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
- Does guest-host mapped multipath
- Supports plenty of virtual disks mapped to the guest w/o need for a
pci slot per each virtio-blk
- 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.
Or would virtio-blk maintainers give a reason why they're unwilling to
have it fixed?
I dont consider virtio-blk broken. It just doesnt cover everything.
Christian
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