Date: Thu, 14 Jul 2005 22:25:09 +0200 From: Matthias Buelow <mkb@incubus.de> To: David Sze <dsze@distrust.net> Cc: freebsd-stable@freebsd.org, freebsd-questions@freebsd.org Subject: Re: dangerous situation with shutdown process Message-ID: <20050714202509.GB23666@drjekyll.mkbuelow.net> In-Reply-To: <20050714201236.GA15856@mail.distrust.net> References: <42D6B117.5080302@plab.ku.dk> <20050714191449.A8A615D07@ptavv.es.net> <20050714195253.GA23666@drjekyll.mkbuelow.net> <20050714201236.GA15856@mail.distrust.net>
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David Sze wrote: >> Until a journalled fs that uses write request barriers is available >> for FreeBSD, you better had a reliable UPS. > >How do OS-level request barriers help if the disk reorders pending >writes in its cache? By separating journal updates from the corresponding metadata (and/or data) actions, and by guaranteeing (by flushing the cache, or a singular disabling/enabling of the wb cache at the barrier) that the journal is updated on disk before the actions take place. This imposes an ordering on the journal vs. action requests, which is what a journalled fs needs for filesystem integrity. It doesn't really matter if the disk reorders writes within those two blocks, the only thing that really matters is that the journal update is completed before metadata (or data) updates take place. With softupdates, as far as I understand, that doesn't work, because there is no journal. All requests must be in the order that softupdates decrees. You'd have to issue a barrier request after every write request, which would be equivalent to disabling the wb cache. mkb.
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