[ClusterLabs] Minimum configuration for dynamically adding a node to a cluster

[Ferenc Wágner]

Nikhil Utane <nikhil.subscribed at gmail.com> writes:

> Would like to know the best and easiest way to add a new node to an already
> running cluster.
>
> Our limitation:
> 1) pcsd cannot be used since (as per my understanding) it communicates over
> ssh which is prevented.
> 2) No manual editing of corosync.conf

If you use IPv4 multicast for Corosync 2 communication, then you needn't
have a nodelist in corosync.conf.  However, if you want a quorum
provider, then expected_votes must be set correctly, otherwise a small
partition booting up could mistakenly assume it has quorum.  In a live
system all corosync daemons will recognize new nodes and increase their
"live" expected_votes accordingly.  But they won't write this back to
the config file, leading to lack of information on reboot if they can't
learn better from their peers.

> So what I am thinking is, the first node will add nodelist with nodeid: 1
> into its corosync.conf file.
>
> nodelist {
>     node {
>       ring0_addr: node1
>       nodeid: 1
>     }
> }
>
> The second node to be added will get this information through some other
> means and add itself with nodeid: 2 into it's corosync file.
> Now the question I have is, does node1 also need to be updated with
> information about node 2?

It'd better, at least to exclude any possibility of clashing nodeids.

> When i tested it locally, the cluster was up even without node1 having
> node2 in its corosync.conf. Node2's corosync had both. If node1 doesn't
> need to be told about node2, is there a way where we don't configure the
> nodes but let them discover each other through the multicast IP (best
> option).

If you use IPv4 multicast and don't specify otherwise, the node IDs are
assigned according to the ring0 addresses (IPv4 addresses are 32 bit
integers after all).  But you still have to update expected_votes.

> Assuming we should add it to keep the files in sync, what's the best way to
> add the node information (either itself or other) preferably through some
> CLI command?

There's no corosync tool to update the config file.  An Augeas lense is
provided for corosync.conf though, which should help with the task (I
myself never tried it).  Then corosync-cfgtool -R makes all daemons in
the cluster reload their config files.
-- 
Feri