[Users] Two fence devices per server.

[Perry Myers]

On 02/12/2012 11:18 AM, Nathan Stratton wrote:
> On Sun, 12 Feb 2012, Livnat Peer wrote:
> 
>> Hi Nathan,
>>
>> Currently OE (oVirt engine) supports only one power management device
>> per server.
> 
> Most servers today take more then one power supply, in fact none of ours
> even come with 1 supply options. How are people dealing with this
> problem today? Is it possible to put both supplies on one strip and use
> oVirt to fence both outlets? If not, are people using some other device
> to feed multiple supplies from one outlet?

It is possible to put both power supplies on a single external APC
device, however that sort of negates the redundancy of having dual power
supplies (to some degree).  Ideally you'd want each power supply on a
separate circuit and putting them on the same circuit means that both
power supplies will fail if the circuit fails.

I think that oVirt Engine needs the ability to configure more than one
fencing device per server, with logic to determine if either fence
device is sufficient to power off the machine, or if both are required.

The cluster stack (rgmanager or pacemaker based) in Fedora and other
distributions contains this sort of logic.  So in those software systems
you could create two (or more) fence devices per host with boolean logic
like:

fence_apc APC1 && fence_apc APC2
This means that in order for fencing to be successful, both devices must
be successfully powered off.  This would handle a dual power supply
situation (dual primary)

vs.

fence_ipmi || fence_apc
This would be used in a situation where the ipmi device is the primary
fencing device and if that succeeds, there is no need to issue the
fence_apc command.  (single primary with backup)

So similar logic is probably needed in oVirt.  Probably would be
appropriate to file an RFE here.

> We have looked at IPMI fence, but we have found it not to be very reliable.

IPMI is a protocol/standard that is implemented by many different
hardware devices.  Which specific hardware device were you using?  What
made you think it was unreliable?

Also note that there are many BMC style fencing devices out there so
you're not necessarily restricted to just using the IPMI protocol.

Many people use IPMI based fence devices in production HA clusters, and
by and large I haven't heard a lot of complaints about them.

Perry