[libvirt] VirtIO-SCSI disks limitation

[Osier Yang]

[[ 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.

[1] http://www.redhat.com/archives/libvir-list/2012-November/msg00537.html

Regards,
Osier

>
> [the issue as been discussed as part of: http://gerrit.ovirt.org/#/c/20630]
>
> Thanks,
> Daniel
>
>
> ----- Original Message -----
>> From: "Dave Allan" <dallan at redhat.com>
>> To: "Daniel Erez" <derez at redhat.com>
>> Cc: "Ayal Baron" <abaron at redhat.com>, "Osier Yang" <jyang at redhat.com>, "John Ferlan" <jferlan at redhat.com>
>> Sent: Monday, November 25, 2013 8:19:42 PM
>> Subject: Re: VirtIO-SCSI disks limitation
>>
>> Hi Daniel,
>>
>> Talk to Osier Yang and John Ferlan (cc'd).
>>
>> Dave
>>
>>
>> On Mon, Nov 25, 2013 at 12:48:45PM -0500, Daniel Erez wrote:
>>> Hi Dave,
>>>
>>> I'm an engineer at oVirt team and I'm working on VirtIO-SCSI integration.
>>> I would appreciate it if you could refer me to a point of contact at
>>> libvirt.
>>> In specific, I need to know if there's any hardcoded limitation for the
>>> number of disks per VirtIO-SCSI controller.
>>>
>>> Best Regards,
>>> Daniel