Re: [Qemu-devel] Anyone seeing huge slowdown launching qemu with Linux 2.6.35?

[Paolo Bonzini <pbonzini@redhat.com>]

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