Re: stonith vs. quorums

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On 07/26/2012 03:29 PM, Antonis Christofides wrote:
> Hello,
> 
> let me see if I've understood correctly. Assume the following cluster:
> 
> 
>     Internet              Internet
>       |                     |
>       |                     |
>      node1 -------------- node2
>  (drbd primary)         (drbd secondary)
> 
> Now the network is partitioned. node2 can't communicate with node1. So
> it thinks node1 is down. So it makes itself a drbd primary, and clients
> are now writing data to node2. But node1 is not really down, it's just
> disconnected from node2. Some clients are still accessing node1 and
> writing to it. Result: chaos. So we use stonith to guard against this
> issue, but stonith requires a second, independent communication channel
> to node1. I have no experience on this but I guess "independent" is a
> big word.
> 
> Instead:
> 
>     Internet              Internet
>       |                     |
>       |                     |
>      node1 -------------- node2
>  (drbd primary)         (drbd secondary)
>       |                     |
>       |                     |
>       +-------- node3 ------+
> 
> 
> The network is partitioned; say node1 is disconnected from both node2
> and node3. Node1, seeing that it no longer has quorum (it is alone),
> switches itself to drbd secondary. node2, seeing that node1 is no longer
> there and that together with node3 it has a quorum, makes itself a drbd
> primary.

Yes, if you don't ignore quorum this is also fine. But at least do
resource-level fencing in drbd do not allow an out-of-date node to be
promoted.

Nevertheless, without stonith you can't recover automatically from
failures like stop-errors.

Regards,
Andreas

-- 
Need help with Pacemaker/Corosync/DRBD?
http://www.hastexo.com/now

> 
> This looks to me simpler and easier to achieve (but again, I have no
> experience). Isn't it a valid alternative solution, that makes stonith
> unnecessary? I'm asking because in some places in the Pacemaker or drbd
> documentation it says "don't do this without stonith!"; it doesn't say
> "don't do this without either stonith or quorum!", and I was therefore
> wondering whether I've understood something wrong.
> _______________________________________________
> discuss mailing list
> discuss@xxxxxxxxxxxx
> http://lists.corosync.org/mailman/listinfo/discuss
> 




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