[Pacemaker] DRBD Recovery Policies

[Lars Ellenberg]

On Sat, Mar 13, 2010 at 12:05:31AM +0100, Menno Luiten wrote:
> Op 12-3-2010 23:32, Lars Ellenberg schreef:
> >
> >Of course it does.
> >  ;)
> >
> >It has to.
> >It would not survive a local disk failure on a Primary, otherwise.
> >
> >Though obviously this has performance impact,
> >reading from remote is most often less performant than reading locally.
> >
> >(To be technically correct,
> >it does not care about files,
> >but only about blocks.)
> >
> >>I believe that this is handled by DRBD by fencing the Master/Slave
> >>resource during resync using Pacemaker. See
> >>http://www.drbd.org/users-guide/s-pacemaker-fencing.html. This would
> >>prevent Node A to promote/start services with outdated data
> >>(fence-peer), and it would be forced to wait with takeover until the
> >>resync is completed (after-resync-target).
> 
> So, if I understand correctly, the URL I linked from the
> documentation is actually optional in a Pacemaker configuration? I
> assumed DRBD didn't play these clever tricks and a resource without
> fencing would end up 'time-warp'-ed, as mentioned on the docs page.
> In that case, wouldn't it be a good idea to explain this behavior on
> that page?

Fencing is not there to prevent you from going Primary on a SyncTarget.
Obviously a SyncTarget has access to good data (the SyncSource).

Fencing is there to preven you from
 * losing the replication link 
 * keep working for "ages" on current Primary
 * experiencing a power outage
 * coming back with only the former Secondary, which has not been
   updated for some time
 * after timeout (and possibly stonith and what not)
   it decides it is alone, and starts services
 
You just went online with stale data.
That is the time warp the User's Guide talks about.

With fencing, when losing the replication link,
the secondary would know that it is outdated,
and later refuse to be promoted.

-- 
: Lars Ellenberg
: LINBIT | Your Way to High Availability
: DRBD/HA support and consulting http://www.linbit.com

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