[DRBD-user] DRBD and KVM for a HA-Cluster ?

Digimer linux at alteeve.com
Fri Jan 7 22:09:24 CET 2011

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

On 01/07/2011 04:00 PM, Lentes, Bernd wrote:
> VM1A and VM1B are identical VM's, offering the same service !

Here is the confusion. When you say "identical", you are referring to
the underlying data being mirrored on both nodes, right? That is, one VM
(VM1) that runs on *either* Node 1 *or* Node 2, right?

If you are thinking of two separate instances, however, that is not

Otherwise, yes, that would work perfectly.

> The vm's are installed in the plain DRBD, without any filesystem. Each vm has just to be created once, because DRBD, responsible for the replication, takes care that the vm "appears"
> on the other node. Right ? Is DRBD able to replicate that, although there is no filesystem ?

Yes. DRBD is a block device level application. It doesn't care at all
about what might be using it. Think of DRBD as just another hard drive.

> Is this scenario senseful ?


> In this scenario, is it possible to run VM1A and VM1B simultaneously, in a DRBD dual primary mode ? I don't think so.
> I hope my scenario is clear.

Absolutely not. :)

VM1 is VM1. It's data will always be identical on both nodes, but only
one node will actually be hosting the VM at any one time. Trying to
start it up on the second Node would be a *very* bad idea.


Should you run into a split brain, you will need to manually run
'drbdadm invalidate rX' on the node that the VM was /not/ running on.
Let's say that VM1 (DRBD r1) and VM3 (DRBD r3) were running on Node 1
and VM2 (DRBD r2) was running on Node 2. Now the DRBD resources
split-brain (ie: network fault). You would now need to run:

Node 1:
- drbdadm invalidate r2

Node 2:
- drbdadm invalidate r1
- drbdadm invalidate r3

Once that is done, you could reconnect and the DRBD resources would
re-sync. Does this make sense?

E-Mail: digimer at alteeve.com
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