Date: Mon, 22 Jun 2009 01:17:44 +0000 From: Nathanael Hoyle <nhoyle@hoyletech.com> To: Andrew Snow <andrew@modulus.org>, Dan Naumov <dan.naumov@gmail.com>, freebsd-fs@freebsd.org Subject: Re: ufs2 / softupdates / ZFS / disk write cache Message-ID: <4A3EDBB8.6010402@hoyletech.com> In-Reply-To: <4A3EB902.8080503@modulus.org> References: <570433.20373.qm@web37308.mail.mud.yahoo.com> <4A3E9D81.1060406@modulus.org> <cf9b1ee00906211534i76fc8fb4r29b3469af8a2fd7c@mail.gmail.com> <cf9b1ee00906211536i37973627ub86948aea63b4156@mail.gmail.com> <4A3EB902.8080503@modulus.org>
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Andrew Snow wrote: > Dan Naumov wrote: >>>> Or: >>>> B) use SCSI instead of ATA disks >>>> C) use UFS+gjournal instead of UFS+SU >>>> D) use ZFS instead of UFS+SU >>> All of these solutions still involve disabling of write cache, with >>> a performance hit of varying degrees. (I have tried all of those >>> except gjournal!) > > B) SCSI drives come with write caching disabled by default. But here, > the performance loss is partially made up by Tagged Command Queueing > and faster spindle speeds > > C) gjournal needs to flush the disk cache regularly to maintain > consistence. It doesn't need to do it as often but on a write-heavy > system it isn't ideal for performance because it flushes everything in > the cache and not just the journal. > > D) ZFS - same as (C) > As a minor nitpick to point D, IIRC it is possible to explicitly place the ZIL on a different device than the pool it is for. In this case, if the ZIL is on a dedicated device, then it is possible to flush only the ZIL, rather than all data pending in cache for the zpool. I realize it's a minor distinction / special case, but the option is worth mentioning. -Nathanael
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