[libvirt] VirtIO-SCSI disks limitation

Daniel P. Berrange berrange at redhat.com
Tue Nov 26 10:29:55 UTC 2013


On Tue, Nov 26, 2013 at 06:24:02PM +0800, Osier Yang wrote:
> [[ TO libvir-list ]]
> 
> Hi, Daniel,
> 
> I'm going to share the thread to public list for further discussion.
> Hope you
> don't mind.
> 
> On 26/11/13 02:37, Daniel Erez wrote:
> >Hi Osier,
> >
> >It seems there's a limitation in libvirt that allows up to six disks in a
> >virtio-scsi controller. I.e. when sending more than six disks, libvirt
> >automatically creates a new controller but of type virtual LSI Logic SCSI.
> >Is this behavior a known issue?
> 
> For narrow SCSI bus, we allow 6 disks indeed.
> 
> For wide SCSI bus,  we allow 15 disks (not including the controller
> itself on unit 7).
> 
> I'm doubting if we have problem on detecting if it supports wide SCSI
> bus though, since as far as I see from the user cases, it's always
> narrow SCSI bus.
> 
> >Shouldn't libvirt allow up to 256 disks
> >per controller or at least create a new controller of type virtio-scsi when needed?
> 
> The controller model for virtio-scsi controller is lsilogic, which we can't
> change simply, since it might affect the existing guests.
> 
> There was the similar discussion in libvir-list before [1].
> 
> But auto generation for controller is quite old, which I'm also not quite
> clear about. I'd like see another discussion to make it more clear whether
> we should do some work for upper layer app (e.g. oVirt).
> 
> Basicly two points:
> 
> *  Should we do some changes on the maximum units for a SCSI controller,
>     I.e. Should 7 (narrow bus) 16 (wide bus) be changed to other numbers?
>     I'm afraid the changes could affect existing guests though.
> 
> *  Do we really want to put the burden on users, I.E, let them create the
>    controller explicitly. For use cases like one wants to add many
> disks for
>    a guest, they need to know whether it's narrow SCSI bus or wide SCSI
>    bus first (which we don't expose outside), and then do the calculation
>    to know when to create a new SCSI controller.
> 
> @Daniel, am I correct on your problems?  Please comments if it doesn't
> cover all your thoughts.

The logic for auto-assignment of disks to controllers is never going
to make everyone happy. If people really care about the mapping they
should define it explicitly by providing a controller address with
their disk XML.

Daniel
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