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


xen-users-bounces@xxxxxxxxxxxxxxxxxxx wrote on 20.10.2008 20:37:57:

> "Simon Talbot" <simont@xxxxxxxxx>

> Sent by: xen-users-bounces@xxxxxxxxxxxxxxxxxxx
>

> 20.10.2008 20:39
>
> To

>
> "John Madden" <jmadden@xxxxxxxxxxx>, <xen-users@xxxxxxxxxxxxxxxxxxx>

>
> cc

>
> Pekka.Panula@xxxxxxxx, Ferenc Wagner <wferi@xxxxxxx>

>
> Subject

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

>
> 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

I tested also with paravirtualized CentOS 5.2 32-bit and i did not got any corruption during 7-zip or with zip. I did migrate atleast 4 times during compression and then i did verify compressed file and they all say it was ok.

I did get corrupted with Windows 2003 standard server, with HVM drivers and with GPLPV drivers. Compression program was 7-zip latest beta i think, maybe 7-zip does not like virtualized servers.

Storage is IBM DS-series and Xen servers are on BladeCenter H HS21 cards, so HBA is "QLogic Corp. ISP2422-based 4Gb Fibre Channel to PCI-X HBA (rev 02)", driver: 8.02.00-k5-rhel5.2-04

Anyone else running Xen with similar system and does your HVM Windows DomU migrate without filesystem corruptions?

BTW: I did test also Xen 3.3.0 but interestingly you cant migrate it with 3.2.1 Xen, is this bug or should it work?

Terveisin/Regards,
  Pekka Panula, Net Servant Oy
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