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Date: Thu, 09 Aug 2012 15:15:41 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] virtio-scsi vs. virtio-blk
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To: ronnie sahlberg <ronniesahlberg@gmail.com>
Cc: Stefan Hajnoczi <stefanha@gmail.com>, qemu-devel <qemu-devel@nongnu.org>, Stefan Priebe - Profihost AG <s.priebe@profihost.ag>

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.

Paolo