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Subject: Re: [Qemu-devel] virtio-scsi vs. virtio-blk
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Stefan Hajnoczi <stefanha@gmail.com>, qemu-devel <qemu-devel@nongnu.org>, ronnie sahlberg <ronniesahlberg@gmail.com>


Am 09.08.2012 15:15, schrieb Paolo Bonzini:
> Il 09/08/2012 14:52, ronnie sahlberg ha scritto:
>>>>
>>>> guest uses noop right now. Disk Host is nexentastor running open solaris. I
>>>> use libiscsi right now so the disks are not visible in both cases
>>>> (virtio-blk and virtio-scsi) to the host right now.
>>>>
>> And if you mount the disks locally on the host using open-iscsi, and
>> access them as /dev/sg* from qemu, what performance do you get?
>
> Good question.
>
>> virtio-blk would first go to scsi emulation and then call out to
>> block/iscsi.c to translate back to scsi commands to send to libiscsi
>>
>> while virtio-scsi (I think) would treat libiscsi as a generic scsi
>> passthrough device.  I.e. all commands just go straight through
>> bdrv_aio_ioctl(SG_IO)
>
> I think he's not using scsi-block or scsi-generic, because 1.0 libiscsi
> didn't support that.
>
> scsi-generic would indeed incur some overhead because it does not do
> scatter/gather I/O directly, but scsi-hd/scsi-block do not have this
> overhead.  In any case, that should be visible through the output of
> perf if it is significant.

Thanks for your help and replies. I'm a little bit lost on all these 
comments. So what to check / do next?

Stefan