[DRBD-user] Fencing & split brain related questions

Digimer lists at alteeve.ca
Fri Mar 14 15:44:54 CET 2014

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

On 14/03/14 05:34 AM, khaled atteya wrote:
> A- In DRBD Users's guide , in explanation of "resource-only" which one
> of fencing policy , they said:
> "If a node becomes a disconnected primary, it tries to fence the
> peer's disk. This is done by calling the fence-peer handler. The handler
> is supposed to reach the
> other node over alternative communication paths and call 'drbdadm
> outdate minor' there."
> My question is : if the handler can't reach the other node for any
> reason ,what will happen ?

I always use 'resource-and-stonith', which blocks until the fence action 
was a success. As for the fence handler, I always pass the requests up 
to the cluster manager. To do this, I use 'rhcs_fence' on Red Hat 
clusters (cman + rgmanager) or crm-fence-peer.sh on corosync + pacemaker 

In either case, the fence action does not try to log into the other 
node. Instead, it uses an external device, like IPMI or PDUs, and forces 
the node off.

> B- In active/passive mode , are these directives have effect:
> Are these directives "after-sb-0pri , after-sb-1pri  , after-sb-2pri"
> have effects in Active/passive mode or only in Active/Active mode ?
> If they have effects , what if i don't set them , is their default value
> for each ?

It doesn't matter what mode you are in, it matters what happened during 
the time that the nodes were split-brained. If both nodes were secondary 
during the split-brain, 0pri policy is used. If one node was Primary and 
the other remained secondary, 1pri policy is used. If both nodes were 
primary, even for a short time, 2pri is used.

The reason the policy doesn't matter so much is because the roles 
matter, not how they got there. For example, if you or someone else 
assumed the old primary was dead and manually promoted the secondary, 
you have a two-primary split-brain, despite the normal mode of operation.

> C- can I use SBD fencing with drbd+pacemaker rather than IPMI or PDU?

No, I do not believe so. The reason being that if the nodes split-brain, 
both will think they have access to the "SAN" storage. Where as with a 
real (external) SAN, it's possible to say "only one node is allowed to 
talk and the other is blocked. There is no way for one node to block 
access to the other node's local DRBD data.

IPMI/PDU fencing is certainly the way to go.

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