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

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Message-ID: <4F313526.2050907@redhat.com>
Date:	Tue, 07 Feb 2012 15:28:54 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Christian Hoff <christian.hoff@...ibm.com>
CC:	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 02/07/2012 02:59 PM, Christian Hoff wrote:
> Instead the format has some disadvantages:
> - It uses up 8 bytes where 3 bytes would be sufficient in order to store
> both the target ID and LUN number information
> - The format limits us to 255 target IDs. I agree that the LUN limit is
> probably more a theoretical and not a practical one, but 255 target IDs
> could become a limitation in the future.

It also provides better upwards-compatibility in case the limitations 
are actually hit.  If I had used "uint8_t target; uint16_t lun;" an 
extension would require a feature bit and a new struct.  With 8-bytes, 
you can just expand the definition.  That pretty much sums it up.

But again, I don't think the limitations are serious.  A MegaSAS header 
has room for 256 targets too, VMWare has only 15, Hyper-V has 1 (and 2 
channels, but I think that's an off-by-one), and you can always have 
multiple HBAs on the same guest.

> Nonetheless I think that virtio-scsi is a useful project and addresses
> many of the limitations imposed by virtio-block. The fact that I am still
> persisting has more to do with interest in the project rather than wanting
> to keep the code from going upstream.

No problem. :)

Paolo
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