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

[Mike Snitzer <snitzer@redhat.com>]

On Wed, Aug 04 2010 at 10:16pm -0400,
Jun'ichi Nomura <j-nomura@ce.jp.nec.com> wrote:

> Hi Christoph,
> 
> (08/04/10 17:54), Christoph Hellwig wrote:
> > On Wed, Aug 04, 2010 at 01:57:37PM +0900, Kiyoshi Ueda wrote:
> >>> -		if (unlikely(dm_rq_is_flush_request(rq))) {
> >>> +		if (rq->cmd_flags & REQ_FLUSH) {
> >>>  			BUG_ON(md->flush_request);
> >>>  			md->flush_request = rq;
> >>>  			blk_start_request(rq);
> >>
> >> Current request-based device-mapper's flush code depends on
> >> the block-layer's barrier behavior which dispatches only one request
> >> at a time when flush is needed.
> >> In other words, current request-based device-mapper can't handle
> >> other requests while a flush request is in progress.
> >>
> >> I'll take a look how I can fix the request-based device-mapper to
> >> cope with it.  I think it'll take time for carefull investigation.
> > 
> > Given that request based device mapper doesn't even look at the
> > block numbers from what I can see just removing any special casing
> > for REQ_FLUSH should probably do it.
> 
> Special casing is necessary because device-mapper may have to
> send multiple copies of REQ_FLUSH request to multiple
> targets, while normal request is just sent to single target.

Yes, request-based DM is meant to have all the same capabilities as
bio-based DM.  So in theory it should support multiple targets but in
practice it doesn't.  DM's multipath target is the only consumer of
request-based DM and it only ever clones a single flush request
(num_flush_requests = 1).

So why not remove all of request-based DM's barrier infrastructure and
simply rely on the revised block layer to sequence the FLUSH+WRITE
request for request-based DM?

Given that we do not have a request-based DM target that requires
cloning multiple FLUSH requests its unused code that is delaying DM
support for the new FLUSH+FUA work (NOTE: bio-based DM obviously still
needs work in this area).

Once we have a need for using request-based DM for something other than
multipath we can take a fresh look at implementing rq-based FLUSH+FUA.

Mike

p.s. I know how hard NEC worked on request-based DM's barrier support;
so I'm not suggesting this lightly.  For me it just seems like we're
carrying complexity in DM that hasn't ever been required.