ssd for zfs

[Richard Kojedzinszky]

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 
now.

Regards,

Kojedzinszky Richard

On Wed, 27 Nov 2013, Tom Evans wrote:

> 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
>