[Pacemaker] setup advice

[Stefano Sasso]

Hello folks,
  I have the following setup in mind, but I need some advice and one hint
on how to realize a particular function.

I have a N (>= 2) nodes cluster, with data storage on postgresql.
I would like to manage postgres master-slave replication in this way: one
node is the "master", one is the "slave", and the others are "standby"
nodes.
If the master fails, the slave becomes the master, and one of the standby
becomes the slave.
If the slave fails, one of the standby becomes the new slave.
If one of the "standby" fails, no problem :)
I can correctly manage this configuration with ms and a custom script
(using
ocf:pacemaker:Stateful as example). If the cluster is already operational,
the failover works fine.

My problem is about cluster start-up: in fact, only the previous running
master and slave own the most updated data; so I would like that the new
master should be the "old master" (or, even, the old slave), and the new
slave should be the "old slave" (but this one is not mandatory). The
important thing is that the new master should have up-to-date data.
This should happen even if the servers are booted up with some minutes of
delay between them. (users are very stupid sometimes).

My idea is the following:
the MS resource is not started when the cluster comes up, but on startup
there will only be one "arbitrator" resource (started on only one node).
This resource reads from somewhere which was the previous master and the
previous slave, and it wait up to 5 minutes to see if one of them comes up.
In positive case, it forces the MS master resource to be run on that node
(and start it); in negative case, if the wait timer expired, it start the
master resource on a random node.

Is that possible? How can avoid a single resource to start on cluster boot?
Or, could you advise another way to do this setup?

I hope I was clear, my english is not so good :)
thank you so much,
   stefano

-- 
Stefano Sasso
http://stefano.dscnet.org/
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