[Pacemaker] Fixed! - Re: Problem with dual-PDU fencing node with redundant PSUs

[Digimer]

On 06/28/2013 07:01 AM, Andrew Beekhof wrote:
> 
> On 28/06/2013, at 8:46 PM, Lars Marowsky-Bree <lmb at suse.com> wrote:
> 
>> On 2013-06-28T20:21:22, Andrew Beekhof <andrew at beekhof.net> wrote:
>>
>>>> It looks correct, but not quite sane. ;-) That seems not to be
>>>> something you can address, though. I'm thinking that fencing topology
>>>> should be smart enough to, if multiple fencing devices are specified, to
>>>> know how to expand them to "first all off (if off fails anywhere, it's a
>>>> failure), then all on (if on fails, it is not a failure)". That'd
>>>> greatly simplify the syntax.
>>> The RH agents have apparently already been updated to support multiple ports.
>>> I'm really not keen on having the stonith-ng doing this.
>>
>> I'd agree, but it's not multiple ports on the same device, it's multiple
>> ports on *different* devices. I don't think a single fencing agent can
>> handle that - it really looks like something only the higher level can
>> cope with.
> 
> True, it wouldn't handle that case but the case itself seems needlessly complex to me.
> Particularly since we've gotten by until very recently with single devices.

The problem is that failing the PDU will leave the node unfenceable
because both the IPMI method and the PDU method will not respond. Thus,
the PDU (or the UPS behind it) become single points of failure. Yes,
they fail very rarely, but they do fail and hey, I'm paranoid. :)

Using two PDUs (on two UPSes connected to two different circuits) means
that failing a UPS/PDU won't take out IPMI, so at least one fence method
will always be available. You would have to have multiple simultaneous
failures for fencing to fail in such a configuration.

For what it's worth, every cluster I have in production has dual-PDUs
for just this reason. It's a configuration I intend to keep for the
foreseeable future.

-- 
Digimer
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