[Pacemaker] RHEL/CentOS 6.4: corosync -> CMAN migration

[Andrew Beekhof]

On Sun, Mar 10, 2013 at 8:35 PM, Stefan Förster
<cite+pacemaker at incertum.net> wrote:
> Hello world,
>
> according to [1] using the pacemaker plugin with corosync is
> deprecated and will cease to be supported at some point in the future
> (on RHEL!).

A small but important distinction...
The plugin has never been supported on RHEL. What it changing is that
at some point it will no longer be _supplied_ either.

In fact prior to 6.4, Pacemaker only had Tech Preview status - using
the CMAN plugin instead of our home grown one was key to that
changing.

> Instead, the use of CMAN and running pacemaker as a
> standalone service is recommended.
>
> Now, unfortunately, the former (corosync with pcmk plugin) is exactly
> what we are using on two fileservice clusters, so we need a migration
> path with no downtime. I'm going to do some extensive testing later
> this week, but I'd be grateful for a bit of advance help. I think what
> I will try is:
>
> 1. Put pacemaker into maintenance mode.
> 2. Stop corosync, remove corosync.conf and the config snippet loading
>    pacemaker from within corosync.
> 3. Install cman and ccs.
> 4. Generate new cluster.conf, using appropriate tools.
> 5. Start cman, start pacemaker.
>
> The two crucial questions are:
>
> 1. In step (2), will pacemaker stop resources when it's brought down,
>    despite being in maintenance mode?

No.
_However_, you will want to stop any cluster filesystems as they're
going die anyway.
Both OCFS2 and GFS2 require the dlm which requires corosync/cman.
Without those running, the filesystems can't function and will panic.

> 2. In step (5), will the pacemaker resource configuration be
>    preserved throughout this migration?

Absolutely.  The only thing that changes is corosync.conf
All the resource related configuration stays in cib.xml and is unchanged.

>
> Any insights would be greatly appreciated.
>
>
> Cheers
> Stefan
>
> [1] https://access.redhat.com/knowledge/docs/en-US/Red_Hat_Enterprise_Linux/6/html/6.4_Technical_Notes/pacemaker.html
>
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