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    SubjectRe: virtio scsi host draft specification, v3
    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.

    >> 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.


    Dr. Hannes Reinecke zSeries & Storage +49 911 74053 688
    SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
    GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)
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