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 h...@suse.de +49 911 74053 688 SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg) _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linux-foundation.org/mailman/listinfo/virtualization
