On 8 jul 2010, at 17.23, Garrett D'Amore wrote: > You want the write cache enabled, for sure, with ZFS. ZFS will do the > right thing about ensuring write cache is flushed when needed.
That is not for sure at all, it all depends on what "the right thing" is, which depends on the application and/or what other measures there are for redundancy. Many raid controllers will respond to a flush [to persistant storage] by doing nothing, since the data already is in its write cache buffer which (hopefully) is battery backed or something. They will typically NOT flush the data to the drives and issue flush commands to the disks, and wait for that to finish, before responding. This means, that if your raid controller dies on you, some of your most recently written data will be gone. It may very well be vital metadata, and in the zfs world it may be data or metadata from several of the latest txgs, that is gone. Potentially, it could leave your file system corrupt, or arbitrary pieces of your data could be lost. Maybe that is acceptable in the application, or maybe this is compensated for with other means as multiple raid controllers with the data mirrored over all of them which could reduce the risks by a lot, but you have to evaluate each separate case by itself. /ragge > 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. >> >> > > > _______________________________________________ > zfs-discuss mailing list > zfs-discuss@opensolaris.org > http://mail.opensolaris.org/mailman/listinfo/zfs-discuss _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
