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

Ulrich Windl Ulrich.Windl at rz.uni-regensburg.de
Wed Apr 19 02:38:16 EDT 2017

>>> Digimer <lists at alteeve.ca> schrieb am 18.04.2017 um 19:08 in Nachricht
<26e49390-b384-b46e-4965-eba5bfe59636 at alteeve.ca>:
> 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
> 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
> work.
>> 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
> 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...).

Slow is relative: If it takes 3 seconds from issuing the reset command until
the node is dead, it's fast enough for most cases. Even a switched PDU has some
delays: The command has to be processed, the relay may "stick" a short moment,
the power supply's capacitors have to discharge (if you have two power supplys,
both need to)...  And iLOs don't really like to be powered off.


> -- 
> Digimer
> Papers and Projects: https://alteeve.com/w/ 
> "I am, somehow, less interested in the weight and convolutions of
> Einstein’s brain than in the near certainty that people of equal talent
> have lived and died in cotton fields and sweatshops." - Stephen Jay Gould
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