[DRBD-user] Setup question using DRBD, cluster FS & KVM

Trey Dockendorf treydock at gmail.com
Fri Nov 25 21:04:49 CET 2011

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

On Fri, Nov 25, 2011 at 9:53 AM, Florian Haas <florian at hastexo.com> wrote:

> On 11/25/11 06:23, Trey Dockendorf wrote:
> > After reading the documents linked previously, most of the DRBD user
> > guide and documents related to Pacemaker, I still have a few conceptual
> > questions.
> >
> > First, I've seen lots of mention of the requirement to to sync the XML
> > data for each VM.  Is this necessary will KVM or Pacemaker be handling
> > this?  Right now I typically store all VM disks at /vmstore which has
> > the same security context as /var/lib/libvirt/images.  Do I also need to
> > replicate the directory containing the domain XML files?
> Yes. You can do that by putting those into a separate directory in
> /vmstore as well (or on NFS, for that matter), and then use
> ocf:heartbeat:symlink to manage your symlinks from /etc/libvirt/qemu. Or
> your can just use csync2 to keep the configurations in sync.
> > The other questions is do Pacemaker and other services for clustering
> > live on the replicated servers or on external systems? As a follow-up ,
> > if they live on the replicated servers, do I have to take measures to
> > ensure those services stay in sync?
> I'm not sure what you mean here exactly, but if your question boils down
> to "do I run Pacemaker on my DRBD nodes?", then yes, definitely so.
> > Right now I have a good idea of how to structure all this, but just a
> > few low level concepts I have yet to fully understand.  So far I think I
> > can achieve the necessary failover and live migration for the VMs using
> > 2 servers with /vmstore replicated with DRBD.  /vmstore will live on top
> > of GFS2 to allow active/active operation.
> What do you use for fencing?
> Florian
> --
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Excellent, thanks for the clarification.

Correct, I wasn't sure if Pacemaker lives on the DRBD nodes.  With
Pacemaker on the DRBD nodes, is there any syncing necessary or will CMAN
handle that?  I'm getting that from

For fencing I would be using STONITH.  Though that seems "extreme". Does
STONITH essentially create a failure when the conflicting system is
shutdown, such that services from it will be brought online on the healthy

Also, one specific use case I haven't seen much information on is what it
is that actually performs the VM migration in KVM once a node fails.  Based
on what I've read it would seem that's Pacemaker's job, but haven't seen
that specific scenario mentioned yet.

- Trey
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