[DRBD-user] Resolving split brain

Digimer lists at alteeve.ca
Sat Jun 4 08:26:32 CEST 2016

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

On 03/06/16 12:38 PM, David Pullman wrote:
> We have a two node cluster that has Pacemaker and DRBD 8.4 on RHEL 6
> configured in HA. The application is a monitoring system that has a web
> front end and uses Postgresql as a backend. The Postgres db uses the
> DRBD device for it's datastore.
> I'm trying to get a configuration that will deterministically resolve
> split brain when it happens. The first priority is to have one and only
> one node running the HA stack and have that updating the database with
> monitoring data. The second priority is, in the event of a split brain,
> to resolve to the most recent content.

A *much* better approach is to build your system to not split-brain in
the first place. Configure fencing/stonith in your resource manager
(pacemaker or cman), then configure DRBD to hook into it via the
fence-handler scripts (and set 'fencing resource-and-stonoith;').

> I've looked at the automatic split brain recovery in the docs, and tried
> a couple of things but I think I'm missing something to get the
> resolution in the case of two standalones. Also, I'm thinking based on
> some other list entries that fencing is the way to go, but I'm not sure
> how to proceed.

There is no way to determine what node has "better" data.


Critical but small data is written to node 1, say accounting data or
something. Few inodes change but the value of that data is great. Later,
on the other node, someone uploads a distro ISO. Many inodes change but
they are easily replaced and have no real value.

Do you discard the node with the older changes?

Do you discard the node with the fewest changes?

Both would result in important data being lost.

> I'm getting split brain on occasion when the I/O between the nodes goes
> down. We have two switches in a redundant configuration connecting the
> nodes. For unrelated reasons I can't change the interconnect.
> Any suggestions, referrals to docs, etc., would be greatly appreciated.

Fencing. 100% required, and will prevent split brains entirely.

Papers and Projects: https://alteeve.ca/w/
What if the cure for cancer is trapped in the mind of a person without
access to education?

More information about the drbd-user mailing list