[Pacemaker] Properly fencing Postgres

[Martin Aspeli]

Hi Dejan,

Thanks for all the help!

>>>>   - The postgres data would need fencing when failing over, from what
>>>> I understand. I read the notes that using an on-board device like
>>>> Dell's DRAC to implemenet STONITH is not a good idea. We don't have
>>>> the option at this stage to buy a UPS-based solution (we do have
>>>> UPS, but it can't be used to cut power to individual servers). We do
>>>> have two pairs of NICs in each server, one of which would be used
>>>> "crossover" between master and slave.
>>> The problem with lights-out devices such as DRAC is that if they
>>> lose power then fencing doesn't work. But if you have them
>>> connected to UPS which is reliable then DRAC should be OK.
>> Yeah, it's backed up by a diesel generator, so that should be fine, I guess.
>>
>> On the other hand, if the power supply blows up on the server, it
>> may still go wrong. That's probably acceptable risk, though.
>
> Well, probably not. I hope that your servers have dual power
> supplies. Those things do tend to break. Some lights-out devices
> come equipped with battery too.

They should have dual PSUs, at least

>> Hence, we don't have init scripts, but I've written them before, so
>> I'm assuming writing basic OCF scripts wouldn't be that hard.
>
> Nope, though you have to make sure that they actually work ;-)
> There's also an RA which is called "anything" and may run any
> daemon or process (hence the name :)

Yeah, I found that. That's probably where we'll start.

> All this reminds me of LVS or similar. But if the slave can't
> serve requests, don't get the point of it. To start faster?

The slave serves requests from the Zope instances, which are part of the 
load balancing algorithm in HAProxy. HAProxy is only active on one node 
and would fail over to the slave node only if it failed on master. Ditto 
with Postgres + memcached.

Martin

-- 
Author of `Professional Plone Development`, a book for developers who
want to work with Plone. See http://martinaspeli.net/plone-book