[DRBD-user] DRBD with KVM for live migrations

Bart Coninckx bart.coninckx at telenet.be
Sun Oct 30 20:41:28 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 10/30/11 20:34, Trey Dockendorf wrote:
>     Hi,
>     a cluster software like Pacemaker could serve your purpose very
>     well. Do incorporate STONITH though, as you will need dual primary DRBD.
>     Mind you Pacemaker is NOT easy, requires a lot of study and reading.
>     Share image files could be done with OCFS2 for example (or any
>     clustering file system). You then need to create a Pacemaker
>     resource handling this. Personally I never used OCFS2, but there are
>     lots of eaxmples around.
>     HTH,
>     B.
> Looking at the man page for STONITH I'm not sure I understand how to
> incorporate that into this particular situation.  When I put DRBD in
> dual primary that will be only for the purpose of live migration, but I
> don't really plan to use live migrations at first for failover.  My
> initial purpose is for maintenance, to allow me to reboot one node
> (after kernel update) and move all the VMs to the other node.  Once I've
> got all that working smoothly then I'd move to having it serve a
> failover function.

If you don't need live migration, you can safely forget about dual 
primary. It is actually even better since it is (or rather can be) a can 
or worms.

No reason to go for OCFS2 either, unless you want shared storage for 
your config files (though NFS might serve that purpose too).
LVM with a regular file system is fine, very good for backup.

But do keep in mind that a master/slave system (or primary/secondary 
DRBD) will in no way allow you to live migrate. You can however always 
add it later, though converting from ext4 to OCFS2 probably will require 
backup and restore.

> Would OCFS2 be used in conjunction with DRBD?  From a few articles I've
> found, I thought that what could be done is create a LVM that stores all
> the qcow2 images (part of my current deployment already) named
> lv_vmstore.  Since I'm running CentOS 6 I've been formatting that ext4.
>   Would I not then use DRBD to replicate lv_vmstore across both nodes?
> One catch to all this I think I forgot to include in my initial email is
> I have no shared storage.  I only have 2 physical hosts with
> approximately 1TB isolated for lv_vmstore.  Someday once budgets allow I
> may have a SAN, but for now I am trying to facilitate live migration
> without one.

DRBD is a technology that would allow you to be without a SAN.
But do you or don't you need live migration (as you now mention you do)?

> Thanks
> - Trey


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