Re: [RFC] relaxed barrier semantics

[Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>]

On Wed, 28 Jul 2010 11:28:59 +0200, Christoph Hellwig <hch@lst.de> wrote:
> On Wed, Jul 28, 2010 at 11:17:06AM +0200, Tejun Heo wrote:
> > 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.

With regard to nilfs, barrier is applied to writeback of super block
since it saves position of a recent log and this log needs to be
written to the platter prior to the super block.

And so, I think a pre-flush + a FUA write can be used instead of
draining for the barrier use in nilfs.

Thanks,
Ryusuke Konishi