Date: Tue, 31 Aug 2010 19:07:28 -0400 From: Martin Cracauer <cracauer@cons.org> To: Benjamin Kaduk <kaduk@mit.edu> Cc: freebsd-fs@freebsd.org, Martin Cracauer <cracauer@cons.org> Subject: Re: fsync(2) and on-disk write-back cache Message-ID: <20100831230728.GA36384@cons.org> In-Reply-To: <alpine.GSO.1.10.1008311210091.9337@multics.mit.edu> References: <20100830225841.GA9363@cons.org> <20100831160840.GA74125@icarus.home.lan> <alpine.GSO.1.10.1008311210091.9337@multics.mit.edu>
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Benjamin Kaduk wrote on Tue, Aug 31, 2010 at 12:12:04PM -0400: > 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. I just went the for-dummies way and annotated all relevant BIO_FLUSH places with debug print statements. They don't seem to be called when doing an fsync on a file in a local filesystem. ufs (no softupdates) -> old-style SCSI disk. I'll snoop around some more, try it on ZFS/SATA and do some timing tests. > 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. I know, that's why the whole question is a little academic. Martin -- %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Martin Cracauer <cracauer@cons.org> http://www.cons.org/cracauer/
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