On 6 maj 2010, at 08.17, Pasi Kärkkäinen wrote: > 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..
Entirely right! This is the case for many local user writes anyway, since many applications doesn't sync the written data to disk. But if you have an application, protocol and/or user that demands or expects persistant storage, disabling ZIL of course could be fatal in case of a crash. Examples are mail servers and NFS servers. /ragge _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
