Date: Tue, 31 Aug 2010 12:12:04 -0400 (EDT) From: Benjamin Kaduk <kaduk@MIT.EDU> To: Martin Cracauer <cracauer@cons.org> Cc: freebsd-fs@freebsd.org Subject: Re: fsync(2) and on-disk write-back cache Message-ID: <alpine.GSO.1.10.1008311210091.9337@multics.mit.edu> In-Reply-To: <20100831160840.GA74125@icarus.home.lan> References: <20100830225841.GA9363@cons.org> <20100831160840.GA74125@icarus.home.lan>
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On Tue, 31 Aug 2010, Jeremy Chadwick wrote: > 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. It is probably also worth noting that disks have been known to lie about having actually flushed bits from their internal cache to the platter. -Ben Kaduk
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