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

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Date:	Tue, 7 Feb 2012 14:59:53 +0100
From:	Christian Hoff <christian.hoff@...ibm.com>
To:	BORNTRAE@...ux.vnet.ibm.com
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-scsi@...r.kernel.org, mst@...hat.com,
	Paolo Bonzini <pbonzini@...hat.com>, rusty@...tcorp.com.au
Subject: Re: Pe: [PATCH v5 1/3] virtio-scsi: first version

Hello Paolo,

On 07/02/12 13:31, Paolo Bonzini wrote:
> The structure of the LUN is defined by SAM, not by me. 

Just had a look at the SAM LUN structure. It appears that you are using 
the SCSI format for hierarchical LUNs(SAM 5 chapter 4.7.6) in order to 
indicate that the LUN is actually attached to the host system and not to 
the Qemu guest operating system itself.

What I am wondering is why you are doing this? The hierarchical SCSI 
format is only retained while the LUN number is being transmitted over the 
virtqueue, so there does not seem to be a technical necessity for using 
this format in order to transmit SCSI target ID and LUN number information 
to the host.

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.

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. Thank you for your work on this 
topic.

Mit freundlichen Grüßen / Kind regards

Christian Hoff

Student - Applied Computer Science

Phone:
49-16098976-950
 IBM Deutschland

E-Mail:
christian.hoff@...ibm.com
 Am Fichtenberg 1

 71083 Herrenberg

 Germany

IBM Deutschland GmbH / Vorsitzender des Aufsichtsrats: Martin Jetter
Geschäftsführung: Martina Koederitz (Vorsitzende), Reinhard Reschke, 
Dieter Scholz, Gregor Pillen, Joachim Heel, Christian Noll
Sitz der Gesellschaft: Ehningen / Registergericht: Amtsgericht Stuttgart, 
HRB 14562 / WEEE-Reg.-Nr. DE 99369940 

From:
BORNTRAE@...ux.vnet.ibm.com
To:
Paolo Bonzini <pbonzini@...hat.com>
Cc:
Christian Hoff/Germany/IBM@...DE, linux-kernel@...r.kernel.org, 
linux-scsi@...r.kernel.org, rusty@...tcorp.com.au, mst@...hat.com, 
kvm@...r.kernel.org
Date:
07/02/2012 14:18
Subject:
Re: Pe: [PATCH v5 1/3] virtio-scsi: first version

On 07/02/12 13:31, Paolo Bonzini wrote:
> The structure of the LUN is defined by SAM, not by me. 

Ok, got it.

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