On 08/04/2010 06:49 PM, Anthony Liguori wrote:
Right, the only question is, to you inject your own bus or do you
just reuse SCSI. On the surface, it seems like reusing SCSI has a
significant number of advantages. For instance, without changing the
guest's drivers, we can implement PV cdroms or PC tape drivers.
If you want multiple LUNs per virtio device SCSI is obviously a good
choice, but you will need something more (like the config space Avi
mentioned). My position is that getting this "something more" right is
considerably harder than virtio-blk.
Maybe it will be done some day, but I still think that not having
virtio-scsi from day 1 was actually a good thing. Even if we can learn
from xenbus and all that.
What exactly would keep us from doing that with virtio-blk? I thought
that supports scsi commands already.
I think the toughest change would be making it appear as a scsi device
within the guest. You could do that to virtio-blk but it would be a
flag day as reasonable configured guests will break.
Having virtio-blk device show up as /dev/vdX was a big mistake. It's
been nothing but a giant PITA. There is an amazing amount of software
that only looks at /dev/sd* and /dev/hd*.
That's another story and I totally agree here, but not reusing /dev/sd*
is not intrinsic in the design of virtio-blk (and one thing that Windows
gets right; everything is SCSI, period).
Paolo
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