> From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss- > boun...@opensolaris.org] On Behalf Of Edward Ned Harvey > > > From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss- > > boun...@opensolaris.org] On Behalf Of Dave Vrona > > > > > 2) ZIL write cache. It appears some have disabled the write cache on > > the X-25E. This results in a 5 fold performance hit but it > eliminates > > a potential mechanism for data loss. Is this valid? If I can mirror > > ZIL, I imagine this is no longer a concern?
Ahh, I see there may have been some confusion there, because your question wasn't asked right. ;-) "Disabling ZIL" is not the same thing as "disabling write cache." Those two terms are not to be mixed. The write cache is either the volatile memory on the disk, or the presumably nonvolatile memory in the HBA. You should never enable volatile disk write cache. You should only enable the HBA writeback cache if (a) the HBA has nonvolatile memory, such as battery backed up, and (b) you don't have a dedicated ZIL log device. The ZIL, generally speaking, should not be disabled. There are some cases where it's ok, but generally speaking don't do it. The justification is thus: Disabling the ZIL makes would-be sync writes into async writes, which are faster, but prone to disappearance caused by ungraceful system shutdown. If you trust your applications to only issue sync writes when they have a reason they need to, and to do async writes whenever that's ok, then you should not disable your ZIL. The only time to disable your ZIL is when you believe your applications are performing sync writes unnecessarily hurting their own performance, and you're not worried about losing the latest ~30 seconds of supposedly already written data after an ungraceful shutdown. PS, if you are in the latter case, if you do disable your ZIL, then there's no point in either HBA writeback, or ZIL log device. The ZIL log device is only used for sync writes, and it will be 100% unused if you disable ZIL. Also, HBA writeback does not benefit ZFS for async writes. _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
