[Pacemaker] OCF_RESKEY_CRM_meta_{ordered,notify,interleave}

Florian Haas florian at hastexo.com
Fri Mar 30 04:34:29 EDT 2012

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" and "meta attributes the RA
shouldn't care about." Can't we just have a rule that _all_ meta
attributes, like parameters, are just always available in the RA
environment with the OCF_RESKEY_CRM_meta_ prefix?


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