Re: [PATCH] block: fix DISCARD_BARRIER requests

[Matthew Wilcox <matthew@wil.cx>]

On Thu, Jun 17, 2010 at 10:10:18AM +0200, Jens Axboe wrote:
> On 2010-06-17 09:54, Christoph Hellwig wrote:
> > Filesystems assume that DISCARD_BARRIER are full barriers, so that they
> > don't have to track in-progress discard operation when submitting new I/O.
> > But currently we only treat them as elevator barriers, which don't
> > actually do the nessecary queue drains.
> > 
> > Also remove the unlikely around both the DISCARD and BARRIER requests -
> > the happen far too often for a static mispredict.
> 
> Thanks, applied. There was a recent problem report on btrfs using
> discard, could possibly explain it if Chris assumed it was a full
> barrier.

If it was on real hardware (ie a SATA disc), it can't be this problem,
since TRIM isn't NCQ so there was already an implicit queue drain ahead
and behind the TRIM.  I hear rumours of an NCQ TRIM coming 'RSN', but
haven't seen any details on it yet.

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."