Re: [dm-devel] [PATCH, RFC 2/2] dm: support REQ_FLUSH directly

[Mike Snitzer <snitzer redhat com>]

On Mon, Aug 30 2010 at  4:33am -0400,
Tejun Heo <tj kernel org> wrote:

> On 08/30/2010 06:45 AM, Jun'ichi Nomura wrote:
> > Hi Mike,
> > 
> > (08/27/10 23:13), Mike Snitzer wrote:
> >>> If there will be no need for supporting a request-based target
> >>> with num_flush_requests > 1, the special handling of flush
> >>> can be removed.
> >>>
> >>> And since there is no such target in the current tree,
> >>> I don't object if you remove that part of code for good reason.
> >>
> >> OK, certainly something to keep in mind.  But _really_ knowing the
> >> multipath FLUSH+FUA performance difference (extra special-case code vs
> >> none) requires a full FLUSH conversion of request-based DM anyway.
> >>
> >> In general, request-based DM's barrier/flush code does carry a certain
> >> maintenance overhead.  It is quite a bit of distracting code in the core
> >> DM which isn't buying us anything.. so we _could_ just remove it and
> >> never look back (until we have some specific need for num_flush_requests
> >>> 1 in rq-based DM).
> > 
> > So, I'm not objecting to your idea.
> > Could you please create a patch to remove that?
> 
> I did that yesterday.  Will post the patch soon.

I did it yesterday also, mine builds on your previous DM patchset...

I'll review your recent patchset, from today, to compare and will share
my findings.

I was hoping we could get the current request-based code working with
your new FLUSH+FUA work without removing support for num_flush_requests
(yet).  And then layer in the removal to give us the before and after so
we would know the overhead associated with keeping/dropping
num_flush_requests.  But like I said earlier "we _could_ just remove it
and never look back".

Thanks,
Mike