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

To: John Madden <jmadden@xxxxxxxxxxx>
Subject: Re: [Xen-users] Re: cLVM on Debian/Lenny
From: Jan Kalcic <jandot@xxxxxxxxxxxxxx>
Date: Mon, 20 Apr 2009 16:19:46 +0200
Cc: Jan Kalcic <jandot@xxxxxxxxxxxxxx>, Olivier Le Cam <Olivier.LeCam@xxxxxxxxxxxxxxxxxxxxx>, Javier Guerra <javier@xxxxxxxxxxx>, Ferenc Wagner <wferi@xxxxxxx>, xen-users@xxxxxxxxxxxxxxxxxxx, Thiago Camargo Martins Cordeiro <thiagocmartinsc@xxxxxxxxx>
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John Madden wrote:
>> I think of cLVM as the LVM with the cluster extension. Any setting on
>> a
>> shared storage is visible by all the nodes of the cluster.
>> But what exactly does this mean? At which level (VG or LV) does the
>> locking library work? Can I enable (active) the same VG on different
>> nodes? If yes, how cLVM prevent split-brain? I would eventually be
>> able
>> to load a LV only on one node?
> cLVM just keeps LVM changes consistent across the cluster.  It's a
> system of locks (via dlm) that prevents one node from making LVM changes
> while another node is making them.  You can mount a LV on multiple nodes
> simultaneously and completely trash your data if you like -- clvm
> doesn't prevent this -- and in fact this behavior is necessary if you're
> going to use a cluster filesystem such as OCFS or GFS.
> John
cLVM does not provide a lock manager which manages *access* (I know
EVMS2 does for instance) to a LV or VG from the nodes of a cluster. As
you said, It just keeps LVM changes consistent. Let me say it provides a
DLM at volume level. To prevent split-brain I need a DLM at file system
level, of course, so a cluster-aware file system like OCFS2 is needed.

But this does not make sense to me. Where are the pros of using LVM for
my VMs if they eventually reside as file on a (cluster) file system?  I
loose all the features I want from LVM (snapshot and resizing).

I think of a cluster where the VMs are using a physical block device
(shared) based on LVM which provides me the LVM features as above and
manage access to the volume it self.

Does it make sense or I am completely "out"?


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