Re: [dm-devel] [PATCH 5/5] dm: implement REQ_FLUSH/FUA support

[Christoph Hellwig <hch lst de>]

On Tue, Aug 17, 2010 at 11:33:52AM +0200, Tejun Heo wrote:
> > That logic wasn't just about retries (at least not in the latest
> > kernel).  With commit 708e929513502fb0 the -EOPNOTSUPP checking also
> > serves to optimize the barrier+discard case (when discards aren't
> > supported).
> 
> With the patch applied, there's no second flush.  Those requests would
> now be REQ_FLUSH + REQ_DISCARD.  The first can't be avoided anyway and
> there won't be the second flush to begin with, so I don't think this
> worsens anything.

In fact the pre-flush is completely superflous for discards, but that's a
separate discussion and should not be changed as part of this patchset but
rather explicitly.