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

[Anthony Liguori]

On 08/04/2010 11:48 AM, Alexander Graf wrote:
> On 04.08.2010, at 18:46, Anthony Liguori wrote:
>
>    
> > On 08/04/2010 11:44 AM, Avi Kivity wrote:
> >      
> > > On 08/04/2010 03:53 PM, Anthony Liguori wrote:
> > >        
> > > > So how do we enable support for more than 20 disks?  I think a virtio-scsi is 
> > > > inevitable..
> > > >          
> > > Not only for large numbers of disks, also for JBOD performance.  If you have 
> > > one queue per disk you'll have low queue depths and high interrupt rates.  
> > > Aggregating many spindles into a single queue is important for reducing 
> > > overhead.
> > >        
> > 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.
> >      
> 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*.

Regards,

Anthony Liguori

> Alex
>
>