linux-kernel - Re: Pe: [PATCH v5 1/3] virtio-scsi: first version

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Date:	Mon, 13 Feb 2012 09:57:50 +0200
From:	Dor Laor <dlaor@...hat.com>
To:	Christian Borntraeger <borntraeger@...ibm.com>
CC:	James Bottomley <James.Bottomley@...senpartnership.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Christian Hoff <christian.hoff@...ibm.com>,
	borntrae@...ux.vnet.ibm.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org,
	mst@...hat.com, rusty@...tcorp.com.au
Subject: Re: Pe: [PATCH v5 1/3] virtio-scsi: first version

On 02/13/2012 09:05 AM, Christian Borntraeger wrote:
> On 12/02/12 21:16, James Bottomley wrote:
>> Well, no-one's yet answered the question I had about why.
>
> Just to give one example from a different angle:
> In the big datacenters tape libraries are still very important, and lots
> of them have a scsi attachement. virtio-blk certainly is not the right
> way to handle those. Furthermore it seems even pretty hard to craft
> a virtio-tape since most of those libraries have vendor specific library
> controls (via sg). We would need to duplicate scsi generic (hint, hint :-)
>
>> virtio-scsi seems to be a basic duplication of virtio-blk except that it seems to
>> fix some problems virtio-blk has.  Namely queue parameter discover,
>> which virtio-blk doesn't seem to do.  There may also be a reason to cut
>> the stack lower down.  Error handling is most often cited for this, but
>> no-one's satisfactorily explaned why it's better to do error handling in
>> the guest instead of the host.
>>
>> Could someone please explain to me why you can't simply fix virtio-blk?
>
> I dont think that virtio-scsi will replace virtio-blk everywhere. For non-scsi
> block devices, image files or logical volumes virtio-blk seems to be the right
> approach, I think.

+1

virtio-scsi is superior w.r.t:
   - Device support: tapes, cdroms, other
   - Does guest-host mapped multipath
   - Supports plenty of virtual disks mapped to the guest w/o need for a
     pci slot per each virtio-blk
   - offload fancy/new/sophisticated scsi commands from the guest to the
     storage array w/o need for qemu implementation. Example XCOPY.

There are some more goodies like ability to support windows guest 
clustering w/o hacky versions of scsi pass through over virtio-blk.
virtio-blk is also a candidate to change the request based towards bio 
based implementation, so sticking to it does not buy us too much.

>
>> Or would virtio-blk maintainers give a reason why they're unwilling to
>> have it fixed?
>
> I dont consider virtio-blk broken. It just doesnt cover everything.
>
> Christian
>
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