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. Thomas
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