ssd for zfs

[Tom Evans]

On Wed, Nov 27, 2013 at 8:51 AM, Richard Kojedzinszky <krichy at cflinux.hu> 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 synchronized
> 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 is
> 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 improve
> 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 only
increase iops by bundling writes - if your underlying pool is write
saturated already, then a ZIL can't help - any data written to the ZIL
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