Date: Fri, 29 Sep 2000 18:10:07 +0000 From: Tony Finch <dot@dotat.at> To: Terry Lambert <tlambert@primenet.com> Cc: John Sconiers <jrs@enteract.com>, Brad Knowles <blk@skynet.be>, Johann Visagie <wjv@cityip.co.za>, chat@FreeBSD.ORG Subject: Re: SGI releases XFS under GPL Message-ID: <20000929181007.A3345@hand.dotat.at> In-Reply-To: <200009290115.SAA06192@usr05.primenet.com> References: <Pine.NEB.3.96.1000928091033.6947A-100000@shell-1.enteract.com> <200009290115.SAA06192@usr05.primenet.com>
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Terry Lambert <tlambert@primenet.com> wrote: > >A common misconception about soft updates is that you can get >the same failure recovery that you would get from journalling >or logging. [...] > >The flaw with this theory is that a power failure is not the >only type of crash you could have, and running after any >crash that can corrupt any portion of the disk (e.g. most >disks corrupt sectors if power is lost during a write, and >in the evnt of a kernel panic, you don't know what data was >corrupted in core, then erroneously written to disk before >the actual panic, etc.), puts you at risk of further disk >corruption and user space software failures. In the worst >case, the crash was the result of a hardware failure of the >disk subsystem (disk, controller, cables, terminator, etc.). >So it is impossible to recover without an exterior log of >the events leading up to the crash (this is how the WAFL >file system from Network Appliance works: it uses an NVRAM >intention log). I'm not sure this argument is entirely sound: it also applies to journalling filesystems where the log is kept on disk. Even if the journal isn't on disk (the WAFL case) you can still be vulnerable to hardware failures -- I've heard of a NetApp becoming completely unusable after a disk decided to fail silently, the only way to recover being to copy the parts of the filesystem that didn't crash the NetApp to another machine. Tony. -- en oeccget g mtcaa f.a.n.finch v spdlkishrhtewe y dot@dotat.at eatp o v eiti i d. fanf@covalent.net To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-chat" in the body of the message
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