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

[Jan Kalcic <jandot@xxxxxxxxxxxxxx>]

Fajar A. Nugraha wrote:
> On Mon, Apr 20, 2009 at 9:19 PM, Jan Kalcic <jandot@xxxxxxxxxxxxxx> wrote:
>   
>> 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"?
>>     
>
> I think you're looking at it the wrong way. You still need a separate
> locking mechanism if you want to prevent a domU backend (be it LVM or
> file) from being used by two or more dom0 at the same time.
>
> Cluster file system (like GFS) can create a shared filesystem that can
> be mounted on two or more nodes at the same time, but it does NOT
> prevent you from (lets say) having a file-backed vm, located on GFS,
> that is used by two (or more) dom0 to as domU backend. End result:
> domU corruption.
>
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>   

Right, I think I almost get the point.

cLVM can't say node1 is accessing this LV so node2 can't and vice versa.
There is not a cluster lock mechanism of this type (what is called
Cluster Segment Manager in EVMS2) Moreover, if there was this kind of
lock mechanism, it would not work for VMs live migration just because
multiple node can access to the same LV simultaneously.

Instead, cLVM can notifies all the nodes whenever a LVM change occurs.
That's it.

Thanks,
Jan



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