Re: [Xen-users] Xen on two node DRBD cluster with Pacemaker

[Jean Baptiste Favre <xen-users@xxxxxxxxxxx>]

Hello Bart,
Answers inline.


Le 19/01/2011 21:49, Bart Coninckx a écrit :
> On Wednesday 19 January 2011 21:15:46 Jean Baptiste FAVRE wrote:
>> Hello Bart,
>> I wrote such an howto some times ago. It's available here in french
>> (http://publications.jbfavre.org/virtualisation/cluster-xen-corosync-pacema
>> ker-drbd-ocfs2.fr) and here in english
>> (http://publications.jbfavre.org/virtualisation/cluster-xen-corosync-pacema
>> ker-drbd-ocfs2.en).
>>
>> It's LVM based but could be easily adapted for img files.
>>
>> Regards,
>> JB
>>
>> Le 19/01/2011 17:54, Bart Coninckx a écrit :
>>> Hi all,
>>>
>>> could somebody point me to what is considered a sound way to offer Xen
>>> guests on a two node DRBD cluster in combination with Pacemaker? I
>>> prefer block devices over images for the DomU's. I understand that for
>>> live migration DRBD 8.3 is needed, but I'm not sure as to what kind of
>>> resource
>>> agents/technologies are advised (LVM,cLVM, ...) and what kind of DRBD
>>> config (seperate devices for each DomU I guess?)
>>> Thank you!
>>> Bart
> 
> Hi Jean,
> 
> thank you for this document, it seems highly educational. Could you please 
> verify if I understand correctly:
> 
> - you use LVM to build your DRBD resources on (not the other way around)
At dom0 level, I have 2 VG:
- system, for... system :)
- XenHosting, for domUs and common Xen related FS

Inside XenHosting, I create LVs:
- One for common stuff (config, kernels, iso, ...).
- One for each domU

Each LV are defined as DRBD resources.

Having one DRBD ressource per domU allow you to migrate them
independently. More, if your cluster grows and get a third server, you
can balance DRBD ressources between them. It's just easier to manage.

Each domU boots on its LV. That means that at domU level, your DRBD
ressource (or LV) is saw as a disk. Then you install your domU system.
The way I install domUs makes me use LV as well

As a summary, you have LV inside DRBD ressource onto LV.
It seems complicated but in fact it's not so tricky :)

If you want to use img files, either you store them on one LV only, and
you'll have to create cluster FS on it, or you use separate LV. In case
of separate LV, I prefer installing domU in LV directly instead of using
img file because it's easier to access LV based FS from dom0 in case
domU crashes.

> - you use a DRBD resource with a OCFS2 filesystem on to offer the ISO's and 
> config files on both nodes for every DomU
Yes, using cluster FS allow you to mount it on each node and provides
concurrent access for each dom0. I choose OCFS2 at the time of writing,
but I had to do it now, I would give a try to GlusterFS.

You can choose another way (like rsync), but I prefer cluster FS as I
don't have to think about synchronisation.

> - for each DomU you create a seperate DRBD resource that is allowed to be 
> dual 
> primary so you can do live migrations.
You MUST allow dual primary if you want to use live migration. So it's
not cluster specific.

> Am I doing all right?  ;-)
Yep, seems ;)

Regards,
JB

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