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Re: [Xen-users] Re: cLVM on Debian/Lenny

To: Javier Guerra <javier@xxxxxxxxxxx>
Subject: Re: [Xen-users] Re: cLVM on Debian/Lenny
From: Jan Kalcic <jandot@xxxxxxxxxxxxxx>
Date: Mon, 20 Apr 2009 22:08:08 +0200
Cc: Jan Kalcic <jandot@xxxxxxxxxxxxxx>, Olivier Le Cam <Olivier.LeCam@xxxxxxxxxxxxxxxxxxxxx>, John Madden <jmadden@xxxxxxxxxxx>, Ferenc Wagner <wferi@xxxxxxx>, xen-users@xxxxxxxxxxxxxxxxxxx, Thiago Camargo Martins Cordeiro <thiagocmartinsc@xxxxxxxxx>
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Javier Guerra wrote:
> right.  they manage very different things (cLVM gives you blockdevices, while 
> OCFS gives you files); but Xen can use either one for the same purpose 
> (storage for DomUs), and neither of them will prevent several nodes clashing 
> each other when using the same 'thing' (file or blockdevice) at the same 
> time.  just like a non-cluster filesystem can't prevent two different 
> processes from corrupting a datafile by writing at the same time.
> for that there are some scripts for HA (hearbeat2) that manage Xen instances 
> as 'cluster resources', and make sure that each one is running on exactly one 
> node.
That's very clear and you got it, HA. With Xen in HA, like any other
kind of resource, you can obviously start manually a domU from console
or virt-manager and so you are not sure it is running on one node only.
Of course, you don't manage clustered resources (usually services or
whatever) without hearbeat, otherwise you end up to face strange and
unpredictable behavior. What would be really useful here, is a kind of
lock which prevent a domU already running on a dom0 to be started on
another domU.I wrongly expected this was managed by cLVM and now which
everything is explained I see the reason why it cannot be done and I
look forward to see such a kind of mechanism implemented in Xen.


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