[Pacemaker] DRBD and fencing

[Matthew Palmer]

On Wed, Mar 10, 2010 at 02:32:05PM +0800, Martin Aspeli wrote:
> Florian Haas wrote:
>> On 03/09/2010 06:07 AM, Martin Aspeli wrote:
>>> Hi folks,
>>>
>>> Let's say have a two-node cluster with DRBD and OCFS2, with a database
>>> server that's supposed to be active on one node at a time, using the
>>> OCFS2 partition for its data store.
>> *cringe* Which database is this?
>
> Postgres.
>
> Why are you cringing? From my reading, I had gathered this was a pretty
> common setup to support failover of Postgres without the luxury of a
> SAN. Are you saying it's a bad idea?

PgSQL on top of DRBD is OK.  PgSQL on top of OCFS2 is a disaster waiting to
gnaw your leg off.

> Mmm, you're not:  
> http://fghaas.wordpress.com/2007/06/26/when-not-to-use-drbd :-)
>
> Or is it OCFS2 you're objecting to? We're using this because there are a  
> few shared files ("blobs" in our CMS) that get written by processes on  
> both nodes. This is very infrequent, though.

Split them -- put PostgreSQL on a regular filesystem and mount it before
starting the database, and run a separate dual-primary for your blobs.

> Also note that this database will see relatively few write transactions  
> compared to read transactions, if that makes a difference.

Cluster filesystems suck at high IO request rates, regardless of whether
they're reads or writes.

- Matt