[ClusterLabs] Fencing in a 3 node cluster

Nicolas S. lists at tropicdreams.net
Fri Jul 10 03:39:20 EDT 2015

10 juillet 2015 08:46 "Digimer" <lists at alteeve.ca> a écrit:

> 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 ?
> Yes.
> 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.
OK Thank you for the explanation between the 2 concepts. 

>> - 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.
Actually the problem is that this test (and very low-cost) platform is hosted elsewhere.
I just found that there is a REST API to maybe reboot the nodes. But for that Ill have to write a fence_agent.

I'm not familiar with python or perl, I'm just experienced in bash programming.
Is it possible to launch a simple shell script (making a curl) as fence agent ? 

>> - 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.
If this demo cluster works I ll be able to have owned machines with idrac/ipmi so it will be easier.
Setting up the demo cluster is harder that the production one :) 

>> - 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
> (http://nodeassassin.org).
>> Regards,
>> Nicolas.
> --
> Digimer
> Papers and Projects: https://alteeve.ca/w
> What if the cure for cancer is trapped in the mind of a person without
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