Re: [RFC] relaxed barrier semantics

[Christoph Hellwig <hch@lst.de>]

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
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.

I'll just need more feedback from ext4, gfs2, btrfs and nilfs folks.
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.