On Wed, May 05, 2010 at 11:32:23PM -0400, Edward Ned Harvey wrote: > > From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss- > > boun...@opensolaris.org] On Behalf Of Robert Milkowski > > > > if you can disable ZIL and compare the performance to when it is off it > > will give you an estimate of what's the absolute maximum performance > > increase (if any) by having a dedicated ZIL device. > > I'll second this suggestion. It'll cost you nothing to disable the ZIL > temporarily. (You have to dismount the filesystem twice. Once to disable > the ZIL, and once to re-enable it.) Then you can see if performance is > good. If performance is good, then you'll know you need to accelerate your > ZIL. (Because disabled ZIL is the fastest thing you could possibly ever > do.) > > Generally speaking, you should not disable your ZIL for the long run. But > in some cases, it makes sense. > > Here's how you determine if you want to disable your ZIL permanently: > > First, understand that with the ZIL disabled, all sync writes are treated as > async writes. This is buffered in ram before being written to disk, so the > kernel can optimize and aggregate the write operations into one big chunk. > > No matter what, if you have an ungraceful system shutdown, you will lose all > the async writes that were waiting in ram. > > If you have ZIL disabled, you will also lose the sync writes that were > waiting in ram (because those are being handled as async.) > > In neither case do you have data or filesystem corruption. >
ZFS probably is still OK, since it's designed to handle this (?), but the data can't be OK if you lose 30 secs of writes.. 30 secs of writes that have been ack'd being done to the servers/applications.. > The risk of running with no ZIL is: In the case of ungraceful shutdown, in > addition to the (up to 30 sec) async writes that will be lost, you will also > lose up to 30 sec of sync writes. > -- Pasi _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
