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


From: ronnie sahlberg
Subject: Re: [Qemu-devel] virtio-scsi vs. virtio-blk
Date: Fri, 10 Aug 2012 22:04:06 +1000

On Fri, Aug 10, 2012 at 9:57 PM, Stefan Priebe - Profihost AG
<address@hidden> 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.



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