Re: [Qemu-devel] virtio scsi host draft specification, v3

[Hannes Reinecke]

On 06/29/2011 12:07 PM, Christoph Hellwig wrote:
> On Wed, Jun 29, 2011 at 10:39:42AM +0100, Stefan Hajnoczi wrote:
> > I think we're missing a level of addressing.  We need the ability to
> > talk to multiple target ports in order for "list target ports" to make
> > sense.  Right now there is one implicit target that handles all
> > commands.  That means there is one fixed I_T Nexus.
> >
> > If we introduce "list target ports" we also need a way to say "This
> > CDB is destined for target port #0".  Then it is possible to enumerate
> > target ports and address targets independently of the LUN field in the
> > CDB.
> >
> > I'm pretty sure this is also how SAS and other transports work.  In
> > their framing they include the target port.
>
> Yes, exactly.  Hierachial LUNs are a nasty fringe feature that we should
> avoid as much as possible, that is for everything but IBM vSCSI which is
> braindead enough to force them.
Yep.

> > The question is whether we really need to support multiple targets on
> > a virtio-scsi adapter or not.  If you are selectively mapping LUNs
> > that the guest may access, then multiple targets are not necessary.
> > If we want to do pass-through of the entire SCSI bus then we need
> > multiple targets but I'm not sure if there are other challenges like
> > dependencies on the transport (Fibre Channel, SAS, etc) which make it
> > impossible to pass through bus-level access?
>
> I don't think bus-level pass through is either easily possible nor
> desirable.  What multiple targets are useful for is allowing more
> virtual disks than we have virtual PCI slots.  We could do this by
> supporting multiple LUNs, but given that many SCSI ressources are
> target-based doing multiple targets most likely is the more scabale
> and more logical variant.  E.g. we could much more easily have one
> virtqueue per target than per LUN.
The general idea here is that we can support NPIV.
With NPIV we'll have several scsi_hosts, each of which is assigned a 
different set of LUNs by the array.
With virtio we need to able to react on LUN remapping on the array 
side, ie we need to be able to issue a 'REPORT LUNS' command and 
add/remove LUNs on the fly. This means we have to expose the 
scsi_host in some way via virtio.

This is impossible with a one-to-one mapping between targets and 
LUNs. The actual bus-level pass-through will be just on the SCSI 
layer, ie 'REPORT LUNS' should be possible. If and how we do a LUN 
remapping internally on the host is a totally different matter.
Same goes for the transport details; I doubt we will expose all the 
dingy details of the various transports, but rather restrict 
ourselves to an abstract transport.

Cheers,

Hannes
--
Dr. Hannes Reinecke                   zSeries & Storage
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