RE: [Xen-users] Shared SAN disk LUN between 2 servers and migrationproblem

["Simon Talbot" <simont@xxxxxxxxx>]

We have a 6 server Xen cluster, with shared SAN storage (directly
presenting LUNs to the host OSs). Live migration works well for both PV
and HVM domains, the only times that we have observed file system
corruption is when heartbeat has mistakenly brought the same VM up on
two machines. 

Apart from that we had a test harness performing around 10,000 live
migrations randomly around the 6 server cluster without a single
incident, the domain ran perfectly throughout the test.

Once scenario I could imagine problems in however, is if your OS/HBA is
performing caching on the write operations to the SAN, so say for
example you have the domain started on Server A and heavy IO is taking
place. You live migrate to Server B and dirty data is still in the cache
of Server A being written to the SAN. Server B then starts running and
continues the heavy writes before Server A is fully flushed, this then
potentially leads to the two servers effectively both fighting over the
same device for a short period of time until Server A's cache is fully
flushed.

I must stress that we have **NOT** experienced this problem and in
general the Host OS cache is fully flushed as part of the Live
Migration, but it could be a problem in some situations where something
in the data path is performing caching that the Host OS/Xen has no
knowledge of.

What Servers/SAN/HBAs etc. are you using?

Simon

Simon Talbot MEng, ACGI 
(Chief Engineer) 
Tel: 020 3161 6001
Fax: 020 3161 6011

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-----Original Message-----
From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx
[mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of John Madden
Sent: 16 October 2008 14:07
To: xen-users@xxxxxxxxxxxxxxxxxxx
Cc: Pekka.Panula@xxxxxxxx; Ferenc Wagner
Subject: Re: [Xen-users] Shared SAN disk LUN between 2 servers and
migrationproblem

> > Do i need one more layer to setup migration working? It seems now
that if
> > i migrate between servers i get filesystem corruptions.
>
> I run a similar setup but for PV linux guests.  (Live) migration
works.
> I've never tried it with HVM, but can't see any obvious obstacle.

Agreed.  No cluster filesystems or extra locking (DLM, etc.) is needed
for 
live migration itself to work in a shared-disk scenario.  I've never
tried it 
with HVM either though. 

John



-- 
John Madden
Sr. UNIX Systems Engineer
Ivy Tech Community College of Indiana
jmadden@xxxxxxxxxxx

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