it is relatively easy to pipe it through SSH. To keep the contents of the file system encrypted in transit and on the remote system, consider using PEFS.
A few settings and security precautions must be completed first. Only the necessary steps required for the
zfs send operation are shown here. For more information on SSH, see OpenSSH.
This configuration is required:
Passwordless SSH access between sending and receiving host using SSH keys
Normally, the privileges of the
rootuser are needed to send and receive streams. This requires logging in to the receiving system as
root. However, logging in as
rootis disabled by default for security reasons. The ZFS Delegation system can be used to allow a non-
rootuser on each system to perform the respective send and receive operations.
On the sending system:
# zfs allow -u someuser send,snapshot mypool
To mount the pool, the unprivileged user must own the directory, and regular users must be allowed to mount file systems. On the receiving system:
# sysctl vfs.usermount=1 vfs.usermount: 0 -> 1 # echo vfs.usermount=1 >> /etc/sysctl.conf # zfs create recvpool/backup # zfs allow -u someuser create,mount,receive recvpool/backup # chown someuser /recvpool/backup
The unprivileged user now has the ability to receive and mount datasets, and the home dataset can be replicated to the remote system:
% zfs snapshot -r mypool/home@monday % zfs send -R mypool/home@monday | ssh someuser@backuphost zfs recv -dvu recvpool/backup
A recursive snapshot called monday is made of the file system dataset home that resides on the pool mypool. Then it is sent with
zfs send -R to include the dataset, all child datasets, snapshots, clones, and settings in the stream. The output is piped to the waiting
zfs receive on the remote host backuphost through SSH. Using a fully qualified domain name or IP address is recommended. The receiving machine writes the data to the backup dataset on the recvpool pool. Adding
zfs recv overwrites the name of the pool on the receiving side with the name of the snapshot.
-u causes the file systems to not be mounted on the receiving side. When
-v is included, more detail about the transfer is shown, including elapsed time and the amount of data transferred.
20.4.8. Dataset, User, and Group Quotas
Dataset quotas are used to restrict the amount of space that can be consumed by a particular dataset. Reference Quotas work in very much the same way, but only count the space used by the dataset itself, excluding snapshots and child datasets. Similarly, user and group quotas can be used to prevent users or groups from using all of the space in the pool or dataset.
The following examples assume that the users already exist in the system. Before adding a user to the system, make sure to create their home dataset first and set the
/home/bob. Then, create the user and make the home directory point to the dataset’s
mountpoint location. This will properly set owner and group permissions without shadowing any pre-existing home directory paths that might exist.
To enforce a dataset quota of 10 GB for storage/home/bob:
# zfs set quota=10G storage/home/bob
To enforce a reference quota of 10 GB for storage/home/bob:
# zfs set refquota=10G storage/home/bob
To remove a quota of 10 GB for storage/home/bob:
# zfs set quota=none storage/home/bob
The general format is
userquota@user=size, and the user’s name must be in one of these formats:
POSIX compatible name such as joe.
POSIX numeric ID such as 789.
SID name such as email@example.com.
SID numeric ID such as S-1-123-456-789.
For example, to enforce a user quota of 50 GB for the user named joe:
# zfs set userquota@joe=50G
To remove any quota:
# zfs set userquota@joe=none
User quota properties are not displayed by
The general format for setting a group quota is:
To set the quota for the group firstgroup to 50 GB, use:
# zfs set groupquota@firstgroup=50G
To remove the quota for the group firstgroup, or to make sure that one is not set, instead use:
# zfs set groupquota@firstgroup=none
As with the user quota property, non-
root users can only see the quotas associated with the groups to which they belong. However,
root or a user with the
groupquota privilege can view and set all quotas for all groups.
To display the amount of space used by each user on a file system or snapshot along with any quotas, use
zfs userspace. For group information, use
zfs groupspace. For more information about supported options or how to display only specific options, refer to zfs(1).
Users with sufficient privileges, and
root, can list the quota for storage/home/bob using:
# zfs get quota storage/home/bob
Reservations guarantee a minimum amount of space will always be available on a dataset. The reserved space will not be available to any other dataset. This feature can be especially useful to ensure that free space is available for an important dataset or log files.
The general format of the
reservation property is
reservation=size, so to set a reservation of 10 GB on storage/home/bob, use:
# zfs set reservation=10G storage/home/bob
To clear any reservation:
# zfs set reservation=none storage/home/bob
The same principle can be applied to the
refreservation property for setting a Reference Reservation, with the general format
This command shows any reservations or refreservations that exist on storage/home/bob:
# zfs get reservation storage/home/bob # zfs get refreservation storage/home/bob
ZFS provides transparent compression. Compressing data at the block level as it is written not only saves space, but can also increase disk throughput. If data is compressed by 25%, but the compressed data is written to the disk at the same rate as the uncompressed version, resulting in an effective write speed of 125%. Compression can also be a great alternative to Deduplication because it does not require additional memory.
ZFS offers several different compression algorithms, each with different trade-offs. With the introduction of LZ4 compression in ZFS v5000, it is possible to enable compression for the entire pool without the large performance trade-off of other algorithms. The biggest advantage to LZ4 is the early abort feature. If LZ4 does not achieve at least 12.5% compression in the first part of the data, the block is written uncompressed to avoid wasting CPU cycles trying to compress data that is either already compressed or uncompressible. For details about the different compression algorithms available in ZFS, see the Compression entry in the terminology section.
The administrator can monitor the effectiveness of compression using a number of dataset properties.
# zfs get used,compressratio,compression,logicalused mypool/compressed_dataset NAME PROPERTY VALUE SOURCE mypool/compressed_dataset used 449G - mypool/compressed_dataset compressratio 1.11x - mypool/compressed_dataset compression lz4 local mypool/compressed_dataset logicalused 496G -
The dataset is currently using 449 GB of space (the used property). Without compression, it would have taken 496 GB of space (the
logicalused property). This results in the 1.11:1 compression ratio.
Compression can have an unexpected side effect when combined with User Quotas. User quotas restrict how much space a user can consume on a dataset, but the measurements are based on how much space is used after compression. So if a user has a quota of 10 GB, and writes 10 GB of compressible data, they will still be able to store additional data. If they later update a file, say a database, with more or less compressible data, the amount of space available to them will change. This can result in the odd situation where a user did not increase the actual amount of data (the
logicalused property), but the change in compression caused them to reach their quota limit.
Compression can have a similar unexpected interaction with backups. Quotas are often used to limit how much data can be stored to ensure there is sufficient backup space available. However since quotas do not consider compression, more data may be written than would fit with uncompressed backups.
20.4.11. Zstandard Compression
In OpenZFS 2.0, a new compression algorithm was added. Zstandard (Zstd) offers higher compression ratios than the default LZ4 while offering much greater speeds than the alternative, gzip. OpenZFS 2.0 is available starting with FreeBSD 12.1-RELEASE via sysutils/openzfs and has been the default in FreeBSD 13-CURRENT since September 2020, and will by in FreeBSD 13.0-RELEASE.
Zstd provides a large selection of compression levels, providing fine-grained control over performance versus compression ratio. One of the main advantages of Zstd is that the decompression speed is independent of the compression level. For data that is written once but read many times, Zstd allows the use of the highest compression levels without a read performance penalty.
Even when data is updated frequently, there are often performance gains that come from enabling compression. One of the biggest advantages comes from the compressed ARC feature. ZFS’s Adaptive Replacement Cache (ARC) caches the compressed version of the data in RAM, decompressing it each time it is needed. This allows the same amount of RAM to store more data and metadata, increasing the cache hit ratio.
ZFS offers 19 levels of Zstd compression, each offering incrementally more space savings in exchange for slower compression. The default level is
zstd-3 and offers greater compression than LZ4 without being significantly slower. Levels above 10 require significant amounts of memory to compress each block, so they are discouraged on systems with less than 16 GB of RAM. ZFS also implements a selection of the Zstd_fast_ levels, which get correspondingly faster but offer lower compression ratios. ZFS supports
zstd-fast-100 in increments of 10, and finally
zstd-fast-1000 which provide minimal compression, but offer very high performance.
If ZFS is not able to allocate the required memory to compress a block with Zstd, it will fall back to storing the block uncompressed. This is unlikely to happen outside of the highest levels of Zstd on systems that are memory constrained. The sysctl
kstat.zfs.misc.zstd.compress_alloc_fail counts how many times this has occurred since the ZFS module was loaded.
When enabled, deduplication uses the checksum of each block to detect duplicate blocks. When a new block is a duplicate of an existing block, ZFS writes an additional reference to the existing data instead of the whole duplicate block. Tremendous space savings are possible if the data contains many duplicated files or repeated information. Be warned: deduplication requires an extremely large amount of memory, and most of the space savings can be had without the extra cost by enabling compression instead.
To activate deduplication, set the
dedup property on the target pool:
# zfs set dedup=on pool
Only new data being written to the pool will be deduplicated. Data that has already been written to the pool will not be deduplicated merely by activating this option. A pool with a freshly activated deduplication property will look like this example:
# zpool list NAME SIZE ALLOC FREE CKPOINT EXPANDSZ FRAG CAP DEDUP HEALTH ALTROOT pool 2.84G 2.19M 2.83G - - 0% 0% 1.00x ONLINE -
DEDUP column shows the actual rate of deduplication for the pool. A value of
1.00x shows that data has not been deduplicated yet. In the next example, the ports tree is copied three times into different directories on the deduplicated pool created above.
# for d in dir1 dir2 dir3; do > mkdir $d && cp -R /usr/ports $d & > done
Redundant data is detected and deduplicated:
# zpool list NAME SIZE ALLOC FREE CKPOINT EXPANDSZ FRAG CAP DEDUP HEALTH ALTROOT pool 2.84G 20.9M 2.82G - - 0% 0% 3.00x ONLINE -
DEDUP column shows a factor of
3.00x. Multiple copies of the ports tree data was detected and deduplicated, using only a third of the space. The potential for space savings can be enormous, but comes at the cost of having enough memory to keep track of the deduplicated blocks.
Deduplication is not always beneficial, especially when the data on a pool is not redundant. ZFS can show potential space savings by simulating deduplication on an existing pool:
# zdb -S pool Simulated DDT histogram: bucket allocated referenced ______ ______________________________ ______________________________ refcnt blocks LSIZE PSIZE DSIZE blocks LSIZE PSIZE DSIZE ------ ------ ----- ----- ----- ------ ----- ----- ----- 1 2.58M 289G 264G 264G 2.58M 289G 264G 264G 2 206K 12.6G 10.4G 10.4G 430K 26.4G 21.6G 21.6G 4 37.6K 692M 276M 276M 170K 3.04G 1.26G 1.26G 8 2.18K 45.2M 19.4M 19.4M 20.0K 425M 176M 176M 16 174 2.83M 1.20M 1.20M 3.33K 48.4M 20.4M 20.4M 32 40 2.17M 222K 222K 1.70K 97.2M 9.91M 9.91M 64 9 56K 10.5K 10.5K 865 4.96M 948K 948K 128 2 9.50K 2K 2K 419 2.11M 438K 438K 256 5 61.5K 12K 12K 1.90K 23.0M 4.47M 4.47M 1K 2 1K 1K 1K 2.98K 1.49M 1.49M 1.49M Total 2.82M 303G 275G 275G 3.20M 319G 287G 287G dedup = 1.05, compress = 1.11, copies = 1.00, dedup * compress / copies = 1.16
zdb -S finishes analyzing the pool, it shows the space reduction ratio that would be achieved by activating deduplication. In this case,
1.16 is a very poor space saving ratio that is mostly provided by compression. Activating deduplication on this pool would not save any significant amount of space, and is not worth the amount of memory required to enable deduplication. Using the formula ratio = dedup * compress / copies, system administrators can plan the storage allocation, deciding whether the workload will contain enough duplicate blocks to justify the memory requirements. If the data is reasonably compressible, the space savings may be very good. Enabling compression first is recommended, and compression can also provide greatly increased performance. Only enable deduplication in cases where the additional savings will be considerable and there is sufficient memory for the DDT.
20.4.13. ZFS and Jails
zfs jail and the corresponding
jailed property are used to delegate a ZFS dataset to a Jail.
zfs jail jailid attaches a dataset to the specified jail, and
zfs unjail detaches it. For the dataset to be controlled from within a jail, the
jailed property must be set. Once a dataset is jailed, it can no longer be mounted on the host because it may have mount points that would compromise the security of the host.
20.5. Delegated Administration
A comprehensive permission delegation system allows unprivileged users to perform ZFS administration functions. For example, if each user’s home directory is a dataset, users can be given permission to create and destroy snapshots of their home directories. A backup user can be given permission to use replication features. A usage statistics script can be allowed to run with access only to the space utilization data for all users. It is even possible to delegate the ability to delegate permissions. Permission delegation is possible for each subcommand and most properties.
20.5.1. Delegating Dataset Creation
zfs allow someuser create mydataset gives the specified user permission to create child datasets under the selected parent dataset. There is a caveat: creating a new dataset involves mounting it. That requires setting the FreeBSD
vfs.usermount sysctl(8) to
1 to allow non-root users to mount a file system. There is another restriction aimed at preventing abuse: non-
root users must own the mountpoint where the file system is to be mounted.
20.5.2. Delegating Permission Delegation
zfs allow someuser allow mydataset gives the specified user the ability to assign any permission they have on the target dataset, or its children, to other users. If a user has the
snapshot permission and the
allow permission, that user can then grant the
snapshot permission to other users.
20.6. Advanced Topics
There are a number of tunables that can be adjusted to make ZFS perform best for different workloads.
vfs.zfs.arc_max- Maximum size of the ARC. The default is all RAM but 1 GB, or 5/8 of all RAM, whichever is more. However, a lower value should be used if the system will be running any other daemons or processes that may require memory. This value can be adjusted at runtime with sysctl(8) and can be set in /boot/loader.conf or /etc/sysctl.conf.
vfs.zfs.arc_meta_limit- Limit the portion of the ARC that can be used to store metadata. The default is one fourth of
vfs.zfs.arc_max. Increasing this value will improve performance if the workload involves operations on a large number of files and directories, or frequent metadata operations, at the cost of less file data fitting in the ARC. This value can be adjusted at runtime with sysctl(8) and can be set in /boot/loader.conf or /etc/sysctl.conf.
vfs.zfs.arc_min- Minimum size of the ARC. The default is one half of
vfs.zfs.arc_meta_limit. Adjust this value to prevent other applications from pressuring out the entire ARC. This value can be adjusted at runtime with sysctl(8) and can be set in /boot/loader.conf or /etc/sysctl.conf.
vfs.zfs.vdev.cache.size- A preallocated amount of memory reserved as a cache for each device in the pool. The total amount of memory used will be this value multiplied by the number of devices. This value can only be adjusted at boot time, and is set in /boot/loader.conf.
ashift(sector size) that will be used automatically at pool creation time. The value is a power of two. The default value of
2^9 = 512, a sector size of 512 bytes. To avoid write amplification and get the best performance, set this value to the largest sector size used by a device in the pool.
Many drives have 4 KB sectors. Using the default
9with these drives results in write amplification on these devices. Data that could be contained in a single 4 KB write must instead be written in eight 512-byte writes. ZFS tries to read the native sector size from all devices when creating a pool, but many drives with 4 KB sectors report that their sectors are 512 bytes for compatibility. Setting
2^12 = 4096) before creating a pool forces ZFS to use 4 KB blocks for best performance on these drives.
Forcing 4 KB blocks is also useful on pools where disk upgrades are planned. Future disks are likely to use 4 KB sectors, and
ashiftvalues cannot be changed after a pool is created.
In some specific cases, the smaller 512-byte block size might be preferable. When used with 512-byte disks for databases, or as storage for virtual machines, less data is transferred during small random reads. This can provide better performance, especially when using a smaller ZFS record size.
vfs.zfs.prefetch_disable- Disable prefetch. A value of
0is enabled and
1is disabled. The default is
0, unless the system has less than 4 GB of RAM. Prefetch works by reading larger blocks than were requested into the ARC in hopes that the data will be needed soon. If the workload has a large number of random reads, disabling prefetch may actually improve performance by reducing unnecessary reads. This value can be adjusted at any time with sysctl(8).
vfs.zfs.vdev.trim_on_init- Control whether new devices added to the pool have the
TRIMcommand run on them. This ensures the best performance and longevity for SSDs, but takes extra time. If the device has already been secure erased, disabling this setting will make the addition of the new device faster. This value can be adjusted at any time with sysctl(8).
vfs.zfs.vdev.max_pending- Limit the number of pending I/O requests per device. A higher value will keep the device command queue full and may give higher throughput. A lower value will reduce latency. This value can be adjusted at any time with sysctl(8).
vfs.zfs.top_maxinflight- Maximum number of outstanding I/Os per top-level vdev. Limits the depth of the command queue to prevent high latency. The limit is per top-level vdev, meaning the limit applies to each mirror, RAID-Z, or other vdev independently. This value can be adjusted at any time with sysctl(8).
vfs.zfs.l2arc_write_max- Limit the amount of data written to the L2ARC per second. This tunable is designed to extend the longevity of SSDs by limiting the amount of data written to the device. This value can be adjusted at any time with sysctl(8).
vfs.zfs.l2arc_write_boost- The value of this tunable is added to
vfs.zfs.l2arc_write_maxand increases the write speed to the SSD until the first block is evicted from the L2ARC. This "Turbo Warmup Phase" is designed to reduce the performance loss from an empty L2ARC after a reboot. This value can be adjusted at any time with sysctl(8).
vfs.zfs.scrub_delay- Number of ticks to delay between each I/O during a
scrub. To ensure that a
scrubdoes not interfere with the normal operation of the pool, if any other I/O is happening the
scrubwill delay between each command. This value controls the limit on the total IOPS (I/Os Per Second) generated by the
scrub. The granularity of the setting is determined by the value of
kern.hzwhich defaults to 1000 ticks per second. This setting may be changed, resulting in a different effective IOPS limit. The default value is
4, resulting in a limit of: 1000 ticks/sec / 4 = 250 IOPS. Using a value of 20 would give a limit of: 1000 ticks/sec / 20 = 50 IOPS. The speed of
scrubis only limited when there has been recent activity on the pool, as determined by
vfs.zfs.scan_idle. This value can be adjusted at any time with sysctl(8).
vfs.zfs.resilver_delay- Number of milliseconds of delay inserted between each I/O during a resilver. To ensure that a resilver does not interfere with the normal operation of the pool, if any other I/O is happening the resilver will delay between each command. This value controls the limit of total IOPS (I/Os Per Second) generated by the resilver. The granularity of the setting is determined by the value of
kern.hzwhich defaults to 1000 ticks per second. This setting may be changed, resulting in a different effective IOPS limit. The default value is 2, resulting in a limit of: 1000 ticks/sec / 2 = 500 IOPS. Returning the pool to an Online state may be more important if another device failing could Fault the pool, causing data loss. A value of 0 will give the resilver operation the same priority as other operations, speeding the healing process. The speed of resilver is only limited when there has been other recent activity on the pool, as determined by
vfs.zfs.scan_idle. This value can be adjusted at any time with sysctl(8).
vfs.zfs.scan_idle- Number of milliseconds since the last operation before the pool is considered idle. When the pool is idle the rate limiting for
scruband resilver are disabled. This value can be adjusted at any time with sysctl(8).
vfs.zfs.txg.timeout- Maximum number of seconds between transaction groups. The current transaction group will be written to the pool and a fresh transaction group started if this amount of time has elapsed since the previous transaction group. A transaction group my be triggered earlier if enough data is written. The default value is 5 seconds. A larger value may improve read performance by delaying asynchronous writes, but this may cause uneven performance when the transaction group is written. This value can be adjusted at any time with sysctl(8).
20.6.2. ZFS on i386
Some of the features provided by ZFS are memory intensive, and may require tuning for maximum efficiency on systems with limited RAM.
As a bare minimum, the total system memory should be at least one gigabyte. The amount of recommended RAM depends upon the size of the pool and which ZFS features are used. A general rule of thumb is 1 GB of RAM for every 1 TB of storage. If the deduplication feature is used, a general rule of thumb is 5 GB of RAM per TB of storage to be deduplicated. While some users successfully use ZFS with less RAM, systems under heavy load may panic due to memory exhaustion. Further tuning may be required for systems with less than the recommended RAM requirements.
18.104.22.168. Kernel Configuration
Due to the address space limitations of the i386™ platform, ZFS users on the i386™ architecture must add this option to a custom kernel configuration file, rebuild the kernel, and reboot:
This expands the kernel address space, allowing the
vm.kvm_size tunable to be pushed beyond the currently imposed limit of 1 GB, or the limit of 2 GB for PAE. To find the most suitable value for this option, divide the desired address space in megabytes by four. In this example, it is
512 for 2 GB.
22.214.171.124. Loader Tunables
The kmem address space can be increased on all FreeBSD architectures. On a test system with 1 GB of physical memory, success was achieved with these options added to /boot/loader.conf, and the system restarted:
vm.kmem_size="330M" vm.kmem_size_max="330M" vfs.zfs.arc_max="40M" vfs.zfs.vdev.cache.size="5M"
For a more detailed list of recommendations for ZFS-related tuning, see https://wiki.freebsd.org/ZFSTuningGuide.
20.7. Additional Resources
20.8. ZFS Features and Terminology
ZFS is a fundamentally different file system because it is more than just a file system. ZFS combines the roles of file system and volume manager, enabling additional storage devices to be added to a live system and having the new space available on all of the existing file systems in that pool immediately. By combining the traditionally separate roles, ZFS is able to overcome previous limitations that prevented RAID groups being able to grow. Each top level device in a pool is called a vdev, which can be a simple disk or a RAID transformation such as a mirror or RAID-Z array. ZFS file systems (called datasets) each have access to the combined free space of the entire pool. As blocks are allocated from the pool, the space available to each file system decreases. This approach avoids the common pitfall with extensive partitioning where free space becomes fragmented across the partitions.
A storage pool is the most basic building block of ZFS. A pool is made up of one or more vdevs, the underlying devices that store the data. A pool is then used to create one or more file systems (datasets) or block devices (volumes). These datasets and volumes share the pool of remaining free space. Each pool is uniquely identified by a name and a GUID. The features available are determined by the ZFS version number on the pool.
A pool is made up of one or more vdevs, which themselves can be a single disk or a group of disks, in the case of a RAID transform. When multiple vdevs are used, ZFS spreads data across the vdevs to increase performance and maximize usable space.
Transaction Groups are the way changed blocks are grouped together and eventually written to the pool. Transaction groups are the atomic unit that ZFS uses to assert consistency. Each transaction group is assigned a unique 64-bit consecutive identifier. There can be up to three active transaction groups at a time, one in each of these three states:
* Open - When a new transaction group is created, it is in the open state, and accepts new writes. There is always a transaction group in the open state, however the transaction group may refuse new writes if it has reached a limit. Once the open transaction group has reached a limit, or the
ZFS uses an Adaptive Replacement Cache (ARC), rather than a more traditional Least Recently Used (LRU) cache. An LRU cache is a simple list of items in the cache, sorted by when each object was most recently used. New items are added to the top of the list. When the cache is full, items from the bottom of the list are evicted to make room for more active objects. An ARC consists of four lists; the Most Recently Used (MRU) and Most Frequently Used (MFU) objects, plus a ghost list for each. These ghost lists track recently evicted objects to prevent them from being added back to the cache. This increases the cache hit ratio by avoiding objects that have a history of only being used occasionally. Another advantage of using both an MRU and MFU is that scanning an entire file system would normally evict all data from an MRU or LRU cache in favor of this freshly accessed content. With ZFS, there is also an MFU that only tracks the most frequently used objects, and the cache of the most commonly accessed blocks remains.
L2ARC is the second level of the ZFS caching system. The primary ARC is stored in RAM. Since the amount of available RAM is often limited, ZFS can also use cache vdevs. Solid State Disks (SSDs) are often used as these cache devices due to their higher speed and lower latency compared to traditional spinning disks. L2ARC is entirely optional, but having one will significantly increase read speeds for files that are cached on the SSD instead of having to be read from the regular disks. L2ARC can also speed up deduplication because a DDT that does not fit in RAM but does fit in the L2ARC will be much faster than a DDT that must be read from disk. The rate at which data is added to the cache devices is limited to prevent prematurely wearing out SSDs with too many writes. Until the cache is full (the first block has been evicted to make room), writing to the L2ARC is limited to the sum of the write limit and the boost limit, and afterwards limited to the write limit. A pair of sysctl(8) values control these rate limits.
ZIL accelerates synchronous transactions by using storage devices like SSDs that are faster than those used in the main storage pool. When an application requests a synchronous write (a guarantee that the data has been safely stored to disk rather than merely cached to be written later), the data is written to the faster ZIL storage, then later flushed out to the regular disks. This greatly reduces latency and improves performance. Only synchronous workloads like databases will benefit from a ZIL. Regular asynchronous writes such as copying files will not use the ZIL at all.
Unlike a traditional file system, when data is overwritten on ZFS, the new data is written to a different block rather than overwriting the old data in place. Only when this write is complete is the metadata then updated to point to the new location. In the event of a shorn write (a system crash or power loss in the middle of writing a file), the entire original contents of the file are still available and the incomplete write is discarded. This also means that ZFS does not require a fsck(8) after an unexpected shutdown.
Dataset is the generic term for a ZFS file system, volume, snapshot or clone. Each dataset has a unique name in the format poolname/path@snapshot. The root of the pool is technically a dataset as well. Child datasets are named hierarchically like directories. For example, mypool/home, the home dataset, is a child of mypool and inherits properties from it. This can be expanded further by creating mypool/home/user. This grandchild dataset will inherit properties from the parent and grandparent. Properties on a child can be set to override the defaults inherited from the parents and grandparents. Administration of datasets and their children can be delegated.
A ZFS dataset is most often used as a file system. Like most other file systems, a ZFS file system is mounted somewhere in the systems directory hierarchy and contains files and directories of its own with permissions, flags, and other metadata.
In addition to regular file system datasets, ZFS can also create volumes, which are block devices. Volumes have many of the same features, including copy-on-write, snapshots, clones, and checksumming. Volumes can be useful for running other file system formats on top of ZFS, such as UFS virtualization, or exporting iSCSI extents.
The copy-on-write (COW) design of ZFS allows for nearly instantaneous, consistent snapshots with arbitrary names. After taking a snapshot of a dataset, or a recursive snapshot of a parent dataset that will include all child datasets, new data is written to new blocks, but the old blocks are not reclaimed as free space. The snapshot contains the original version of the file system, and the live file system contains any changes made since the snapshot was taken. No additional space is used. As new data is written to the live file system, new blocks are allocated to store this data. The apparent size of the snapshot will grow as the blocks are no longer used in the live file system, but only in the snapshot. These snapshots can be mounted read only to allow for the recovery of previous versions of files. It is also possible to rollback a live file system to a specific snapshot, undoing any changes that took place after the snapshot was taken. Each block in the pool has a reference counter which keeps track of how many snapshots, clones, datasets, or volumes make use of that block. As files and snapshots are deleted, the reference count is decremented. When a block is no longer referenced, it is reclaimed as free space. Snapshots can also be marked with a hold. When a snapshot is held, any attempt to destroy it will return an
Snapshots can also be cloned. A clone is a writable version of a snapshot, allowing the file system to be forked as a new dataset. As with a snapshot, a clone initially consumes no additional space. As new data is written to a clone and new blocks are allocated, the apparent size of the clone grows. When blocks are overwritten in the cloned file system or volume, the reference count on the previous block is decremented. The snapshot upon which a clone is based cannot be deleted because the clone depends on it. The snapshot is the parent, and the clone is the child. Clones can be promoted, reversing this dependency and making the clone the parent and the previous parent the child. This operation requires no additional space. Since the amount of space used by the parent and child is reversed, existing quotas and reservations might be affected.
Every block that is allocated is also checksummed. The checksum algorithm used is a per-dataset property, see
Each dataset has a compression property, which defaults to off. This property can be set to one of a number of compression algorithms. This will cause all new data that is written to the dataset to be compressed. Beyond a reduction in space used, read and write throughput often increases because fewer blocks are read or written.
* LZ4 - Added in ZFS pool version 5000 (feature flags), LZ4 is now the recommended compression algorithm. LZ4 compresses approximately 50% faster than LZJB when operating on compressible data, and is over three times faster when operating on uncompressible data. LZ4 also decompresses approximately 80% faster than LZJB. On modern CPUs, LZ4 can often compress at over 500 MB/s, and decompress at over 1.5 GB/s (per single CPU core).
* LZJB - The default compression algorithm. Created by Jeff Bonwick (one of the original creators of ZFS). LZJB offers good compression with less CPU overhead compared to GZIP. In the future, the default compression algorithm will likely change to LZ4.
* GZIP - A popular stream compression algorithm available in ZFS. One of the main advantages of using GZIP is its configurable level of compression. When setting the
When set to a value greater than 1, the
Checksums make it possible to detect duplicate blocks of data as they are written. With deduplication, the reference count of an existing, identical block is increased, saving storage space. To detect duplicate blocks, a deduplication table (DDT) is kept in memory. The table contains a list of unique checksums, the location of those blocks, and a reference count. When new data is written, the checksum is calculated and compared to the list. If a match is found, the existing block is used. The SHA256 checksum algorithm is used with deduplication to provide a secure cryptographic hash. Deduplication is tunable. If
Instead of a consistency check like fsck(8), ZFS has
ZFS provides very fast and accurate dataset, user, and group space accounting in addition to quotas and space reservations. This gives the administrator fine grained control over how space is allocated and allows space to be reserved for critical file systems.
Quotas limit the amount of space that a dataset and all of its descendants, including snapshots of the dataset, child datasets, and the snapshots of those datasets, can consume.
A reference quota limits the amount of space a dataset can consume by enforcing a hard limit. However, this hard limit includes only space that the dataset references and does not include space used by descendants, such as file systems or snapshots.
User quotas are useful to limit the amount of space that can be used by the specified user.
The group quota limits the amount of space that a specified group can consume.
Reservations of any sort are useful in many situations, such as planning and testing the suitability of disk space allocation in a new system, or ensuring that enough space is available on file systems for audio logs or system recovery procedures and files.
When a disk fails and is replaced, the new disk must be filled with the data that was lost. The process of using the parity information distributed across the remaining drives to calculate and write the missing data to the new drive is called resilvering.
A pool or vdev in the
Individual devices can be put in an
A pool or vdev in the
A pool or vdev in the