Chapter 20. The Z File System (ZFS)
ZFS is an advanced file system designed to solve major problems found in previous storage subsystem software.
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. ZFS calculates checksums and writes them along with the data. When reading that data later, ZFS recalculates the checksums. If the checksums do not match, meaning detecting one or more data errors, ZFS will attempt to automatically correct errors when ditto-, mirror-, or parity-blocks are available.
Pooled storage: adding physical storage devices to a pool, and allocating storage space from that shared pool. Space is available to all file systems and volumes, and increases by adding new storage devices to the pool.
Performance: caching mechanisms provide increased performance. ARC is an advanced memory-based read cache. ZFS provides a second level disk-based read cache with L2ARC, and a disk-based synchronous write cache named ZIL.
A complete list of features and terminology is in ZFS Features and Terminology.
20.1. What Makes ZFS Different
More than a file system, ZFS is fundamentally different from traditional file systems. 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 exist on a single disk alone at a time. If there were two disks then creating two separate file systems was necessary. A traditional hardware RAID configuration avoided this problem by presenting the operating system with a single logical disk made up of the space provided by physical disks on top of which the operating system placed a file system. Even with software RAID solutions like those provided by GEOM, the UFS file system living on top of the RAID believes it’s dealing with a single device. ZFS' combination of the volume manager and the file system solves this and allows the creation of file systems that all share a pool of available storage. One big advantage of ZFS' awareness of the physical disk layout is that existing file systems grow automatically when adding extra disks to the pool. This new space then becomes available to the file systems. ZFS can also apply different properties to each file system. This makes it useful to create separate file systems and datasets instead of a single monolithic file system.
20.2. Quick Start Guide
FreeBSD can mount ZFS pools and datasets 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 creating and mounting of the
example pool, and that is now accessible as a file system.
Create files for users to browse:
# 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
This pool is not using any advanced ZFS features and properties yet. 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.
Disable compression 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
mount shows the pool and file systems:
# 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)
Use ZFS datasets like any file system after creation.
Set other available features on a per-dataset basis when needed.
The example below creates a new file system called
It assumes the file system contains important files and configures it to store two copies of each data block.
# zfs create example/data # zfs set copies=2 example/data
df to see the data and space usage:
# 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 all file systems in the pool have the same available space.
df in these examples shows that the file systems use the space they need and all draw from the same pool.
ZFS gets rid of concepts such as volumes and partitions, and allows several file systems to share the same pool.
To destroy the file systems and then the pool that is no longer needed:
# zfs destroy example/compressed # zfs destroy example/data # zpool destroy example
Disks fail. One way to avoid data loss from disk failure is to use 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 two disks are available, ZFS mirroring provides redundancy if required. Refer to zpool(8) for more details.
The previous example created the
This example makes a new file system called
home in that pool:
# zfs create storage/home
Enable compression and store an extra copy of directories and files:
# 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.
Create a file system snapshot to roll back to later:
# zfs snapshot storage/home@08-30-08
ZFS creates snapshots of a dataset, not a single directory or file.
@ character is a delimiter between the file system name or the volume name.
Before deleting an important directory, back up the file system, then roll back to an earlier snapshot in which the directory still exists:
# 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 snapshot taken:
# ls /storage/home/.zfs/snapshot
Write a script to take regular snapshots of user data. Over time, snapshots can use up a lot of disk space. Remove the previous snapshot using the command:
# zfs destroy storage/home@08-30-08
After testing, make /storage/home the real /home with 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. Add daily status updates about the created file systems to the nightly periodic(8) runs by adding this line to /etc/periodic.conf:
20.2.3. Recovering RAID-Z
Every software RAID has a method of monitoring its
View the status of RAID-Z devices using:
# zpool status -x
If all pools are Online and everything is normal, the message shows:
all pools are healthy
If there is a problem, perhaps a disk being in the Offline state, the pool state will look like this:
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
"OFFLINE" shows the administrator took da1 offline using:
# zpool offline storage da1
Power down the computer now and replace da1. Power up the computer and return da1 to the pool:
# zpool replace storage da1
Next, check the status again, this time without
-x to display all pools:
# 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. Creating file systems automatically enables them.
Disabling Checksums is possible but not recommended! Checksums take little storage space and provide data integrity. Most ZFS features will not work properly with checksums disabled. Disabling these checksums will not increase performance noticeably.
Verifying the data checksums (called scrubbing) ensures integrity of the
storage pool with:
# 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.
Since scrubbing is I/O intensive, ZFS allows a single scrub to run at a time.
After scrubbing completes, view the status 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
Displaying the completion date of the last scrubbing helps decide when to start another. Routine scrubs help protect data from silent corruption and ensure the integrity of the pool.
ZFS administration uses two main utilities.
zpool utility controls the operation of the pool and allows adding, removing, replacing, and managing disks.
zfs utility allows creating, destroying, and managing datasets, both file systems and volumes.
20.3.1. Creating and Destroying Storage Pools
Creating a ZFS storage pool (zpool) requires permanent decisions, as the pool structure cannot change after creation. The most important decision is which types of vdevs to group the physical disks into. See the list of vdev types for details about the possible options. After creating the pool, most vdev types do not allow adding disks to the vdev. The exceptions are mirrors, which allow adding new disks to the vdev, and stripes, which upgrade to mirrors by attaching a new disk to the vdev. Although adding new vdevs expands a pool, the pool layout cannot change after pool creation. Instead, back up the data, destroy the pool, and recreate it.
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
To create more than one vdev with a single command, specify 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 use partitions rather than whole disks. Putting ZFS in a separate partition allows the same disk to have other partitions for other purposes. In particular, it allows adding partitions with bootcode and file systems needed for booting. This allows booting from disks that are also members of a pool. ZFS adds 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, using the replacement disk.
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
Destroy a pool that is no longer needed to reuse the disks.
Destroying a pool requires unmounting the file systems in that pool first.
If any dataset is in use, the unmount operation fails without destroying the pool.
Force the pool destruction with
This can cause undefined behavior in applications which had open files on those datasets.
20.3.2. Adding and Removing Devices
Two ways exist for adding disks to a zpool: attaching a disk to an existing vdev with
zpool attach, or adding vdevs to the pool with
Some vdev types allow adding disks to the vdev after creation.
A pool created with a single disk lacks redundancy.
It can detect corruption but can not repair it, 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, use
zpool attach to add a new disk to the vdev, creating a mirror.
zpool attach to add new disks to a mirror group, increasing redundancy and read performance.
When partitioning the disks used for the pool, replicate the layout of the first disk on to the second.
gpart backup and
gpart restore 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 resilvering finishes 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.
Adding vdevs provides higher performance by distributing writes across the vdevs.
Each vdev provides its own redundancy.
Mixing vdev types like
RAID-Z is possible but discouraged.
Adding a non-redundant vdev to a pool containing mirror or RAID-Z vdevs risks the data on the entire pool.
Distributing writes means a failure of the non-redundant disk will result in the loss of a fraction of every block written to the pool.
ZFS stripes data 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. ZFS allocates space so that each vdev reaches 100% full at the same time. Having vdevs with different amounts of free space will lower performance, as more data writes go to the less full vdev.
When attaching new 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
Removing vdevs from a pool is impossible and removal of disks from a mirror is exclusive if there is enough remaining redundancy. If a single disk remains in a mirror group, that group ceases to be a mirror and becomes 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 ZFS detects a read, write, or checksum error, the corresponding error count increases.
status output shows the configuration and status of each device in the pool and the status of the entire pool.
Actions to take 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 detecting an error, ZFS increases the read, write, or checksum error counts.
Clear the error message and reset the counts with
zpool clear mypool.
Clearing the error state can be important for automated scripts that alert the administrator when the pool encounters an error.
Without clearing old errors, the scripts may fail to report further errors.
20.3.5. Replacing a Functioning Device
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 the data from the old disk to the new one.
After the operation completes, ZFS disconnects the old disk 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 resilvering finishes before rebooting. When booting from the pool 'zroot', update the boot code on the newly attached disk 'ada2p3'. Assuming GPT partitioning is used and [.filename]#da0# is the new boot disk, 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. The data is still available, but with reduced performance because ZFS computes missing data from the available redundancy. To restore the vdev to a fully functional state, replace the failed physical device. ZFS is then instructed to begin the resilver operation. ZFS recomputes data on the failed device from available redundancy and writes it to the replacement device. After completion, the vdev returns to online status.
If the vdev does not have any redundancy, or if devices have failed and there is not enough redundancy to compensate, the pool enters the faulted state. Unless enough devices can reconnect the pool becomes inoperative requiring a data restore from backups.
When replacing a failed disk, the name of the failed disk changes to the GUID of the new disk.
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
Routinely scrub pools, ideally at least once every month.
scrub operation is 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 keep it from slowing down 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
To cancel a scrub operation if needed, run
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 configuration with two disks where one drive is starting to malfunction and cannot properly store the data any more. This is worse when the data was not accessed for a long time, as with long term archive storage. Traditional file systems need to run commands that check and repair the data like fsck(8). These commands take time, and in severe cases, an administrator has to decide which repair operation to perform. When ZFS detects a data block with a mismatched checksum, it tries to read the data from the mirror disk. If that disk can provide the correct data, ZFS will give that to the application and correct the data on the disk with the wrong checksum. This happens without any interaction from a system administrator during normal pool operation.
The next example shows this self-healing behavior by creating a mirrored pool of disks /dev/ada0 and /dev/ada1.
# 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 -
Copy some important data to the pool to protect from data errors using the self-healing feature and create a checksum of the pool 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
Simulate data corruption by writing random data to the beginning of one of the disks in the mirror. To keep ZFS from healing the data when detected, export the pool before the corruption and import it again afterwards.
This is a dangerous operation that can destroy vital data, shown here for demonstration alone. Do not try it during normal operation of a storage pool. Nor should this intentional corruption example run on any disk with a file system not using ZFS on another partition in it. Do not use any other disk device names other than the ones that are part of the pool. Ensure proper backups of the pool exist and test them 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.
To find the device with the wrong checksum, look for one whose
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
ZFS detected the error and handled it 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
Generate checksums before and after the intentional tampering while the pool data still matches. This shows how ZFS is capable of detecting and correcting any errors automatically when the checksums differ. Note this is possible with 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; do not disable them for any reason. ZFS requires no fsck(8) or similar file system consistency check program to detect and correct this, and keeps the pool available while there is 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 a wrong checksum on ada1, shown by the
(repairing) output from
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 scrubbing operation completes with all the data synchronized from ada0 to ada1, clear the error messages 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, with all error counts now zero.
20.3.9. Growing a Pool
The smallest device in each vdev limits the usable size of a redundant pool. Replace the smallest device 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 replacing the 1 TB drive 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 grows to 2 TB.
Start expansion by using
zpool online -e on each device.
After expanding all devices, the extra space becomes available to the pool.
20.3.10. Importing and Exporting Pools
Export pools before moving them to another system.
ZFS unmounts all datasets, marking each device as exported but still locked to prevent use by other disks.
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, use
zpool export -f to force exporting the 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.
If this is undesired behavior, use
zpool import -N to prevent it.
zpool import -o sets temporary properties for this specific import.
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, force the import using
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 importing a pool from a system using an older version, manually upgrade the pool to the latest ZFS version to support newer features. Consider whether the pool may ever need importing on an older system before upgrading. Upgrading is a one-way process. Upgrade older pools is possible, but downgrading pools with newer features is not.
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 are 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 to see what new features the upgrade provides, as well as which features are already supported.
Upgrade a pool to support new 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
Update the boot code on systems that boot from a pool to support the new pool version.
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
ZFS records commands that change the pool, including creating datasets, changing properties, or replacing a disk.
Reviewing history about a pool’s creation is useful, as is checking 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 altering the pool in some way along with a timestamp.
zfs list are not included.
When specifying no pool name, ZFS displays history of all pools.
zpool history can show even more information when providing the options
-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
Show more details by adding
Showing history records in a long format, including information like the name of the user who issued the command and the hostname on which the change happened.
# 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.
myzfsbox is also shown in the commands after the pool’s creation.
The hostname display becomes important when exporting the pool from one system and importing on another.
It’s possible to distinguish the commands issued on the other system by the hostname recorded for each command.
Combine both options to
zpool history to give the most detailed information possible for any given pool.
Pool history provides valuable information when tracking down the actions performed or when needing more detailed output 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, read and write operations performed per second, and I/O bandwidth used. By default, ZFS monitors and displays all pools in the system. Provide a pool name to limit monitoring to 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 see I/O activity, specify a number as the last parameter, indicating an interval in seconds to wait between updates. The next statistic line prints after each interval. Press Ctrl+C to stop this continuous monitoring. Give a second number on the command line after the interval to specify the total number of statistics to display.
Display even more detailed I/O statistics with
Each device in the pool appears with a statistics line.
This is useful for seeing read and write operations 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
ZFS can split a pool consisting of one or more mirror vdevs into two pools.
Unless otherwise specified, ZFS detaches the last member of each mirror and creates a new pool containing the same data.
Be sure to make a dry run of the operation with
This displays the details of the requested operation without actually performing it.
This helps confirm that the operation will do what the user intends.
zfs utility can create, destroy, and manage all existing ZFS datasets within a pool.
To manage the pool itself, use
20.4.1. Creating and Destroying Datasets
Unlike traditional disks and volume managers, space in ZFS is not preallocated.
With traditional file systems, after partitioning and assigning the space, there is no way to add a new file system without adding a new disk.
With ZFS, creating new file systems is possible 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.
Nesting datasets within each other is possible and child datasets will inherit properties from their ancestors.
Delegate, replicate, snapshot, jail allows administering and destroying each dataset as a unit.
Creating a separate dataset for each different type or set of files has advantages.
The drawbacks to having a large number of datasets are that some commands like
zfs list will be slower, and that mounting of hundreds or even thousands of datasets will 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 the files on the dataset, as it does not involve scanning the files and updating the corresponding metadata.
Destroy the 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 minutes to appear in the pool.
zpool get freeing poolname to see the
freeing property, that shows which datasets are having their blocks freed in the background.
If there are child datasets, like snapshots or other datasets, destroying the parent is impossible.
To destroy a dataset and its children, use
-r to recursively destroy the dataset and its children.
-n -v to list datasets and snapshots destroyed by this operation, without actually destroy anything.
Space reclaimed by destroying snapshots is also shown.
20.4.2. Creating and Destroying Volumes
A volume is a special dataset type. Rather than mounting as a file system, expose it as a block device under /dev/zvol/poolname/dataset. This allows using the volume for other file systems, to back the disks of a virtual machine, or to make it available to other network hosts using protocols like iSCSI or HAST.
Format a volume with any file system or 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 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 minutes to reclaim the free space in the background.
20.4.3. Renaming a Dataset
To change the name of a dataset, use
To change the parent of a dataset, use this command as well.
Renaming a dataset to have a different parent dataset will change the value of those properties inherited from the parent dataset.
Renaming a dataset unmounts then remounts it in the new location (inherited from the new parent dataset).
To prevent this behavior, use
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
Renaming snapshots uses the same command.
Due to the nature of snapshots, rename cannot change their parent dataset.
To rename a recursive snapshot, specify
-r; this will also rename all snapshots with the same name in child datasets.
# 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 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.
zfs inherit reverts most properties to their inherited values.
User-defined properties are also possible.
They become part of the dataset configuration and provide further information about the dataset or its contents.
To distinguish these custom properties from the ones supplied as part of ZFS, use a colon (
:) 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
If the custom property is not defined in any of the parent datasets, this option removes it (but the pool’s history still records the change).
# 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 #
184.108.40.206. Getting and Setting Share Properties
Two commonly used and useful dataset properties are the NFS and SMB share options. Setting these defines if and how ZFS shares datasets on the network. At present, FreeBSD supports setting NFS sharing alone. 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
Set other options for sharing datasets through NFS, such as
To set options on 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), ZFS creates snapshots fast by preserving older versions of the data on disk.
If no snapshots exist, ZFS reclaims space for future use when data is rewritten or deleted.
Snapshots preserve disk space by recording just the differences between the current dataset and a previous version.
Allowing snapshots on whole datasets, not on individual files or directories.
A snapshot from a dataset duplicates everything contained in it.
This includes the file system properties, files, directories, permissions, and so on.
Snapshots use no extra space when first created, but consume space as the blocks they reference change.
Recursive snapshots taken with
-r create snapshots with the same name on the dataset and its children, providing a consistent moment-in-time snapshot of the file systems.
This can be important when an application has files on related datasets or that depend 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 as a quick way of backing up the current state of the file system when performing a risky action like a software installation or a system upgrade. If the action fails, rolling back to the snapshot returns the system to the same state when creating the snapshot. If the upgrade was successful, delete the snapshot to free up space. Without snapshots, a failed upgrade often requires restoring backups, which is tedious, time consuming, and may require downtime during which the system is unusable. Rolling back to snapshots is fast, even while the system is running in normal operation, with little or no downtime. The time savings are enormous with multi-terabyte storage systems considering the time required to copy the data from backup. Snapshots are not a replacement for a complete backup of a pool, but offer a quick and easy way to store a dataset copy at a specific time.
220.127.116.11. Creating Snapshots
To create snapshots, use
zfs snapshot dataset@snapshotname.
-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, append
-t snapshot to
-t all displays both file systems and snapshots.
Snapshots are not mounted directly, showing no path in the
ZFS does not mention available disk space in the
AVAIL column, as snapshots are read-only after their creation.
Compare the snapshot to the original dataset:
# 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 the changes (delta) made and not the complete file system contents all over again. This means that snapshots take little space when making changes. Observe space usage even more by copying a file to the dataset, then creating 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 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 show the changes between itself and the snapshot taken afterwards.
18.104.22.168. Comparing Snapshots
ZFS provides a built-in command to compare the differences in content between two snapshots.
This is helpful with a lot of snapshots taken over time when the user wants to see how the file system has changed over time.
zfs diff lets a user find the latest snapshot that still contains a file deleted by accident.
Doing this for the two snapshots 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 change type:
Adding the path or file.
Deleting the path or file.
Modifying the path or file.
Renaming the path or file.
Comparing the output with the table, it becomes clear that ZFS added passwd
after creating the snapshot
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.
22.214.171.124. Snapshot Rollback
When at least one snapshot is available, roll back to it at any time.
Most often this is the case when the current state of the dataset is no longer and if preferring an older version.
Scenarios such as local development tests gone wrong, botched system updates hampering the system functionality, or the need to restore deleted files or directories are all too common occurrences.
To roll back a snapshot, use
zfs rollback snapshotname.
If a lot of changes are present, the operation will take a long 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 rolled back, the dataset has the same state as it had when the snapshot was originally taken.
Rolling back to a snapshot discards all other data in that dataset not part of the snapshot.
Taking a snapshot of the current state of the dataset before rolling back to a previous one is a good idea when requiring some data later.
This way, the user can roll back and forth between snapshots without losing data that is still valuable.
In the first example, roll back a snapshot because of a careless
rm operation that removes too much data than 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 notices the removal of extra files and wants them back. ZFS provides an easy way to get them back using rollbacks, when performing snapshots of important data 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. Rolling back to a snapshot taken much earlier with other snapshots taken afterwards is also possible. 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 delete these snapshots.
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 confirm that this is the desired action.
If that is the intention, and understanding the consequences of losing all intermediate snapshots, issue the command:
# 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 the removal of the intermediate snapshots as a result of
zfs rollback -r.
126.96.36.199. Restoring Individual Files from Snapshots
Snapshots live in a hidden directory under the parent dataset: .zfs/snapshots/snapshotname.
By default, these directories will not show even when executing a standard
ls -a .
Although the directory doesn’t show, access it 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
Restore individual files 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 like the snapshots taken earlier to make it easier to identify them. The next example shows how to restore a file from the hidden .zfs directory by copying it from the snapshot containing 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
Even if the the
snapdir property is set to hidden, running
ls .zfs/snapshot will still list the contents of that directory.
The administrator decides whether to display these directories.
This is a per-dataset setting.
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 change after creation. Copying files into and removing them from snapshot directories are both disallowed 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.
written property of a snapshot tracks the space the snapshot uses.
To destroy snapshots and reclaim the space, use
zfs destroy dataset@snapshot.
-r recursively removes all snapshots with the same name under the parent dataset.
-n -v to the command displays a list of the snapshots to be deleted and an estimate of the space it would reclaim without performing the actual destroy operation.
20.4.6. Managing Clones
A clone is a copy of a snapshot treated more like a regular dataset.
Unlike a snapshot, a clone is writeable and mountable, and has its own properties.
After creating a clone using
zfs clone, destroying the originating snapshot is impossible.
To reverse the child/parent relationship between the clone and the snapshot use
Promoting a clone makes the snapshot become a child of the clone, rather than of the original parent dataset.
This will change how ZFS accounts for the space, but not actually change the amount of space consumed.
Mounting the clone anywhere within the ZFS file system hierarchy is possible, not only below the original location of the snapshot.
To show the clone feature use this example dataset:
# 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 change, create a read/write clone of a snapshot. After achieving the desired result in the clone, promote the clone to a dataset and remove the old file system. Removing the parent dataset 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
Creating a clone makes it an exact copy of the state the dataset as it was when taking the snapshot.
Changing the clone independently from its originating dataset is possible now.
The connection between the two is the snapshot.
ZFS records this connection in the property
Promoting the clone with
zfs promote makes the clone an independent dataset.
This removes the value of the
origin property and disconnects the newly independent dataset from the snapshot.
This example shows 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.
To do this,
zfs destroy the old dataset first and then
zfs rename the clone to the old dataset name (or to 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 an ordinary dataset. It contains all the data from the original snapshot plus the files added to it like loader.conf. Clones provide useful features to ZFS users in different scenarios. For example, provide jails as snapshots containing different sets of installed applications. Users can clone these snapshots and add their own applications as they see fit. Once satisfied with the changes, promote the clones to full datasets and provide them 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 feature, storing this data on another pool connected to the local system is possible, as is sending 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 show 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 writing and reading data happens on a regular basis. Using a second standby pool backup 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. Use a snapshot to provide a consistent file system version to replicate. After creating a snapshot of mypool, copy it to the backup pool by replicating snapshots. This does not include changes made since the most recent snapshot.
# zfs snapshot mypool@backup1 # zfs list -t snapshot NAME USED AVAIL REFER MOUNTPOINT mypool@backup1 0 - 43.6M -
Now that a snapshot exists, use
zfs send to create a stream representing the contents of the snapshot.
Store this stream as a file or receive it on another pool.
Write the stream to standard output, but redirect to a file or pipe or an error appears:
# 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 sent snapshot, which means the data contained in the snapshot, not 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.
To create and send these snapshots automatically, use a cron(8) job.
Instead of storing the backups as archive files, ZFS can receive them as a live file system, allowing direct access to the backed up data.
To get to the actual data contained in those streams, use
zfs receive 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.
Use the data directly on the receiving pool after the transfer is complete.
It is only possible to replicate a dataset 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 -
188.8.131.52. Incremental Backups
zfs send can also determine the difference between two snapshots and send individual differences between the two.
This saves disk space and transfer time.
# 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 -
Create a second snapshot called replica2.
This second snapshot contains changes made to the file system between now and the previous snapshot, replica1.
zfs send -i and indicating the pair of snapshots generates an incremental replica stream containing the changed data.
This succeeds 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 replicated the changed data rather than the entirety of replica1. Sending the differences alone took much less time to transfer and saved disk space by not copying the whole pool each time. This is useful when replicating over a slow network or one charging per transferred byte.
A new file system, backup/mypool, is available with the files and data from the pool mypool.
-P copies the dataset properties including compression settings, quotas, and mount points.
-R copies all child datasets of the dataset along with their properties.
Automate sending and receiving to create regular backups on the second pool.
184.108.40.206. 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 when sending the streams over the internet to a remote host. Use SSH to securely encrypt data sent over a network connection. Since ZFS requires redirecting the stream from standard output, piping it through SSH is easy. To keep the contents of the file system encrypted in transit and on the remote system, consider using PEFS.
Change some settings and take security precautions first.
This describes the necessary steps required for the
zfs send operation; for more information on SSH, see OpenSSH.
Change the configuration as follows:
Passwordless SSH access between sending and receiving host using SSH keys
ZFS requires the privileges of the
rootuser to send and receive streams. This requires logging in to the receiving system as
Security reasons prevent
rootfrom logging in by default.
Use the ZFS Delegation system 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 need permission 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 can receive and mount datasets now, and replicates the home dataset to the remote system:
% zfs snapshot -r mypool/home@monday % zfs send -R mypool/home@monday | ssh someuser@backuphost zfs recv -dvu recvpool/backup
Create a recursive snapshot called monday of the file system dataset home on the pool mypool.
zfs send -R includes the dataset, all child datasets, snapshots, clones, and settings in the stream.
Pipe the output through SSH to the waiting
zfs receive on the remote host backuphost.
Using an IP address or fully qualified domain name is good practice.
The receiving machine writes the data to the backup dataset on the recvpool pool.
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 mount on the receiving side.
-v shows more details about the transfer, including the elapsed time and the amount of data transferred.
20.4.8. Dataset, User, and Group Quotas
Use Dataset quotas to restrict the amount of space consumed by a particular dataset. Reference Quotas work in much the same way, but count the space used by the dataset itself, excluding snapshots and child datasets. Similarly, use user and group quotas to prevent users or groups from using up all 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
Then, create the user and make the home directory point to the dataset’s
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 see the quotas associated with the groups to which they belong.
A user with the
groupquota privilege or
root 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
For group information, use
For more information about supported options or how to display specific options alone, refer to zfs(1).
Privileged users and
root can list the quota for storage/home/bob using:
# zfs get quota storage/home/bob
Reservations guarantee an always-available amount of space on a dataset. The reserved space will not be available to any other dataset. This useful feature ensures 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 applies 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 written at the block level saves space and also increases disk throughput. If data compresses by 25% the compressed data writes 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 extra memory.
ZFS offers different compression algorithms, each with different trade-offs. The introduction of LZ4 compression in ZFS v5000 enables compressing 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 header part of the data, ZFS writes the block 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 see the effectiveness of compression using 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 using 449 GB of space (the used property).
Without compression, it would have taken 496 GB of space (the
This results in a 1.11:1 compression ratio.
Compression can have an unexpected side effect when combined with User Quotas.
User quotas restrict how much actual space a user consumes on a dataset after compression.
If a user has a quota of 10 GB, and writes 10 GB of compressible data, they will still be able to store more 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 data storage to ensure there is enough backup space available. Since quotas do not consider compression ZFS may write more data than would fit with uncompressed backups.
20.4.11. Zstandard Compression
OpenZFS 2.0 added a new compression algorithm. 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 since 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 written once but read often, Zstd allows the use of the highest compression levels without a read performance penalty.
Even with frequent data updates, enabling compression often provides higher performance. 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. 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 much slower.
Levels above 10 require large amounts of memory to compress each block and systems with less than 16 GB of RAM should not use them.
ZFS uses a selection of the Zstd_fast_ levels also, which get correspondingly faster but supports lower compression ratios.
zstd-fast-100 in increments of 10, and
zstd-fast-1000 which provide minimal compression, but offer high performance.
If ZFS is not able to get the required memory to compress a block with Zstd, it will fall back to storing the block uncompressed.
This is unlikely to happen except at the highest levels of Zstd on memory constrained systems.
ZFS counts how often this has occurred since loading the ZFS module with
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 a new reference to the existing data instead of the whole duplicate block. Tremendous space savings are possible if the data contains a lot of duplicated files or repeated information. Warning: deduplication requires a large amount of memory, and enabling compression instead provides most of the space savings without the extra cost.
To activate deduplication, set the
dedup property on the target pool:
# zfs set dedup=on pool
Deduplicating only affects new data written to the pool. Merely activating this option will not deduplicate data already written to the pool. 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 deduplicated yet.
The next example copies some system binaries three times into different directories on the deduplicated pool created above.
# for d in dir1 dir2 dir3; do > mkdir $d && cp -R /usr/bin $d & > done
To observe deduplicating of redundant data, use:
# 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
Detecting and deduplicating copies of the data uses 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 when the data in 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 activating deduplication would achieve.
In this case,
1.16 is a poor space saving ratio mainly provided by compression.
Activating deduplication on this pool would not save any 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 good.
Good practice is to enable compression first as compression also provides greatly increased performance.
Enable deduplication in cases where savings are considerable and with enough available memory for the DDT.
20.4.13. ZFS and Jails
zfs jail and the corresponding
jailed property to delegate a ZFS dataset to a Jail.
zfs jail jailid attaches a dataset to the specified jail, and
zfs unjail detaches it.
To control the dataset from within a jail, set the
ZFS forbids mounting a jailed dataset 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 need permission to create and destroy snapshots of their home directories. A user performing backups can get permission to use replication features. ZFS allows a usage statistics script to run with access to only the space usage data for all users. Delegating the ability to delegate permissions is also possible. 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.
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.
Another restriction aimed at preventing abuse: non-
root users must own the mountpoint where mounting the file system.
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
Adjust tunables to make ZFS perform best for different workloads.
vfs.zfs.arc_max- Upper size of the ARC. The default is all RAM but 1 GB, or 5/8 of all RAM, whichever is more. Use a lower value if the system runs any other daemons or processes that may require memory. Adjust this value at runtime with sysctl(8) and set it in /boot/loader.conf or /etc/sysctl.conf.
vfs.zfs.arc_meta_limit- Limit the amount of the ARC 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. Adjust this value at runtime with sysctl(8) in /boot/loader.conf or /etc/sysctl.conf.
vfs.zfs.arc_min- Lower 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. Adjust this value at runtime with sysctl(8) and 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. Set this value at boot time and in /boot/loader.conf.
ashift(sector size) 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.
Common drives have 4 KB sectors. Using the default
9with these drives results in write amplification on these devices. Data contained in a single 4 KB write is instead written in eight 512-byte writes. ZFS tries to read the native sector size from all devices when creating a pool, but 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 with planned disk upgrades. Future disks use 4 KB sectors, and
ashiftvalues cannot change after creating a pool.
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 transfers during small random reads. This can provide better performance when using a smaller ZFS record size.
vfs.zfs.prefetch_disable- Disable prefetch. A value of
1disables it. The default is
0, unless the system has less than 4 GB of RAM. Prefetch works by reading larger blocks than requested into the ARC in hopes to soon need the data. If the workload has a large number of random reads, disabling prefetch may actually improve performance by reducing unnecessary reads. Adjust this value 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. Adjust this value 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. Adjust this value at any time with sysctl(8).
vfs.zfs.top_maxinflight- Upper 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. Adjust this value at any time with sysctl(8).
vfs.zfs.l2arc_write_max- Limit the amount of data written to the L2ARC per second. This tunable extends the longevity of SSDs by limiting the amount of data written to the device. Adjust this value at any time with sysctl(8).
vfs.zfs.l2arc_write_boost- Adds the value of this tunable to
vfs.zfs.l2arc_write_maxand increases the write speed to the SSD until evicting the first block from the L2ARC. This "Turbo Warmup Phase" reduces the performance loss from an empty L2ARC after a reboot. Adjust this value 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. Changing this setting results 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. Recent activity on the pool limits the speed of
scrub, as determined by
vfs.zfs.scan_idle. Adjust this value 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. ZFS determins the granularity of the setting by the value of
kern.hzwhich defaults to 1000 ticks per second. Changing this setting results 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. Other recent activity on the pool limits the speed of resilver, as determined by
vfs.zfs.scan_idle. Adjust this value at any time with sysctl(8).
vfs.zfs.scan_idle- Number of milliseconds since the last operation before considering the pool is idle. ZFS disables the rate limiting for
scruband resilver when the pool is idle. Adjust this value at any time with sysctl(8).
vfs.zfs.txg.timeout- Upper number of seconds between transaction groups. The current transaction group writes to the pool and a fresh transaction group starts if this amount of time elapsed since the previous transaction group. A transaction group may trigger earlier if writing enough data. The default value is 5 seconds. A larger value may improve read performance by delaying asynchronous writes, but this may cause uneven performance when writing the transaction group. Adjust this value 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 upper efficiency on systems with limited RAM.
As a lower value, the total system memory should be at least one gigabyte. The amount of recommended RAM depends upon the size of the pool and which features ZFS uses. A general rule of thumb is 1 GB of RAM for every 1 TB of storage. If using the deduplication feature, a general rule of thumb is 5 GB of RAM per TB of storage to deduplicate. While some users use ZFS with less RAM, systems under heavy load may panic due to memory exhaustion. ZFS may require further tuning for systems with less than the recommended RAM requirements.
220.127.116.11. 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 push beyond the 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
512 for 2 GB.
18.104.22.168. Loader Tunables
Increases the kmem address space on all FreeBSD architectures. A test system with 1 GB of physical memory benefitted from adding these options to /boot/loader.conf and then restarting:
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. Further Resources
20.8. ZFS Features and Terminology
More than a file system, ZFS is fundamentally different. ZFS combines the roles of file system and volume manager, enabling new storage devices to add to a live system and having the new space available on the existing file systems in that pool at once. By combining the traditionally separate roles, ZFS is able to overcome previous limitations that prevented RAID groups being able to grow. A vdev is a top level device in a pool and 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. Used blocks from the pool decrease the space available to each file system. 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 consists 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 ZFS version number on the pool determines the features available.
A pool consists of one or more vdevs, which themselves are a single disk or a group of disks, transformed to a RAID. When using a lot of vdevs, ZFS spreads data across the vdevs to increase performance and maximize usable space. All vdevs must be at least 128 MB in size.
Transaction Groups are the way ZFS groups blocks changes together and writes them to the pool. Transaction groups are the atomic unit that ZFS uses to ensure consistency. ZFS assigns each transaction group 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 - A new transaction group begins in the open state and accepts new writes. There is always a transaction group in the open state, but the transaction group may refuse new writes if it has reached a limit. Once the open transaction group has reached a limit, or reaching 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 how recently object was used, adding new items to the head of the list. When the cache is full, evicting items from the tail of the list makes 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 evicted objects to prevent adding them back to the cache. This increases the cache hit ratio by avoiding objects that have a history of occasional use. Another advantage of using both an MRU and MFU is that scanning an entire file system would evict all data from an MRU or LRU cache in favor of this freshly accessed content. With ZFS, there is also an MFU that 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. RAM stores the primary ARC. 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 increase read speeds for cached files on the SSD instead of having to read from the regular disks. L2ARC can also speed up deduplication because a deduplication table (DDT) that does not fit in RAM but does fit in the L2ARC will be much faster than a DDT that must read from disk. Limits on the data rate added to the cache devices prevents prematurely wearing out SSDs with extra writes. Until the cache is full (the first block evicted to make room), writes to the L2ARC limit to the sum of the write limit and the boost limit, and afterwards limit 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 is stored to disk rather than merely cached for later writes), writing the data to the faster ZIL storage then later flushing it out to the regular disks greatly reduces latency and improves performance. Synchronous workloads like databases will profit from a ZIL alone. Regular asynchronous writes such as copying files will not use the ZIL at all.
Unlike a traditional file system, ZFS writes a different block rather than overwriting the old data in place. When completing this write the metadata updates to point to the new location. When a shorn write (a system crash or power loss in the middle of writing a file) occurs, the entire original contents of the file are still available and ZFS discards the incomplete write. 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 a dataset as well. Child datasets have hierarchical names like directories. For example, mypool/home, the home dataset, is a child of mypool and inherits properties from it. Expand this further by creating mypool/home/user. This grandchild dataset will inherit properties from the parent and grandparent. Set properties on a child to override the defaults inherited from the parent and grandparent. 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 mounts somewhere in the systems directory hierarchy and contains files and directories of its own with permissions, flags, and other metadata.
ZFS can also create volumes, which appear as disk devices. Volumes have a lot of the same features as datasets, 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 goes to new blocks, but without reclaiming the old blocks as free space. The snapshot contains the original file system version and the live file system contains any changes made since taking the snapshot using no other space. New data written to the live file system uses new blocks to store this data. The snapshot will grow as the blocks are no longer used in the live file system, but in the snapshot alone. Mount these snapshots read-only allows recovering of previous file versions. A rollback of a live file system to a specific snapshot is possible, undoing any changes that took place after taking the snapshot. Each block in the pool has a reference counter which keeps track of the snapshots, clones, datasets, or volumes use that block. As files and snapshots get deleted, the reference count decreases, reclaiming the free space when no longer referencing a block. Marking snapshots with a hold results in any attempt to destroy it will returns an
Cloning a snapshot is also possible. A clone is a writable version of a snapshot, allowing the file system to fork as a new dataset. As with a snapshot, a clone initially consumes no new space. As new data written to a clone uses new blocks, the size of the clone grows. When blocks are overwritten in the cloned file system or volume, the reference count on the previous block decreases. Removing the snapshot upon which a clone bases is impossible 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 new space. Since the amount of space used by the parent and child reverses, it may affect existing quotas and reservations.
Every block is also checksummed. The checksum algorithm used is a per-dataset property, see
Each dataset has a compression property, which defaults to off. Set this property to an available compression algorithm. This causes compression of all new data written to the dataset. Beyond a reduction in space used, read and write throughput often increases because fewer blocks need reading or writing.
* LZ4 - Added in ZFS pool version 5000 (feature flags), LZ4 is now the recommended compression algorithm. LZ4 works about 50% faster than LZJB when operating on compressible data, and is over three times faster when operating on uncompressible data. LZ4 also decompresses about 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 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 when writing data. With deduplication, the reference count of an existing, identical block increases, saving storage space. ZFS keeps a deduplication table (DDT) in memory to detect duplicate blocks. The table contains a list of unique checksums, the location of those blocks, and a reference count. When writing new data, ZFS calculates checksums and compares them to the list. When finding a match it uses the existing block. Using the SHA256 checksum algorithm with deduplication provides a secure cryptographic hash. Deduplication is tunable. If
Instead of a consistency check like fsck(8), ZFS has
ZFS provides fast and accurate dataset, user, and group space accounting as well as quotas and space reservations. This gives the administrator fine grained control over space allocation and allows reserving space for critical file systems.
Quotas limit the total size of a dataset and its descendants, including snapshots of the dataset, child datasets, and the snapshots of those datasets.
Volumes do not support quotas, as the
A reference quota limits the amount of space a dataset can consume by enforcing a hard limit. This hard limit includes space referenced by the dataset alone and does not include space used by descendants, such as file systems or snapshots.
User quotas are useful to limit the amount of space 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 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 replacing a failed disk, ZFS must fill the new disk with the lost data. Resilvering is the process of using the parity information distributed across the remaining drives to calculate and write the missing data to the new drive.
A pool or vdev in the
The administrator puts individual devices in an
A pool or vdev in the
A pool or vdev in the