[kwlug-disc] DRBD

[Tim Laurence]

First off, forget about heartbeat. It is the name of an old package which
was replaced with 3 packages; pacemaker, corosync, and "cluster-glue".

Corosync is a communication service that allows multiple servers to
communicate with each other.

Pacemaker is a system that uses corosync and cluster-glue to coordinate
interdependent activities on multiple servers, for example making sure a
service is running somewhere in a cluster of servers but only one place.

DRBD is summarized as "RAID-1 over ethernet". It keeps "block" devices
synchronized across multiple servers.


The reason you see all of these together is because the file-systems that
use block devices such as those presented by DRBD usually assume they are
the only code making changes to that block device. If you mount the
replicated DRBD block device on two servers at once it can become seriously
corrupted because the filesystem drivers on each of the separate servers
are not coordinating their activities. Imagine two authors writing and
revising the same book who are not communicating and are not even aware of
the others existence, then you have a good sense for the problem.

The reason why you see pacemaker and drbd together a lot is that pacemaker
can be used (is is actually built to) ensure a replicated DRBD block device
is only mounted on one server at a time. Also, since DRBD is used when
availability/up-time of the data is important it is also important to not
only make sure the disk is mounted somehwere but you also need to make sure
the service that uses that disk (database server, webserver, nfs-server,
etc) is started on that node and ensure is not running anywhere else. It
can do this all automaticly meaning if you server fails while you are away
you will usually find out about a server failure via an automatic email
mention services are back up rather than a phone call from marketing asking
why the website is down. :)


--Tim


On Thu, May 15, 2014 at 1:42 PM, Jonathan Poole <jpoole at digitaljedi.ca>wrote:

> Corosync is a cluster engine…DRBD is distributed replicated block device
> system..
>
> Most setups i’ve used use a combination of Pacemaker, corosync, and DRBD
> for distributed volumes.
>
>
>
> So, basically DRBD is used for replicating volumes.
>
> On May 15, 2014, at 12:11 PM, Joe Wennechuk <youcanreachmehere at hotmail.com>
> wrote:
>
> Any Have any experience with DRBD. It seems to use heartbeat, and not
> corosync?
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