On Thu, Sep 10, 2015 at 08:12:47AM +0200, Thomas Huth wrote: > On 10/09/15 03:24, David Gibson wrote: > > 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. > > FWIW, I had a quick look at FreeBSD sources here: > > https://svnweb.freebsd.org/base/stable/10/sys/powerpc/pseries/phyp_vscsi.c?revision=259204&view=markup > > ... and as far as I can see, they do not limit the LUNs to 8. > (I only spotted a "cpi->max_lun = ~(lun_id_t)(0);" in there). > So there indeed might also be older guests that support more than 8 LUNs.
Fair enough, you've convinced me. I still think it makes sense downstream only, though. -- 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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