[Linux-HA] Order constraints and migration
andrew at beekhof.net
Tue Oct 6 08:55:38 MDT 2009
On Wed, Sep 23, 2009 at 1:05 PM, Denis Chapligin <chollya at satgate.net> wrote:
> On Wed, 23 Sep 2009 10:53:51 +0200
> Dejan Muhamedagic <dejanmm at fastmail.fm> wrote:
>> The order constraint means: if resource A is not running then
>> resource B can't run either. So, in order to stop A, you have to
>> stop B first. Though there are non-symmetrical constraints in
>> which stop order is not required to be the opposite of the start
> Ok, i got the idea. So, actually i've expected exactly this behaviour,
> but only for start/stop operations.
>> So, if this is not what you want/expect, perhaps you don't need
>> an order constraint. Can you describe the resources and how they
>> depend on each other.
> I have two nodes (n1,n2) with two XEN resources configred on them,
> let call them A and B. The B resource is a virtual machine, running
> webserver and stuff. The A resrouce as a virtual machine, running a
> MySQL server. Web on the B vm requires mysql on the A vm. Therefore, to
> ensure that resources are started in proper order, i've created a order
> "First A then B".
> So my cluster is working good, resource A is running on n1, resource B
> on n2. For some reason i decide to make
> some maintenance on node n1 and tell the crm to set that node to
> 'standby' state. During the next several seconds i see the following
> The B resource is stopped on n2 node
> The A resource is migrated to n2 node
> The B resource is started on n2 node
> I was expecting that B resource wouldn't be touched at all.
But thats what you asked for.
Order (A, B) tells the cluster that B needs A in order to run.
So if A is unavailable at some point, then B must be stopped.
Either B can run without A (in which case, set score=0) or it cant
(stopping B is necessary).
> The A
> resource was unavailable for less then a second and it didn't
> rebooted and it's execution state doesn't changed during migration.
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