Re: [Qemu-devel] [PATCH] spapr: Reduce advertised max LUNs for spapr_vscsi

[Thomas Huth]

On 09/09/15 09:19, David Gibson wrote:
> On Wed, Sep 09, 2015 at 08:25:34AM +0200, Thomas Huth wrote:
>> On 09/09/15 03:22, David Gibson wrote:
>>> The implementation of the PAPR paravirtual SCSI adapter currently
>>> allows up to 32 LUNs (max_lun == 31).  However the adapter isn't really
>>> designed to support lots of devices - the PowerVM implementation only
>>> ever puts one disk per vSCSI controller.
>>
>> Do you know how many LUNs are advertised by PowerVM?
> 
> Well, what do you mean by "advertised".  AFAIK from the point of view
> of the guest, the number of LUNs is advertised per-target, not per
> controller.

I mean, what's the highest LUN number that can be seen by a guest under
PowerVM? Is it always using only one LUN per controller, or is there a
way to change the amount of LUNs? (Sorry if I ask dumb questions ... I
do not have much experience with PowerVM yet)

>>> More specifically, the Linux guest side vscsi driver (the only one we
>>> really care about) is hardcoded to allow a maximum of 8 LUNs.
>>
>> So what about changing the vscsi driver in Linux instead to support more
>> LUNs?
> 
> Doesn't help for existing guests.  Basically what I'm trying to
> achieve is for qemu to reject up-front configurations that are
> unlikely to actually work in the guest.

I just wonder whether it makes sense to change the guest instead. In the
future, if we ever have guests that support more LUNs than 8 (maybe some
non-Linux guests like FreeBSD?), we've got to change QEMU back again...
OTOH, since this is just a one-line fix, it's likely ok to limit this to
8 now - it's easy to revert if we ever need to, so I'm fine with that
change, I just wanted to discuss the other possibilites.

 Thomas

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