[dm-devel] [PATCH UPDATED 4/5] dm: implement REQ_FLUSH/FUA support for request-based dm

[Mike Snitzer]

On Wed, Sep 01 2010 at  1:07pm -0400,
Mike Snitzer <snitzer at redhat.com> wrote:

> On Wed, Sep 01 2010 at 11:35am -0400,
> Mikulas Patocka <mpatocka at redhat.com> wrote:
> 
> > 
> > On Wed, 1 Sep 2010, Mike Snitzer wrote:
> > > 
> > > Mikulas,
> > > 
> > > Current bio-based barrier support also defers IO if a flush is in
> > > progress.  See _dm_request:
> > 
> > I know. But it doesn't hurt with flush/fua requests. It just lowers 
> > performance (it defers i/os when it doesn't have to) but doesn't damage 
> > data.
> > 
> > So I think that we can let it be this way until flush/fua patch is 
> > finalized.
> 
> Neither Tejun nor I see the point in waiting when we have a window of
> time to address the issues now.  We want DM to realize the benefit
> associated with the kernel-wide FLUSH+FUA conversion too.

But we can meet in the middle.  I've reordered the DM FLUSH+FUA patches
so that the more intrusive bio-based relaxed ordering patch is at the
very end.

My hope was that the request-based deadlock I'm seeing would disappear
if that relaxed ordering patch wasn't applied.  Unfortunately, I still
see the hang.

Anyway, I've made the patches available here:
http://people.redhat.com/msnitzer/patches/flush-fua/