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Re: [Xen-users] xen-cluster lvm and locking

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Subject: Re: [Xen-users] xen-cluster lvm and locking
From: Javier Guerra Giraldez <javier@xxxxxxxxxxx>
Date: Mon, 5 Nov 2007 07:35:02 -0500
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On Monday 05 November 2007, Lucas Mayr wrote:
> Hello everybody!
> I want to run several DomUs in a cluster-like environment. The Dom0
> machines have access to a SAN and use LVM for disk management.
> Unfortunately it is possible, that the same DomU can be started on two
> different Dom0s. Without a cluster-aware file system (like gfs) a volume
> which is mounted with read/write access, more then once at a time, the file
> system get screwed.

even with your disk images as files on a cluster filesystem (like GFS), 
starting the same DomU twice would corrupt the image. maybe you mean using 
GFS as the main filesystem for the DomU's themselves... but then there would 
be MAC-address conflicts, and other nasty issues.

what you have to do is to make sure you never start the same DomU twice

> Is there a way to avoid starting a Dom0 simultaneously on more than one
> Dom0?
> I have read about clvm, but i couldn't get a clue, on how it could help me.

you mentioned GFS, if you know that, you know CLVM.

shortly, CLVM is LVM with cluster-wide locks (using the GFS lock managers), so 
that you can still manage the LVM online with all nodes running.  without it 
(plain LVM), any change on the LVM metadata on one node wouldn't propagate to 
the other nodes, with high risk of corrupting the whole volume group.

CLVM makes it 'just work' (after you get it working, of course)

> Maybe the answer is out there, but searching didn't get me to the right
> place, so please forgive me if my problem has been discussed already
> somewhere else in this mailing list.

the most favoured solutions lean more to the HA-EVMS direction, instead of the 
CLVM-GFS stack.  there are some (simple) resource definitions that use HA 
locks to avoid starting a DomU more than once.


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