[dm-devel] RHEL 5.4 multipath doesn't handle virtio block devices

[Hannes Reinecke]

Martin Schwenke wrote:
> On Tue, 30 Mar 2010 14:07:02 +0900, Kiyoshi Ueda <k-ueda at ct.jp.nec.com>
> wrote:
> 
>> Why do you want to use dm-multipath on virtio block devices?
> 
> For consistency rather than failover...  :-)
> 
> I'm using virtual machines to test a clustered NAS environment.  The
> production setup expects to see dm devices from dm-multipath.  So I'm
> using dm-multipath on virtio block devices to simulate use of
> dm-multipath in a SAN environment.  
> 
megasas megasas megasas ...
I posted a megasas HBA emulation for KVM a while back.
Would solve your problem rather elegantly.
But then you'd have to recompile KVM, at which point you could include
the SCSI emulation as well ...

> The RHEL 5.4 version of KVM doesn't contain a SCSI emulation, so the
> only 2 choices for large numbers of block devices are virtio and
> iSCSI.  I'm trying to keep my options open for both of these.
> 
> The fix for virtio block devices seems quite straightforward, so I
> thought I would propose it.  :-)
> 
Problem is, though, that you don't have a unique ID for virtio devices.
EG scsi_id and friends don't work here.
And AFAIR there is no feature flag which would allow you do set one.

How do you distinguish between, say, 2 virtio devices across 2 paths?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
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