Re: [RFC] relaxed barrier semantics

[Jan Kara <jack@suse.cz>]

On Wed 28-07-10 11:28:59, Christoph Hellwig wrote:
> On Wed, Jul 28, 2010 at 11:17:06AM +0200, Tejun Heo wrote:
> > Well, if disabling barrier works around the problem for them (which is
> > basically what was suggeseted in the first message), that's not too
> > bad for short term, I think.
> 
> It's a pretty horrible workaround.  Requiring manual mount options to
> get performance out of a setup which could trivially work out of the
> box is a bad workaround.
> 
> > I'll re-read barrier code and see how hard it would be to implement a
> > proper solution.
> 
> If we move all filesystems to non-draining barriers with pre- and post-
> flushes that might actually be a relatively easy first step.  We don't
> have the complications to deal with multiple types of barriers to
> start with, and it'll fix the issue for devices without volatile write
> caches completely.
> 
> I just need some help from the filesystem folks to determine if they
> are safe with them.
> 
> I know for sure that ext3 and xfs are from looking through them.  And
  Yes, ext3 is safe.

> I know reiserfs is if we make sure it doesn't hit the code path that
> relies on it that is currently enabled by the barrier option.
  Yes, just always writing the commit buffer at the place where we
currently do it in !barrier case should be enough for reiserfs.

> I'll just need more feedback from ext4, gfs2, btrfs and nilfs folks.
  As I wrote in some other email, ext4/jbd2 is OK, unless you mount the
filesystem with async_commit mount option. With that option it does the
same thing as reiserfs in barrier case - i.e., it needs ordering.

> That already ends our small list of barrier supporting filesystems, and
> possibly ocfs2, too - although the barrier implementation there seems
> incomplete as it doesn't seem to flush caches in fsync.
  Well, ocfs2 uses jbd2 for journaling so it supports barriers out of the
box and does not need the ordering. ocfs2_sync_file is actually correct
(although maybe slightly inefficient) because it does
jbd2_journal_force_commit() which creates and immediately commits a
transaction and that implies a barrier.

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