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

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Date:	Mon, 13 Feb 2012 08:05:59 +0100
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	James Bottomley <James.Bottomley@...senpartnership.com>
CC:	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 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.

> 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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