[ClusterLabs] DRBD on asymmetric-cluster

[Ken Gaillot]

On 04/07/2016 10:30 AM, Jason Voorhees wrote:
>> FYI, commands that "move" a resource do so by adding location
>> constraints. The ID of these constraints will start with "cli-". They
>> override the normal behavior of the cluster, and stay in effect until
>> you explicitly remove them. (With pcs, you can remove them with "pcs
>> resource clear".)
> 
> Agree :)
> 
> 
>> Note that colocation constraints only specify that the resources must
>> run together. It does not imply any order in which they must be started.
>> If Web and/or MTA should be started after clusterdataClone, configure
>> explicit ordering constraints for that.
>>
> 
> Agree. So far I haven't created any ordering constraints because it
> isn't important to me, YET, the order for starting services. However I
> have a question... if I don't have any ordering constraints at all, am
> I still able to activate resources no matter the order?

Sort of, but not exactly.

With a colocation constraint "A with B", the cluster must assign B to a
node before it can place A. B does not have to be started, but it does
have to be *able* to be started, in order to be assigned to a node. So
if something prevents B from being started (disabled in config, not
allowed on any online node, etc.), it will not be assigned to a node,
and A will not run.

That doesn't mean that B will be started first, though. If the cluster
needs to start both A and B, it can start them in any order. With an
ordering constraint "B then A", B must be started first, and the start
must complete successfully, before A can be started.

>>> These are the commands I run to create the master/slave resource and
>>> its contraints:
>>>
>>> # pcs cluster cib myfile
>>> # pcs -f myfile resource create clusterdata ocf:linbit:drbd
>>> drbd_resource=clusterdb op monitor interval=30s role=Master op monitor
>>> interval=31s role=Slave
>>> # pcs -f myfile resource master clusterdataClone clusterdata
>>> master-max=1 master-node-max=1 clone-max=2 clone-node-max=1
>>> notify=true
>>> # pcs -f myfile constraint location clusterdataClone rule
>>> score=INFINITY \#uname eq nodo1 or \#uname eq nodo2
>>
>> The above constraint as currently worded will have no effect. It says
>> that clusterdataClone must be located on either nodo1 or nodo2. Since
>> those are your only nodes, it doesn't really constrain anything.
> Ok, the last command (location with rule) was created to allow
> clusterdataClone start at both nodes, because without this rule the
> resource was always in "stopped" status in both nodes. Once I added
> this rule my clusterdataClone resource started automatically but I
> don't understand why it choosed a node to run as Master and the other
> one as Slave. Is it random?

I don't know why the resource would be stopped without this constraint.
Maybe you have an opt-in cluster? But in that case you can use a normal
location constraint, you don't need a rule.

It will choose one as master and one as slave because you have
master-max=1. The choice, as with everything else in pacemaker, is based
on scores, but these are transparent to the user and appears "random".

>> If you want to prefer one node for the master role, you want to add
>> role=master, take out the node you don't want to prefer, and set score
>> to something less than INFINITY.
> Well, I could add a rule to prefer nodo1 over nodo2 to run the Master
> role (in fact, I think I already did it) but what I want it's
> something different: I would like the Master role to follow IPService,
> I mean, clusterdataClone become Master where IPService was previously
> activated.
> 
> Is this possible? Or the only way to configure constraints is that my
> resources (IPService, Web, MTA) follow the Master role of
> clusterdataClone?

I think the latter approach makes more sense and is common. Storage is
more complicated than an IP and thus more likely to break, so it would
seem to be more reliable to follow where storage can successfully start.
The exception would be if the IP is much more important to you than the
storage and is useful without it.

You might want to look at resource sets. The syntax is a bit difficult
to follow but it's very flexible. See pcs constraint colocation/order set.