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

[David Gibson <david@gibson.dropbear.id.au>]

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On Wed, Sep 09, 2015 at 09:29:18AM +0200, Thomas Huth wrote:
> 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)

Um.. I'm not sure, I have very little experience with PowerVM too.  I
think with PowerVM it's usually real SCSI devices being passed
through, rather than disk images, so presumably the SCSI target itself
reports however many LUNs it has.  There may be a limitation in
PowerVM, or in the AIX VIO server I think it typically backends onto,
but I don't know what it is.

Since that limit has been in the guest side driver forever, presumbly
no-one has hit LUNs > 8 in practice.

> >>> 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.

Remember that the spapr-vscsi device exists pretty much entirely to
make transition simpler for existing PowerVM guests.  New guests
(Linux or otherwise) intended to run under KVM should be using
virtio-blk or virtio-scsi.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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