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

[ronnie sahlberg]

You can easily verify if your target supports thin-provisioning via
the UNMAP command.

Download the sg3-utils package
 and either mount the LUN locally via the kernel open-iscsi or apply
the libiscsi patch to sg3-utils to make it iscsi-aware

then use the commands    "sg_unmap"  to try to unmap regions   and
"sg_get_lba_status" to check that the regions are now unmapped.



On Fri, Aug 10, 2012 at 9:54 PM, Stefan Priebe - Profihost AG
<address@hidden> wrote:
> http://www.nexenta.com/corp/products/what-is-openstorage/nexentastor
>
> tells me:
> "SCSI UNMAP as a client-side feature frees up storage in the back end, in
> the context of thin provisioning (a 100-to-one reduction in space for VDI
> when using NexentaStor)."
>
> So i would say nexenta supports it. But i'm using virtio-scsi-pci? I'm
> really sorry to ask so many questions.
>
> Stefan
> Am 10.08.2012 13:20, schrieb 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
>>
>