[ClusterLabs] Fencing in a 3 node cluster

Digimer lists at alteeve.ca
Fri Jul 10 02:45:46 EDT 2015

On 09/07/15 11:37 PM, Nicolas S. wrote:
> Hello,
> I'm working on a 3 node cluster project.
> I didnt want to go to 2 node cluster , I'd rather3 nodes for the
> ressources, and to have a quorum. My 3 machines are identical.
> Each machine exports a disk to the cluster via iscsi (it's to simulate a
> SAN on my test platform).
> For the moment all is running : dlm + clvm (+ cmirror) + gfs2
> I now want to add the fencing to my cluster (for the moment it's
> disabled in the cluster config). My problem is to choose the right
> fencing/stonith.
> My questions :
> - As I have a quorum , do I need fencing ?


Quorum is a tool useful when the cluster nodes are behaving predictably.
Fencing is a tool for when a node has entered an unknown state. They
solve different problems.

> - I can't use an ipmi/drac or so fencing agent, my cluster is on a very
> low-cost test platform it hasn't remote reboot/poweroff access.

Then you will need to use an external device, like a switched PDU.

> - Maybe  I could use the fence_scsi, but how to use it on 3 nodes ?
> Where should I place the fencing disk ? If the node exporting the disk
> fails, there is no more fencing.

This would disconnect the failed VM from the shared storage, but leave
it otherwise alone. Personally, I don't like this because though the
storage is safe, it's possible for the node to still cause trouble. I
much prefer, and always recommend, power fencing.

> - Any other advice will be helpful :)

APC-brand AP7900 switched PDUs are excellent and fast fence devices.
They can often be found used for under $200usd. If this is still too
much, and if this is a learning platform, something I did when I first
learned was to create an arduino-based fence device

> Regards,
> Nicolas.

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