[dm-devel] dm-mpath request merging concerns [was: Re: It's time to put together the schedule]

[Benjamin Marzinski]

On Mon, Feb 23, 2015 at 02:56:03PM -0500, Mike Snitzer wrote:
> On Mon, Feb 23 2015 at  1:34pm -0500,
> Benjamin Marzinski <bmarzins at redhat.com> wrote:
> 
> > On Mon, Feb 23, 2015 at 11:18:36AM -0600, Mike Christie wrote:
> > > 
> > > If the device/transport is fast or the workload is low, the multipath_busy
> > > never returns busy, then we can hit Hannes's issue. For 4 paths, we just
> > > might not be able to fill up the paths and hit the busy check. With only 2
> > > paths, we might be slow enough or the workload is heavy enough to keep the
> > > paths busy and so we hit the busy check and do more merging.
> > 
> > Netapp is seeing this same issue.  It seems like we might want to make
> > multipath_busy more aggressive about returning busy, which would
> > probably require multipath tracking the size and frequency of the
> > requests.  If it determines that it's getting a lot of requests that
> > could have been merged, it could start throttling how fast requests are
> > getting pulled off the queue, even there underlying paths aren't busy.
> 
> the ->busy() checks are just an extra check the shouldn't be the primary
> method for governing the effectiveness of the DM-mpath queue's elevator.
> 
> I need to get back to basics to appreciate how the existing block layer
> is able to have an effective elevator regardless of the device's speed.
> And why isn't request-based DM able to just take advantage of it?

I always thought that at least one of the schedulers always kept
incoming requests on an interal queue for at least a little bit to see
if any merging could happen, even if they could otherwise just be added
to the request queue. but I admit to being a little vague on how exactly
they all work.

Another place where we could break out of constantly pulling requests of
the queue before they're merged is in dm_prep_fn().  If we thought that
we should break and let merging happen, we could return BLKPREP_DEFER.

-Ben

> 
> (my money is on request-based DM being overly clever but we'll see)