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

Digimer lists at alteeve.ca
Wed Apr 19 11:20:42 EDT 2017

On 19/04/17 02:38 AM, Ulrich Windl wrote:
>>>> 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
> 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
>> 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
> 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...).
> 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.
> Ulrich

The way I understand SBD, and correct me if I am wrong, recovery won't
begin until sometime after the watchdog timer kicks. If the watchdog
timer is 60 seconds, then your cluster will hang for >60 seconds (plus
fence delays, etc).

IPMI and PDUs can confirm fence the peer if ~5 seconds (plus fence delays).

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