You want the write cache enabled, for sure, with ZFS. ZFS will do the
right thing about ensuring write cache is flushed when needed.
For the case of a single JBOD, I don't find it surprising that UFS beats
ZFS. ZFS is designed for more complex configurations, and provides much
better data integrity guarantees than UFS (so critical data is written
to the disk more than once, and in areas of the drive that are not
adjacent to improve the chances of recovery in the event of a localized
media failure). That said, you could easily accelerate the write
performance of ZFS on that single JBOD by adding a small SSD log device.
(4GB would be enough. :-)
- Garrett
On Thu, 2010-07-08 at 15:10 +0200, Philippe Schwarz wrote:
> Hi,
>
> With dual-Xeon, 4GB of Ram (will be 8GB in a couple of weeks), two PCI-X
> 3Ware cards 7 Sata disks (750G & 1T) over FreeBSD 8.0 (But i think it's
> OS independant), i made some tests.
>
> The disks are exported as JBOD, but i tried enabling/disabling write-cache .
>
> I tried with UFS and ZFS on the same disk and the difference is
> overwhelming.
>
> With a 1GB file (greater than the ZFS cache ?):
>
> With Writecache disabled
> UFS
> time cp /mnt/ufs/rnd /mnt/ufs/rnd2
> real 2m58.073s
> ZFS
> time cp /zfs/rnd /zfs/rnd2
> real 4m33.726s
>
> On the same card with WCache enabled
> UFS
> time cp /mnt/ufs/rnd /mnt/ufs/rnd2
> real 0m31.406s
> ZFS
> time cp /zfs/rnd /zfs/rnd2
> real 1m0.199s
>
> So, despite the fact that ZFS can be twice slower than UFS, it is clear
> that Write-Cache have to be enabled on the controller.
>
> Any drawback (except that without BBU, i've got a pb in case of power
> loss) in enabling the WC with ZFS ?
>
>
> Thanks.
> Best regards.
>
>
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