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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