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Date:      Thu, 28 Nov 2013 11:55:51 +1100
From:      Jan Mikkelsen <>
Subject:   Re: ssd for zfs
Message-ID:  <>
In-Reply-To: <>
References:  <>

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Using the drive write cache seems like a really bad idea for a ZIL.

The purpose of a ZIL is to keep a log of actions for recovery after a =
system crash. The drive write cache lets the drive lie to the operating =
system about whether or not a write has been made durable. If you have a =
power failure while you have 1400 writes outstanding to the drive you =
might find that you have data loss on restart.

For a ZIL you are best off with a drive that has a supercapacitor to =
ensure all outstanding writes can be completed on power loss. For =
example, the Intel S3700 series.

Performance on your zpool is probably limited by the number of vdevs you =
have. More vdevs give more I/O parallelism. If you only have one vdev =
you will be limited to single drive throughput. Depending on the number =
of drives you have and what you need, you will either need a bunch of =
mirrored vdevs or a bunch of raidz2 vdevs (if you have enough drives).

I made this mistake early on, thinking a raidz2 vdev alone would give =
parallelism. You need multiple vdevs.


Jan Mikkelsen

On 28 Nov 2013, at 1:14 am, wrote:

> -------- Eredeti =FCzenet --------
> T=E1rgy: Re: ssd for zfs
> D=E1tum: 2013-11-27 14:07
> Felad=F3: Richard Kojedzinszky <>
> C=EDmzett: Tom Evans <>
> M=E1solat: FreeBSD FS <>
> Dear FS devs,
> After some investigation, it turned out that when I turn write-cache =
off under linux, the performance drops to 100 on that OS also. But when =
enabled, 1400 IOPS (synchronous) can be achieved. So I would like to see =
the same on FreeBSD as well. Using camcontrol shows that the write cache =
is enabled, but I may assume that something around this is causing the =
performance degradation. But unfortunately I cannot step forward right =
> Regards,
> Kojedzinszky Richard
> On Wed, 27 Nov 2013, Tom Evans wrote:
>> On Wed, Nov 27, 2013 at 8:51 AM, Richard Kojedzinszky =
<> wrote:
>>> Dear fs developers,
>>> Probably this is not the best list to report my issue, but please =
forward it
>>> to where it should get.
>>> I bought an SSD for my ZFS filesystem to use it as a ZIL. I've =
tested it
>>> under linux, and found that it can handle around 1400 random =
>>> write IOPS. Then I placed it into my freebsd 9.2 box, and after =
attaching it
>>> as a ZIL, my zpool only performs 100 (!) write iops. I've attached =
it to an
>>> AHCI controller and to an LSI 1068 controller, on both it behaves =
the same.
>>> So I expect that something in the scsi layer is different, FreeBSD =
>>> handling this device slower, but actually it can handle the 1400 =
iops as
>>> tested under linux.
>>> Please give some advice where to go, how to debug, and how to =
>>> FreeBSD's performance with this drive.
>> The ZIL is only used for synchronous writes. The majority of writes
>> are asynchronous, and the ZIL is not used at all. Plus, a ZIL can =
>> increase iops by bundling writes - if your underlying pool is write
>> saturated already, then a ZIL can't help - any data written to the =
>> has to end up on the pool.
>> Test the SSD by itself under FreeBSD to rule out FreeBSD not working
>> correctly on the SSD (I doubt this though).
>> Cheers
>> Tom
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