fsync(2) and on-disk write-back cache

[Kostik Belousov]

On Tue, Aug 31, 2010 at 07:07:28PM -0400, Martin Cracauer wrote:
> 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.
Be assured that all variants of UFS (sync, async, SU, SU+J) do not
call BIO_FLUSH.
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