Re: [Qemu-devel] virtio-scsi vs. virtio-blk

[ronnie sahlberg]

On Fri, Aug 10, 2012 at 8:30 PM, Paolo Bonzini <address@hidden> wrote:
> Il 10/08/2012 12:28, Stefan Priebe - Profihost AG ha scritto:
>> I'm using iscsi. So no raw or qcow2.
>
> Ok, then you need to use scsi-block as your device instead of scsi-disk
> or scsi-hd.  This will disable the QEMU SCSI emulation and let your VM
> talk directly to the NAS.
>
> CCing Ronnie who may be interested in bug reports since I'm on holiday
> starting "soon".
>

I think it works on any,
You can of course not boot from a if=scsi disk in qemu,

but any '-drive file=iscsi://...,if=scsi' should work as long as it is
not the boot device.

SCSI emulation in qemu picks this up via WRITESAME10/16 and then calls
 bdrv_aio_discard()
block/iscsi.c is invoked for discard and then translates this back to
a SBC UNMAP command it sends to the target.


Now, block/iscsi.c does assume that any target that reports that it
supports thin-provisioning actually implements UNMAP command.
There could be targets that support thin-provision ing that does NOT
support UNMAP and unly support discard via WRITESAME10/16
so at some stage I should send a patch to iscsi.c to check which
commands the target supprots and use one of the supported ones instead
of a blanket
"you say you support thin-provisioning, I take that as confirmation
you support SBC UNMAP"


regards
ronnie sahlberg