[DRBD-user] drbd resource fencing

[Florian Haas]

On Monday 03 December 2007 10:59:34 Dominik Klein wrote:
> Hi
>
> I read the recent thread "Suggestion to prevent split brain situation"
> and this link about drbd resource fencing.
> http://lists.linbit.com/pipermail/drbd-dev/2006-April/000341.html
>
> I ran a little test. I unplugged the DRBD link and the secondary was
> still upToDate. I tried "drbdadm primary" on the secondary and it was
> refused. After that, the resource was marked outdated.

I believe you are mistaking parallelism for causality. :-)

Mind you, I am totally shooting from the hip here as you didn't give us any 
logs or config files to look at. Specifically, I don't know what fencing 
policy you are using, whether you have redundant Heartbeat links, and whether 
you have set up dopd correctly. But I am led to believe that what really 
happened was this: The resource was marked Outdated *before* you attempted to 
make it Secondary. It just so happened by chance that it took the Primary a 
few seconds to outdate the resource on the Secondary, and you only noticed 
the completion of that process when you attempted "drbdadm primary" on the 
outdated node. 

> But now say the primary node fails, not just the DRBD link. How can the
> secondary node become primary when it is marked outdated after having
> lost communication to the primary node?

Counter-question: How would a failed node, while no longer there, be able to 
outdate the surviving node?

> Doesn't this prevent failover to work?

No. :-)

> What did I miss? I cannot imagine this is it.

Could it be that you simply missed the fact that resource fencing is an active 
operation, where the Primary actively outdates its Secondary via a 
communication path outside of DRBD, and that such an operation doesn't 
complete instantaneously?

Cheers,
Florian

-- 
: Florian G. Haas 
: LINBIT Information Technologies GmbH
: Vivenotgasse 48, A-1120 Vienna, Austria