Hi Dor, James & Co, On Mon, 2012-02-13 at 09:57 +0200, Dor Laor wrote: > On 02/13/2012 09:05 AM, Christian Borntraeger wrote: > > On 12/02/12 21:16, James Bottomley wrote: > >> Well, no-one's yet answered the question I had about why. > > > > Just to give one example from a different angle: > > In the big datacenters tape libraries are still very important, and lots > > of them have a scsi attachement. virtio-blk certainly is not the right > > way to handle those. Furthermore it seems even pretty hard to craft > > a virtio-tape since most of those libraries have vendor specific library > > controls (via sg). We would need to duplicate scsi generic (hint, hint :-) > > > >> virtio-scsi seems to be a basic duplication of virtio-blk except that it seems to > >> fix some problems virtio-blk has. Namely queue parameter discover, > >> which virtio-blk doesn't seem to do. There may also be a reason to cut > >> the stack lower down. Error handling is most often cited for this, but > >> no-one's satisfactorily explaned why it's better to do error handling in > >> the guest instead of the host. > >> > >> Could someone please explain to me why you can't simply fix virtio-blk? > > > > I dont think that virtio-scsi will replace virtio-blk everywhere. For non-scsi > > block devices, image files or logical volumes virtio-blk seems to be the right > > approach, I think. > > +1 > > virtio-scsi is superior w.r.t: > - Device support: tapes, cdroms, other AFAICT any type of non TYPE_DISK struct scsi_device passthrough is going to currently require virtio-scsi in order to work. > - Does guest-host mapped multipath The logic that comes with target_core_fabric_configfs.c and the native target control plane gives a host-side (tcm_vhost) fabric driver generic explict/implict ALUA multipath support by default. I think there are some interesting possibilities for paravirtualized ALUA multipath.. 8-) > - Supports plenty of virtual disks mapped to the guest w/o need for a > pci slot per each virtio-blk Ouch, virtio-blk lacks multi-lun per pci slot support..? > - offload fancy/new/sophisticated scsi commands from the guest to the > storage array w/o need for qemu implementation. Example XCOPY. > ... > There are some more goodies like ability to support windows guest > clustering w/o hacky versions of scsi pass through over virtio-blk. > virtio-blk is also a candidate to change the request based towards bio > based implementation, so sticking to it does not buy us too much. > MSFT cluster guests that require SPC-3 PR support can run today with tcm_loop LLD SCSI LUNs + SG_IO/BSG + right megasas QEMU HBA emulation, but I do agree this would be better served by virtio-scsi for guests that require SPC-3 PR support or passthrough. --nab -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
