Date: Sun, 21 Jun 2009 13:03:17 +0300 From: Dan Naumov <dan.naumov@gmail.com> To: Erik Trulsson <ertr1013@student.uu.se> Cc: freebsd-fs@freebsd.org, Kip Macy <kip.macy@gmail.com> Subject: Re: ufs2 / softupdates / ZFS / disk write cache Message-ID: <cf9b1ee00906210303l5b54bfaau28e253ce4e674592@mail.gmail.com> In-Reply-To: <20090621092736.GA92656@owl.midgard.homeip.net> References: <cf9b1ee00906201429y7ec68afdse66be30fc2f75e8f@mail.gmail.com> <20090620231130.GA88907@owl.midgard.homeip.net> <3c1674c90906201808t1854dd46n82213fbd0c1c254c@mail.gmail.com> <cf9b1ee00906201918w1bc7063bw641cfc768ee33398@mail.gmail.com> <20090621092736.GA92656@owl.midgard.homeip.net>
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On Sun, Jun 21, 2009 at 12:27 PM, Erik Trulsson<ertr1013@student.uu.se> wro= te: > On Sun, Jun 21, 2009 at 05:18:39AM +0300, Dan Naumov wrote: >> Uh oh.... After some digging around, I found the following quote: "ZFS >> is designed to work with storage devices that manage a disk-level >> cache. ZFS commonly asks the storage device to ensure that data is >> safely placed on stable storage by requesting a cache flush." at >> http://www.solarisinternals.com/wiki/index.php/ZFS_Evil_Tuning_Guide I >> guess this might be somewhat related to why in the "disk cache >> disabled" scenario, ZFS suffers bigger losses than UFS2. > > If that quote is correct (and I have no real reason to doubt it) then > it should probably be safe to enable the disk's write cache when used wit= h > ZFS. =A0(That would make sense since UFS/FFS was originally designed to w= ork > with an older generation of disks that did not do any significant amount > of write-caching (partly due to having very little cache on them), while > ZFS has been designed to be used on modern hardware, and to be reliable e= ven > on cheap consumer-grade disks.) Actually, now that I think of it, this could be pretty big. If using ZFS on a disk will cause the disk to flush the cache every 5 seconds, wouldn't that mean that the sections of the cache that hold data from the UFS partition get flushed to disk as well, mostly eleminating the entire "disk cache lying =3D softupdates inconsistent" problem altogether? The most important part of this is obviously, whether the "ZFS forces cache flushes every 5 seconds) thing works in all cases (like mine, where I use ZFS on a slice) and not only those where ZFS is given direct access to the disk. Anyone knowledgable in the ways of FreeBSD ZFS implementation care to chip in? :) Sincerely, Dan Naumov
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