On Fri, Aug 10, 2012 at 9:57 PM, Stefan Priebe - Profihost AG <s.pri...@profihost.ag> wrote: > Am 10.08.2012 13:12, schrieb ronnie sahlberg: > >> You want discard to work? > > Yes > > >> You are using qemu 1.0 ? > > actual qemu-kvm git > > >> So you dont have the qemu support for scsi-generic passthrough to iscsi >> devices. > > Why? >
scsi-generic passthrough I think was added for iscsi in 1.1 so in 1.0 your guest will talk scsi to qemu, and invoke the scsi-emulation in qemu. It then will call functions like 'bdrv_aio_discard()" in libiscsi that will translate it back into a scsi command again and pass it to the target. It still works, it just means you have a small degradation of performance compared to if you could send the SCSI CDB straight through to the iscsi target as you can in qemu 1.1 Very likely so small performance hit that you can not even measure it. Biggest difference is cosmetic if you run 'sg_inq' in your guest. in 1.0 it will talk to the qemu scsi emulation layer. in 1.1 when scsi passthrough is use you will talk to the iscsi target.
