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

To: Jan Kalcic <jandot@xxxxxxxxxxxxxx>
Subject: Re: [Xen-users] Re: cLVM on Debian/Lenny
From: Javier Guerra <javier@xxxxxxxxxxx>
Date: Mon, 20 Apr 2009 15:19:03 -0500
Cc: Thiago Camargo Martins Cordeiro <thiagocmartinsc@xxxxxxxxx>, Olivier Le Cam <Olivier.LeCam@xxxxxxxxxxxxxxxxxxxxx>, Ferenc Wagner <wferi@xxxxxxx>, xen-users@xxxxxxxxxxxxxxxxxxx, John Madden <jmadden@xxxxxxxxxxx>
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Jan Kalcic wrote:
> 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.

that's exactly what heartbeat2 does.


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