[ClusterLabs] how important would you consider to have two independent fencing device for each node ?

Digimer lists at alteeve.ca
Tue Apr 18 13:08:18 EDT 2017

On 18/04/17 11:07 AM, Lentes, Bernd wrote:
> Hi,
> i'm currently establishing a two node cluster. Each node is a HP server with an ILO card.
> I can fence both of them, it's working fine.
> But what is if the ILO does not work correctly ? Then fencing is not possible.

Correct. If you only have iLO fencing, then the cluster would hang
(failed fencing is *not* an indication of node death).

> I also have a switched PDU from APC. Each server has two power supplies. Currently one is connected to the normal power equipment, the other to the UPS.
> As a sort of redundancy, if the UPS does not work properly.

That's a fine setup.

> When i'd like to use the switched PDU as a fencing device i will loose the redundancy of two independent power sources, because then i have to connect both power supplies together to the UPS.
> I wouldn't like to do that.

Not if you have two switched PDUs. This is what we do in our Anvil!
systems... One PDU feeds the first PSU in each node and the second PDU
feeds the second PSUs. Ideally both PDUs are fed by UPSes, but that's
not as important. One PDU on a UPS and one PDU directly from mains will

> How important would you consider to have two independent fencing device for each node ? I'd can't by another PDU, currently we are very poor.

Depends entirely on your tolerance for interruption. *I* answer that
with "extremely important". However, most clusters out there have only
IPMI-based fencing, so they would obviously say "not so important".

> Is there another way to create a second fencing device, independent from the ILO card ?
> Thanks.

Sure, SBD would work. I've never seen IPMI not have a watchdog timer
(and iLO is IPMI++), as one example. It's slow, and needs shared
storage, but a small box somewhere running a small tgtd or iscsid should
do the trick (note that I have never used SBD myself...).

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