Date: Mon, 4 Sep 2006 16:18:59 -0700 From: Can Sar <csar@stanford.edu> To: Chuck Swiger <cswiger@mac.com> Cc: FreeBSD-questions@FreeBsd.org Subject: Re: UFS2 fsck Question (semantics of -p) Message-ID: <25D1B58B-AA8F-4CE1-AAF7-768F7A6B35C9@stanford.edu> In-Reply-To: <44F4718F.1010502@mac.com> References: <34A15B01-CECC-478F-8EE8-3AEA839803C7@stanford.edu> <44F4718F.1010502@mac.com>
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We ran our experiment on top of a very simple RAM disk which does not have any caches or anything of that sort. The dmesg log is at http://keeda.stanford.edu/dmesg The resultant images are at: http://keeda.stanford.edu/ufs-umount-image http://keeda.stanford.edu/ufs-mount-sync-image If you run fsck -p on them, fsck will not be able to recover, while fsck without the -p option will be able to. Can On Aug 29, 2006, at 9:55 AM, Chuck Swiger wrote: > Can Sar wrote: > [ ... ] >> Would you consider it an error if the -p option does not fix >> inconsistencies caused by a simple power failure, without any >> hardware or software corruption? > > You're asking an interesting question, but the issue of data > integrity depends not only on the software which comprises the OS, > but also on the hardware being used. > > In particular, the system depends upon the hard drives to reliably > report when data being written actually has been; SCSI drives, > using tagged command queuing, especially in conjunction with a > battery-backup which ensures the drive stays up long enough to > flush it's write cache even if system power is removed, will tend > to fare pretty well. > > IDE drives, by contrast, have a bad habit of lying about whether > data has actually been written to the disk itself rather than > simply making it to the write cache on the drive. (Such drives > ignore the ATA "FLUSH CACHE" command, specificly.) > > In other words, showing that a filesystem can become inconsistent > in a fashion that "fsck -p" cannot correct is interesting and a > concern regardless of the circumstances, but showing it in cases > where you are using battery-backed drives and/or SCSI rather than > IDE is a lot more meaningful. If you are using IDE devices, your > testing will be more meaningful if you disable the IDE write-cache > entirely. Also, you should put your results somewhere, perhaps on > a webpage with links to the filesystem images and a complete dmesg > so that the OS version and hardware being used is well-documented. > > -- > -Chuck >
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