[DRBD-user] Fencing & split brain related questions

khaled atteya khaled.atteya at gmail.com
Sun Mar 16 10:08:23 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.

Thank you Digimer very much for your effort & time.
Would you Please see the question below ? :)

On Fri, Mar 14, 2014 at 4:44 PM, Digimer <lists at alteeve.ca> wrote:

> 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 clusters.
> 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.
> For the previous section , Are you meaning If i  assumed the old primary
was dead and manually promoted the secondary ,split-brain will happen
despite the two nodes can communicate each other ? In other way , would you
please explain this : "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*." , especially
underlined sentence ?

>  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.
> --
> Digimer
> Papers and Projects: https://alteeve.ca/w/
> What if the cure for cancer is trapped in the mind of a person without
> access to education?

System Engineer
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