On 12/1/12 5:46 AM, Hermes Flying wrote:
> Thank you for this!
>
> One last thing I need to clear out before digging into your configuration
> specs etc.
> Since the pacemaker is a fail-over system rather than a load-balancing system
> (like Red Hat) as you say, my understanding is that one of my nodes will have
> the VIP until:
> 1) Tomcat crashes and can not restart (dead for some reason) --> Pacemaker
> migrates VIP
>
> 2) The network communication with the outside network is cut off. -->
> Pacemaker migrates VIP
>
> If these (2) are valid (are they?) then that means that there is no
> primary/backup concept using pacemaker (since I will assign to one of my
> nodes to have the VIP and my installed Load Balancer will distribute the load
> among my 2 Tomcats) and as a result there can not be a split-brain.
In the event of a split brain with Pacemaker, and you don't have any
fencing configured, you will end up with your VIP running on both
systems. Chances in your configuration it won't be a big deal since your
router/firewall/whatever will learn the ARP of one system, so you'll end
up routing traffic properly - But it will be unpredictable, and
difficult to troubleshoot.
>
> Yet you imply that split-brain can occur even with Pacemaker if I don't have
> fencing properly set.
> But how? Since it seems to me that Pacemaker does not have a notion of
> primary/backup. Or you mean something else with "fail-over" system?
For each resource, Pacemaker knows there is a node where it is running,
and 'other' nodes where it is not running (but could if the node running
it failed). So from a resource perceptive, there is an active node and
one or more backups.
>
> Additionally you say that the "coordination" of Pacemaker instances is done
> via corosync which is over network messages right?
> So what happens in the event of communication/network failure but only in the
> communication paths used for corosync coordination and not the communication
> path with the clients? Hope this question makes sense as I am new in your
> facilities.
Split brain. That's why you need a redundant communications network,
plus you need fencing.
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