Date: Tue, 31 Aug 2010 09:08:40 -0700 From: Jeremy Chadwick <freebsd@jdc.parodius.com> To: Martin Cracauer <cracauer@cons.org> Cc: freebsd-fs@freebsd.org Subject: Re: fsync(2) and on-disk write-back cache Message-ID: <20100831160840.GA74125@icarus.home.lan> In-Reply-To: <20100830225841.GA9363@cons.org> References: <20100830225841.GA9363@cons.org>
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On Mon, Aug 30, 2010 at 06:58:42PM -0400, Martin Cracauer wrote: > I always assumed the answer to this question is "of course": > > When doing an fsync (waiting for the commit), do we actually tell the > disk to flush the on-disk write-back cache (if that is in use) to the > platters? > > I just went down some code paths in both FreeBSD and Linux and in both > cases the paths for fsync quickly disappear in the generic > block-by-block flushing code that is also used for regular (non-fsync) > flushing. I didn't see anything aware of the on-disk cache. I don't have an authoritative answer to your question, but this thread seems to imply there's a relation between fsync() and an intentional disk flush (BIO_FLUSH). I'm sure when BIO_FLUSH is called depends on the filesystem as well. Funny timing this topic, given a post over in -stable: http://lists.freebsd.org/pipermail/freebsd-stable/2010-August/058525.html -- | Jeremy Chadwick jdc@parodius.com | | Parodius Networking http://www.parodius.com/ | | UNIX Systems Administrator Mountain View, CA, USA | | Making life hard for others since 1977. PGP: 4BD6C0CB |
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