Re: Pe: [PATCH v5 1/3] virtio-scsi: first version
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On 02/13/2012 02:40 PM, Nicholas A. Bellinger wrote:
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, otherAFAICT 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 multipathThe 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-blkOuch, virtio-blk lacks multi-lun per pci slot support..?
Only if you use the pci multi-function option but that kills standard hot unplug
- 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 kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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