[Pacemaker] OCF_RESKEY_CRM_meta_{ordered,notify,interleave}

Florian Haas florian at hastexo.com
Mon Apr 2 06:05:19 EDT 2012

On Mon, Apr 2, 2012 at 11:54 AM, Andrew Beekhof <andrew at beekhof.net> wrote:
> On Fri, Mar 30, 2012 at 7:34 PM, Florian Haas <florian at hastexo.com> wrote:
>> On Fri, Mar 30, 2012 at 1:12 AM, Andrew Beekhof <andrew at beekhof.net> wrote:
>>> Because it was felt that RAs shouldn't need to know.
>>> Those options change pacemaker's behaviour, not the RAs.
>>> But subsequently, in lf#2391, you convinced us to add notify since it
>>> allowed the drbd agent to error out if they were not turned on.
>> Yes, and for ordered the motivation is exactly the same. Let me give a
>> bit of background info.
>> I'm currently working on an RA for GlusterFS volumes (the server-side
>> stuff, everything client side is already covered in
>> ocf:heartbeat:Filesystem). GlusterFS volumes are composed of "bricks",
>> and for every brick there's a separate process to be managed on each
>> cluster node. When these brick processes fail, GlusterFS has no
>> built-in way to recover, and that's where Pacemaker can be helpful.
>> Obviously, you would run that RA as a clone, on however many nodes
>> constitute your GlusterFS storage cluster.
>> Now, while brick daemons can be _monitored_ individually, they can
>> only be _started_ as part of the volume, with the "gluster volume
>> start" command. And if we "start" a volume simultaneously on multiple
>> nodes, GlusterFS just produces an error on all but one of them, and
>> that error is also a generic one and not discernible from other errors
>> by exit code (yes, you may rant).
>> So, whenever we need to start >1 clone instance, we run into this problem:
>> 1. Check whether brick is already running.
>> 2. No, it's not. Start volume (this leaves other bricks untouched, but
>> fires up the brick daemons expected to run locally).
>> 3. Grumble. A different node just did the same thing.
>> 4. All but one fail on start.
>> Yes, all this isn't necessarily wonderful design (the start volume
>> command could block until volume operations have completed on other
>> servers, or it could error out with a "try again" error, or it could
>> sleep randomly before retrying, or something else), but as it happens
>> configuring the clone as ordered makes all of this evaporate.
>> And it simply would be nice to be able to check whether clone ordering
>> is enabled, during validate.
>>> I'd need more information.  The RA shouldn't need to care I would have
>>> thought. The ordering happens in the PE/crmd, the RA should just do
>>> what its told.
>> Quite frankly, I don't quite get this segregation of "meta attributes
>> we expect to be relevant to the RA"
> The number of which is supposed to be zero.
> I'm not sure "cutting down on questions to the mailing list" is a good
> enough reason for adding additional exceptions.

Well, but you did read the technical reason I presented here?

> The one truly valid exception in my mind is globally-unique, since the
> monitor operation has to work quite differently.

Why are we not supposed to check for things like notify, ordered, allow-migrate?

> My concern with providing them all to RAs is that someone will
> probably start abusing them.

_Everything_ about an RA can be abused. Why is that any concern of
yours? You can't possibly enforce, from Pacemaker, that an RA actually
does what it's supposed to do.


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