[Linux-cluster] question about number of fencing devices needed for a two node cluster

[Jakov Sosic]

On 12/06/2010 10:11 PM, Alvaro Jose Fernandez wrote:
> Hi,
> 
>  
> 
> I would like to know about wheter it would suffice for a two-node RHCS
> cluster a single power switch (APC 7921) fencing device. The power
> switch has 8 power outlets and I intend to use four of them for each
> node's dual power supplies.
> 
>  
> 
> I know it would be desirable to have two devices for a fully redundant
> configuration, but after reading some examples from the docs (they are
> meant for two power switch), I still cannot understand why a single
> power switch connected to both servers and the switch taking power from
> the UPS, would not be a good configuration. There is a single UPS in the
> environment.
> 
>  
> 
> ¿any experiences over this issue?

It's because you still have SPOF. In this case, SPOF is the electronic
module of the powerswitch, so, if the electronics go down, there's no
way to fence the node. It would be better to have for example iDRAC or
IPMI as primary fencing device and APC as secondary.

But, as in many things in IT, you are back to price/performance ratio.
If you really must achieve 5x9 uptime, or else you have money penalty,
then you'll invest in secondary fencing device. For most clusters, one
fencing device is enough, though.


-- 
Jakov Sosic