[DRBD-user] Corosync and DRBD fencing: one or both?

Digimer linux at alteeve.com
Tue Aug 30 18:24:16 CEST 2011


On 08/30/2011 11:25 AM, William Seligman wrote:
> On 8/29/11 4:42 PM, Digimer wrote:
>> On 08/29/2011 03:36 PM, William Seligman wrote:
>>> A general question: I have a Corosync+Pacemaker with DRBD setup on Linux; I'll
>>> give the details if it's relevant. Corosync+Pacemaker controls DRBD start, stop,
>>> and promotion. I've implemented fencing via STONITH as Corosync resources.
>>>
>>> I have not put fencing in the drbd.conf file; I was under the impression that
>>> Corosync+Pacemaker would take of STONITHing a node if there's a DRBD problem. Is
>>> this correct? Or should I have fencing/STONITH configured in both Corosync and
>>> drbd.conf?
>>>
>>> Does the answer change between a primary/secondary versus dual-primary setup?
>>
>> You still want to configure fencing, but you can use the
>> 'crm-fence-peer.sh' handler. Using this with 'resource-and-stonith' will
>> tell DRBD to block I/O until the fence succeeds, preventing it from
>> going dual-primary (even if just for the brief moment between fault and
>> fence).
> 
> I may be dense, but I find the answer ambiguous; perhaps I didn't ask the
> question the right way.
> 
> Let me ask in a differen way: If I have fencing set up in corosync, and corosync
> controls drbd, do I also need fencing in drbd.conf?

Yes you do.

There is the potential for a period of time between the fault and it's
detection by Pacemaker. During this time, if DRBD is not appropriately
configured, both sides could go StandAlone/Primary. Once that happens,
you've got a split-brain.

The 'crm-fence-peer.sh' is used in drbd.conf to let DRBD block IO and
call a fence via pacemaker. This will result in two fence calls, which
is obviously overkill, but that isn't what we're after. The
corresponding "resource-and-stonith" argument is what matters. That is
what will block IO at the DRBD level until the fence call succeeds.

-- 
Digimer
E-Mail:              digimer at alteeve.com
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