Re: [PATCH] multipath-tools: improve processing efficiency for addition and deletion of multipath devices

["Benjamin Marzinski" <bmarzins@redhat.com>]

On Fri, Feb 17, 2017 at 10:48:44AM +0100, Martin Wilck wrote:
> On Fri, 2017-02-17 at 00:21 -0600, Benjamin Marzinski wrote:
> > On Thu, Feb 16, 2017 at 11:38:36PM -0600, Benjamin Marzinski wrote:
> > > > > +
> > > > > +bool
> > > > > +merge_need_stop(struct uevent *earlier, struct uevent *later)
> > > > > +{
> > > > > +	/*
> > > > > +	 * dm uevent do not try to merge with left uevents
> > > > > +	 */
> > > > > +	if (!strncmp(later->kernel, "dm-", 3))
> > > > > +		return true;
> > > > > +
> > > > > +	/*
> > > > > +	 * we can not make a jugement without wwid,
> > > > > +	 * so it is sensible to stop merging
> > > > > +	 */
> > > > > +	if (!earlier->wwid || !later->wwid)
> > > > > +		return true;
> > > > > +	/*
> > > > > +	 * uevents merging stoped
> > > > > +	 * when we meet an opposite action uevent from the
> > > > > same LUN
> > > > > to AVOID
> > > > > +	 * "add path1 |remove path1 |add path2 |remove path2
> > > > > |add
> > > > > path3"
> > > > > +	 * to merge as "remove path1, path2" and "add path1,
> > > > > path2,
> > > > > path3"
> > > > > +	 * OR
> > > > > +	 * "remove path1 |add path1 |remove path2 |add path2
> > > > > |remove
> > > > > path3"
> > > > > +	 * to merge as "add path1, path2" and "remove path1,
> > > > > path2,
> > > > > path3"
> > > > > +	 * SO
> > > > > +	 * when we meet a non-change uevent from the same LUN
> > > > > +	 * with the same wwid and different action
> > > > > +	 * it would be better to stop merging.
> > > > > +	 */
> > > > > +	if (!strcmp(earlier->wwid, later->wwid) &&
> > > > > +	    strcmp(earlier->action, later->action) &&
> > > > > +	    strcmp(earlier->action, "change") &&
> > > > > +	    strcmp(later->action, "change"))
> > > > > +		return true;
> > > > > +
> > > > > +	return false;
> > > > > +}
> > > > 
> > > > I know you discussed this with Ben before, but still have some
> > > > trouble
> > > > with it. 
> > > > 
> > > > The first case should have been reduced to "remove path1 | remove
> > > > path2
> > > > > add path3" by filtering beforehand. I suppose you want to avoid
> > > > > this
> > > > 
> > > > sequence because it could leave us without paths temporarily,
> > > > causing
> > > > multipathd to destroy the map. But I don't understand what "stop
> > > > merging" buys you here - if you process the events one-by-one,
> > > > you may
> > > > also have 0 paths at some point. 
> > > 
> > > Well, because of the filtering , you will never actually stop
> > > merging
> > > in this case, like you mentioned.
> > 
> > Or, if I actually read better, I would see that you will stop
> > merging.
> > I can't think of any problems in theory with merging adds after
> > removes
> > in the simple case at least.
> > 
> > > > In the second case, we know all events in the sequence have the
> > > > same
> > > > WWID; in this case I think it would be safe to filter away
> > > > "remove"
> > > > events by subsequent "add" events, ending up with "add path1| add
> > > > path2| remove path3". But I may be overlooking something here.
> > > 
> > > We can't filter out the remove path events in this case. If you
> > > have
> > > 
> > > add sdb | remove sdb
> > > 
> > > You know that sdb in the remove event is referring to the same
> > > device as
> > > sdb in the add event. If you have
> > > 
> > > remove sdb | add sdb
> > > 
> > > There is no guarantee that sdb in the add event is referring to the
> > > same
> > > LUN as sdb in the remove event. Once the device gets removed that
> > > name
> > > can get reused for anything.
> > 
> > Or you could change the filtering code to verify that the wwids
> > matched.
> 
> AFAICS this is already the case in this code path. merge_needs_stop()
> is run after determining the WWID, and will only ever act on devices
> with the same WWID.
>  

I was talking about filtering out the remove events.  The filtering code
simply looks at kernel names, which is fine if you don't filter out
removes.

> > However, while the device would be for the same LUN, it might be a
> > different I_T Nexus, so you still don't want to filter the remove of
> > that
> > specific device. 
> 
> I figure this could be handled in uev_add_path() by not simply
> returning if an add event for an already existing device was
> encountered. But I agree that would be another, separate, patch.
> 

Assume a device like this (except not a scsi debug one).

mpathi (353333330000007d0) dm-10 Linux   ,scsi_debug      
size=1.0G features='0' hwhandler='0' wp=rw
|-+- policy='service-time 0' prio=1 status=active
| `- 10:0:0:0 sdh 8:112 active ready running
|-+- policy='service-time 0' prio=1 status=enabled
| `- 11:0:0:0 sdi 8:128 active ready running
|-+- policy='service-time 0' prio=1 status=enabled
| `- 13:0:0:0 sdk 8:160 active ready running
`-+- policy='service-time 0' prio=1 status=enabled
  `- 12:0:0:0 sdj 8:144 active ready running

Now imagine that path sdk (13:0:0:0) had already gone away.  After this
you get a string of events that look liek

remove sdj | add sdj

Except the sdj that got removed was (12:0:0:0) and the one the got added
was (13:0:0:0). In this case, skipping the completely and not adding the
new device could get you a bad configuration, since (12:0:0:0) might
have a different priority than (13:0:0:0).  Skipping the remove and also
adding the new device is also wrong because then you still have
(12:0:0:0) in the multipath device, when it doesn't really exist.

I'm pretty sure that we can't ever filter out remove events.

-Ben

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