Re: [RFC] ext4: Don't send extra barrier during fsync if there are no dirty pages.

[Jan Kara <jack@suse.cz>]

On Mon 02-08-10 17:09:39, Darrick J. Wong wrote:
> On Wed, Jul 21, 2010 at 07:16:09PM +0200, Jan Kara wrote:
> >   Hi,
> > 
> > > On Wed, Jun 30, 2010 at 09:21:04AM -0400, Ric Wheeler wrote:
> > > > 
> > > > The problem with not issuing a cache flush when you have dirty meta
> > > > data or data is that it does not have any tie to the state of the
> > > > volatile write cache of the target storage device.
> > > 
> > > We track whether or not there is any metadata updates associated with
> > > the inode already; if it does, we force a journal commit, and this
> > > implies a barrier operation.
> > > 
> > > The case we're talking about here is one where either (a) there is no
> > > journal, or (b) there have been no metadata updates (I'm simplifying a
> > > little here; in fact we track whether there have been fdatasync()- vs
> > > fsync()- worthy metadata updates), and so there hasn't been a journal
> > > commit to do the cache flush.
> > > 
> > > In this case, we want to track when is the last time an fsync() has
> > > been issued, versus when was the last time data blocks for a
> > > particular inode have been pushed out to disk.
> > > 
> > > To use an example I used as motivation for why we might want an
> > > fsync2(int fd[], int flags[], int num) syscall, consider the situation
> > > of:
> > > 
> > > 	fsync(control_fd);
> > > 	fdatasync(data_fd);
> > > 
> > > The first fsync() will have executed a cache flush operation.  So when
> > > we do the fdatasync() (assuming that no metadata needs to be flushed
> > > out to disk), there is no need for the cache flush operation.
> > > 
> > > If we had an enhanced fsync command, we would also be able to
> > > eliminate a second journal commit in the case where data_fd also had
> > > some metadata that needed to be flushed out to disk.
> >   Current implementation already avoids journal commit because of
> > fdatasync(data_fd). We remeber a transaction ID when inode metadata has
> > last been updated and do not force a transaction commit if it is already
> > committed. Thus the first fsync might force a transaction commit but second
> > fdatasync likely won't.
> >   We could actually improve the scheme to work for data as well. I wrote
> > a proof-of-concept patches (attached) and they nicely avoid second barrier
> > when doing:
> > echo "aaa" >file1; echo "aaa" >file2; fsync file2; fsync file1
> > 
> >   Ted, would you be interested in something like this?
> 
> Well... on my fsync-happy workloads, this seems to cut the barrier count down
> by about 20%, and speeds it up by about 20%.
  Nice, thanks for measurement.

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR