[dm-devel] [PATCH v2 1/2] libmultipath: don't reject maps with undefined prio

[Martin Wilck]

On Do, 2018-03-22 at 18:52 -0500, Benjamin Marzinski wrote:
> On Wed, Mar 21, 2018 at 10:34:18AM +0100, Martin Wilck wrote:
> > libmultipath's prio routines can deal with pp->priority ==
> > PRIO_UNDEF
> > just fine. PRIO_UNDEF is just a very low priority. So there's
> > no reason to reject setting up a multipath map because paths have
> > undefined priority.
> > 
> 
> The problem with this is that presumably, if there is a prioritizer
> set,
> then the paths aren't supposed to be in the same pathgroup.  If this
> is
> the case and you end up setting all the paths to the same pathgroup
> because the prioritizer is wrong, you will likely have a bad
> experience.

Sure. This is obviously an unfortunate corner-case. The good news is
that it happens very rarely. The default setting for prio is "const",
which can't fail, so (disregarding FAILED paths) this can happen only
for hardware that claims to support ALUA (or some other algorithm) but
actually doesn't (or fails ALUA repeatedly). There's one concrete
example of such hardware (certain variants of IBM IPR) that has
motivated this patch.

The rationale for my patch is that multipath-tools currently behave
inconsistently. Prio calls are allowed to fail. If prio detection works
initially, but fails later for whatever reason (e.g. ALUA calls
failing), multipathd handles that "gracefully" by assigning such paths
PRIO_UNDEF == -1. This "works fine" (your concerns apply) even if the
prio call fails for every path of the map.

Likewise, even maps for which get_prio() always fails for every path
can be set up manually, using "multipathd add map $WWID". The only case
where multipath/multipathd refuse to set up this kind of map is
coalesce_paths(), i.e. autodetection. We have an additional,
undocumented "blacklist" criterion here - maps that fail get_prio, even
if just for a single path, won't be set up automatically. That doesn't
make a lot of sense to me.

> Either you will frequently be writing to a non-optimized path (which
> can
> auto-trespass and become an optimized one in some arrays, causing
> horrible ping-ponging), or some of your paths will just always keep
> getting restored and then failed, since they require a manual
> trespass
> to be actually usable, but ghost paths are treated as usable by the
> kernel.
> 
> If you just set the priortizer for this device to const (or whatever
> make sense if the device isn't alua) and have detect_prio enabled,
> won't
> it correctly detect an alua prioritizer when it is supported, and
> failback to const when not?

My first idea was similar, but no, AFAICS that doesn't happen. Once you
change pp->prio, it remains "const" until the next reconfigure(), which
may mean "never". Therefore my patch (v2) doesn't touch pp->prio. If
the failure is temporary, pp->priority will eventually be re-calculated 
correctly (note that get_prio() is always called by pathinfo() if 
pp->priority == PRIO_UNDEF). If the failure is permanent, pp->priority
stays at "const -1", and that's ok.

It would be possible to create a complex solution with multiple
prioritizers and fallback algorithms, but IMHO that would be
overengineered for a rare and rather pathological case like this.

Regards,
Martin

> 
> -Ben
> 
> > Signed-off-by: Martin Wilck <mwilck at suse.com>
> > ---
> >  libmultipath/configure.c | 5 -----
> >  1 file changed, 5 deletions(-)
> > 
> > diff --git a/libmultipath/configure.c b/libmultipath/configure.c
> > index fa6e21cb31af..bb3d8771592d 100644
> > --- a/libmultipath/configure.c
> > +++ b/libmultipath/configure.c
> > @@ -993,9 +993,6 @@ int coalesce_paths (struct vectors * vecs,
> > vector newmp, char * refwwid,
> >  			continue;
> >  		}
> >  
> > -		if (pp1->priority == PRIO_UNDEF)
> > -			mpp->action = ACT_REJECT;
> > -
> >  		if (!mpp->paths) {
> >  			condlog(0, "%s: skip coalesce (no paths)",
> > mpp->alias);
> >  			remove_map(mpp, vecs, 0);
> > @@ -1021,8 +1018,6 @@ int coalesce_paths (struct vectors * vecs,
> > vector newmp, char * refwwid,
> >  					mpp->size);
> >  				mpp->action = ACT_REJECT;
> >  			}
> > -			if (pp2->priority == PRIO_UNDEF)
> > -				mpp->action = ACT_REJECT;
> >  		}
> >  		verify_paths(mpp, vecs);
> >  
> > -- 
> > 2.16.1
> 
> 

-- 
Dr. Martin Wilck <mwilck at suse.com>, Tel. +49 (0)911 74053 2107
SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton
HRB 21284 (AG Nürnberg)