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

[Martin Cracauer]

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
-- 
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Martin Cracauer <cracauer at cons.org>   http://www.cons.org/cracauer/