Date: Sun, 21 Jun 2009 11:27:36 +0200 From: Erik Trulsson <ertr1013@student.uu.se> To: Dan Naumov <dan.naumov@gmail.com> Cc: freebsd-fs@freebsd.org, Kip Macy <kip.macy@gmail.com> Subject: Re: ufs2 / softupdates / ZFS / disk write cache Message-ID: <20090621092736.GA92656@owl.midgard.homeip.net> In-Reply-To: <cf9b1ee00906201918w1bc7063bw641cfc768ee33398@mail.gmail.com> References: <cf9b1ee00906201429y7ec68afdse66be30fc2f75e8f@mail.gmail.com> <20090620231130.GA88907@owl.midgard.homeip.net> <3c1674c90906201808t1854dd46n82213fbd0c1c254c@mail.gmail.com> <cf9b1ee00906201918w1bc7063bw641cfc768ee33398@mail.gmail.com>
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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 with ZFS. (That would make sense since UFS/FFS was originally designed to work 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 even on cheap consumer-grade disks.) > > It is quite obvious at this point that disabling disk cache in order > have softupdates live in harmony with disks "lying" about whether disk > cache contents have actually been committed to the disk in not in any > way, shape or form a viable solution to the problem. On a sidenote, is > there any way I can test whether *MY* disk is truthful about writing > cache to disk or not? If you have IDE/SATA disks they will "lie". SCSI/SAS disks won't. SATA disks using NCQ should probably also be safe -- too bad FreeBSD does not support NCQ yet. > > In the past (this was during my previous foray into the FreeBSD world, > circa-2001/2002) I have suffered severe data corruption (leading to an > unbootable system) using UFS2 + softupdates on 2 different occasions > due to power losses and this past experience has me very worried about > the proper way to configure my system to avoid such incidents in the > future. > > > - Sincerely, > Dan Naumov > > -- <Insert your favourite quote here.> Erik Trulsson ertr1013@student.uu.se
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