Re: [Xen-users] High availability and live migration with just LVMand hardware fencing

[Bart Coninckx <bart.coninckx@xxxxxxxxxx>]

On Wednesday 19 May 2010 22:31:29 John Madden wrote:
> > I see ... Well, there are two good reasons I cannot have cLVM: need
> > snapshots and it's simply not available for SELS10, so I'm going to keep
> > on asking if permitted.
> 
> clvm doesn't do snapshots, so you can't do snapshots from dom0.
> Snapshots aren't of much use in dom0, though (assuming you're handing
> LV's to domU's, which have their own caches), so why does that matter?
> Do your snapshots from within your domU's.  cLVM may not be available on
> SLES10 out of the box, but that doesn't mean you can't add it.

Well, for (HVM) Windows guests I see it as a necessity basically: next to 
backups of regular data, snapshotting will allow to take fast complete backups 
of a temprarely suspended OS. 

And indeed, cLVM can be added from source, but while applying for support one 
of the first things Novell asks you to do is to send them a supportconfig 
report, which amongst others lists loaded modules (I suppose cLVM uses a 
module). This might rise problems. 

> > - What if I would use a seperate volume group for every LV (and for every
> > Xen guest)? Would that help? I suppose not really, since the Xen guest
> > should be migratable, meaning that all nodes would need to have a
> > consistent view no all VG's and LV's.
> 
> Metadata changes cause writes to disks.  Not safe.  I've done this
> before with active/passive clusters where you could somewhat assume some
> safety (i.e., lvm isn't even running on the passive node) but I'd never
> do it on

That would indeed seem safe to me, if you have something like Heartbeat manage 
volume activation.

> > - would snapshotting change the LVM meta data?
> 
> Yes, a snapshot is a LV.

OK, it seems that I will be needing cLVM or EVMS after all. At least I'm able 
to avoid something like OCFS2 that would add another layer of complexity. 


Thank you all for your insight on this.


Rgds,


Bart


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