[Pacemaker] [solved] stopping resource stops others in colocation / order sets

[Phil Frost]

On 06/15/2012 11:55 AM, David Vossel wrote:
>> If resC is stopped
>>   
>>   resource stop resC
>>
>> then drbd_nfsexports is demoted, and resB and resC will stop. Why is
>> that? I'd expect that resC, being listed last in both the colocation
>> and
> It is the order constraint.
>
> Order constraints are symmetrical. If you say to do these things in this order
>
> 1. promote drbd
> 2. start resB
> 3. start rscC
>
> Then the opposite is also true.  If you want to demote drbd it the following has to happen first.
>
> 1. stop rscC
> 2. stop resB
> 3. demote drbd
>
> You can get around this by using the symmetrical option for your order constraints.

True, but I wasn't demoting DRBD; I was stopping resource C (mostly to 
see what would happen if for whatever reason, it couldn't run anywhere). 
The order constraint alone doesn't explain the behavior I saw. However, 
I think I've pieced together what was actually happening. This constraint:

colocation colo inf: drbd_nfsexports_ms:Master resB resC


means, very confusingly, that to be a master, resB and resC must be 
active. Also, for resC to be active, resB must be active. In other words 
(with some transitive reduction applied):

drbd -> resC -> resB

It doesn't make any sense, and the documentation is wrong (or at least 
self-conflicting). But that's what it really does mean. What was really 
confusing is that were it not for the :Master modifier, then crm would 
make only one resource set, and it would mean something else. So when I 
was testing with dummy resources, I was only more confused. I made 
another post explaining it more.

So, DRBD can't be master unless resC and resB are active. And as you 
explained, the order constraint will stop resC and resB if DRBD is 
demoted. Combined, that means either all these things run, or none:

- resC is stopped, or can't run anywhere
- (by colocation constraint) DRBD may not be master. Demote it.
- (by order constraint) stop resC (actually a no-op: it's already stopped)
- (by order constraint) stop resB