Xen users - Mar 2007 - Xen and SAN : snapshot XOR live-migration ?

Please tell me if I am wrong :

Xen needs LVM to perform domU snapshots and snapshots must be performed 
by dom0.
By the way, a LVM volume group should not be used by more that one 
kernel at the same time. So if we use a SAN storage, one volume group 
should be activated on only one server and deactivated on the others.
But if we do that, it should not be possible to perform live migration 
of domU.
My conclusion would be that we have to choose between snapshot or 
live-migration, but with xen 3.0.3 we can not use both.

Don''t you think so ?

Regards

  Franck


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On Thu, 22 Mar 2007, Franck ELIE wrote:
> Please tell me if I am wrong :
>
> Xen needs LVM to perform domU snapshots and snapshots must be performed by 
> dom0.
I''m confused. Why Xen would need LVM to perform any snapshots at all?
> By the way, a LVM volume group should not be used by more that one 
> kernel at the same time. So if we use a SAN storage, one volume group 
> should be activated on only one server and deactivated on the others.
AFAIK you can use the LVM volume group in all nodes, but you may not 
change anything. If you need changes (e.g volume size), you must 
deactivate the vg in all but one node where you actually do the change, 
which in your case probably means shutting down all domU''s.

Check CLVM, it is cluster-aware.
> But if we do that, it should not be possible to perform live migration 
> of domU. My conclusion would be that we have to choose between snapshot 
> or live-migration, but with xen 3.0.3 we can not use both.
I''m pretty sure you can achieve both, it just depends how you set up
your
storage.

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On Thu, 2007-03-22 at 23:38 +0100, Franck ELIE wrote:
> Please tell me if I am wrong :
> 
> Xen needs LVM to perform domU snapshots and snapshots must be performed 
> by dom0.
> By the way, a LVM volume group should not be used by more that one 
> kernel at the same time. So if we use a SAN storage, one volume group 
> should be activated on only one server and deactivated on the others.
> But if we do that, it should not be possible to perform live migration 
> of domU.
> My conclusion would be that we have to choose between snapshot or 
> live-migration, but with xen 3.0.3 we can not use both.
> 
> Don''t you think so ?
> 
> Regards
> 
>   Franck
> 
> 
Just use CLVM. While its suitability is still kind of in debate for
heavy production use, its more than ample for such simple tasks if I
understand your need correctly.

Best,
--Tim



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On Mar 23, 2007, at 12:51 AM, Tim Post wrote:
> On Thu, 2007-03-22 at 23:38 +0100, Franck ELIE wrote:
>> Please tell me if I am wrong :
>>
>> Xen needs LVM to perform domU snapshots and snapshots must be  
>> performed
>> by dom0.
>> By the way, a LVM volume group should not be used by more that one
>> kernel at the same time. So if we use a SAN storage, one volume group
>> should be activated on only one server and deactivated on the others.
>> But if we do that, it should not be possible to perform live  
>> migration
>> of domU.
>> My conclusion would be that we have to choose between snapshot or
>> live-migration, but with xen 3.0.3 we can not use both.
>
> Just use CLVM. While its suitability is still kind of in debate for
> heavy production use, its more than ample for such simple tasks if I
> understand your need correctly.
CLVM does not yet support snapshots and mirrors.

-- 
-- Tom Mornini, CTO
-- Engine Yard, Ruby on Rails Hosting
-- Reliability, Ease of Use, Scalability
-- (866) 518-YARD (9273)


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Siim Vahtre wrote :
> On Thu, 22 Mar 2007, Franck ELIE wrote:
>
>> Please tell me if I am wrong :
>>
>> Xen needs LVM to perform domU snapshots and snapshots must be 
>> performed by dom0.
>
> I''m confused. Why Xen would need LVM to perform any snapshots at
all?
OK, I forgot to say that for me, LVM snapshot seems to be a good 
solution in order to perform a pseudo transparent backup (without to 
shutdown or to perform a "pause" of the domU).
But if we do not need to keep domU up 24h/24h, it is possible to use 
other way to perform snapshots ; for example with a basic "xm pause" ;
copy of domU ;  "xm unpause" (if you do not have databases, etc.)

  Franck


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Siim Vahtre wrote:
> AFAIK you can use the LVM volume group in all nodes, but you may not 
> change anything. If you need changes (e.g volume size), you must 
> deactivate the vg in all but one node where you actually do the change, 
> which in your case probably means shutting down all domU''s.
> 
> Check CLVM, it is cluster-aware.
Do you mean that with cLVM you can resize a volume on the fly without 
having to shut down every attached domUs (assuming the file system does 
support that -- which is the case for some like XFS if I understood)?

I''m actually interested on that topic because we are about to get a SAN
for our servers.

Thanks in anticipation,
-- 
Olivier Le Cam
Département des Technologies de l''Information et de la Communication
CRDP de l''académie de Versailles

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>> Check CLVM, it is cluster-aware.
>
> Do you mean that with cLVM you can resize a volume on the fly without 
> having to shut down every attached domUs (assuming the file system does 
> support that
Yes, but you need some kind of cluster framework for it to work.
>  -- which is the case for some like XFS if I understood)?
AFAIK, XFS does not support shrinking.
> I''m actually interested on that topic because we are about to get
a SAN for
> our servers.
Good luck!

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