Date: Tue, 31 Aug 2010 11:38:51 +0200 From: Gary Jennejohn <gljennjohn@googlemail.com> To: Martin Cracauer <cracauer@cons.org> Cc: freebsd-fs@freebsd.org Subject: Re: fsync(2) and on-disk write-back cache Message-ID: <20100831113851.6d449628@ernst.jennejohn.org> In-Reply-To: <20100830225841.GA9363@cons.org> References: <20100830225841.GA9363@cons.org>
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On Mon, 30 Aug 2010 18:58:42 -0400 Martin Cracauer <cracauer@cons.org> 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 may be wrong, but it seems to me that the actual cache flush on the disk is handled at a very low level - in the drivers. At least I'm pretty certain that e.g. the ahci code sends flush commands to the disks. -- Gary Jennejohn
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