Date: Wed, 3 May 2006 15:04:22 +0100 (BST) From: Robert Watson <rwatson@FreeBSD.org> To: Pavel Merdin <freebsd-fs@merdin.com> Cc: freebsd-fs@freebsd.org Subject: Re[2]: Stress testing the UFS2 filesystem Message-ID: <20060503150126.K21389@fledge.watson.org> In-Reply-To: <1408111762.20060503145710@merdin.com> References: <20060502193900.GA94069@peter.osted.lan> <1541458526.20060503003229@merdin.com> <20060502221306.GD95348@xor.obsecurity.org> <44584421.3000807@cs.tu-berlin.de> <20060503072013.GA2926@xor.obsecurity.org> <18034.193.3.141.124.1146642890.squirrel@webmail7.pair.com> <1408111762.20060503145710@merdin.com>
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On Wed, 3 May 2006, Pavel Merdin wrote: >> Actually the filesystems mounts without any problems if fsck is run first > > That's not a bug in this case. It's a feature. And there is nothing new in > this. Background fsck helps saving start time, but it's risky as kernel can > panic if system accesses problematic sector before fsck. So background fsck > should be turned off if one needs reliability (e.g. on servers). Well, the feature and problem are that bgfsck relies on invariants holding true for data written to the disk, so corrects only a narrow set of expected failure modes. I.e., that soft updates really does sequence changes out to the disk such that certain invariants regarding meta-data hold. This allows access to the file system before the checks are complete, but places assumptions on what will be found on disk. Those invariants might fail to hold for a few reasons -- bugs in UFS, hardware failure, and design breakage in the hardware are among the most common. One known problem is that the guarantees provided by recent ATA disks are really very weak with respect to the expectations of currently file systems. Robert N M Watson
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