Date: Wed, 21 Jul 2004 14:51:37 -0700 From: Julian Elischer <julian@elischer.org> To: Alfred Perlstein <alfred@FreeBSD.org> Cc: Nate Lawson <nate@root.org> Subject: Re: cvs commit: src/sys/kern kern_shutdown.c Message-ID: <40FEE569.2010209@elischer.org> In-Reply-To: <200407212045.i6LKjHvX090599@palm.tree.com> References: <200407212045.i6LKjHvX090599@palm.tree.com>
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Stephan Uphoff wrote: >Alfred Perlstein wrote: > > >>* Scott Long <scottl@freebsd.org> [040721 09:57] wrote: >> >> >>>It should be noted that syncing on panic is almost never a good idea. >>>The whole idea of panic() is to signal that the system has gotten into >>>an inconsistent and unrecoverable state. Do you really want to trust it >>>to spam your drive with buffers that are in an unknown state via a set >>>of codepaths that are in an unknown state? It's much better to just >>>step back and let fsck try to repair the damage. I can't remember a >>>single time in the last 4 years when a panic actually successfuly synced >>>out all of the buffers and shutdown the filesystem, so it's not likely >>>that you'll avoid a fsck on reboot with this. >>> >>> >>It's not about avoiding a fsck, it's about recovering the last 30+ seconds >>of disk activity. Ie, files you've just created and such. >> >> > >Locking is disabled during a sync on panic. >( all lockmgr requests succeed) > >Even if the internal file system state is not corrupted >in a panic, multiple threads might be active in the filessystem >using locks to carefully update buffers or enforce the buffer >write order. > >A sync requests trampling through the file systems with >total disregard for any locks can do interesting things >to a filesystem on disk. > >I think adding a "dangerous" warning to the sysctl description >might be useful. >Otherwise it is hard to guess that by trying to save 30 seconds of >data one risks loosing the whole file system. > If you have no sync then you are more likely to have a successful core-dump.. so write a utility that extracts the missing data from the corefile ! :-) > > > Stephan > > >
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