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Re: [Qemu-devel] [PATCH] block/iscsi: use 16 byte CDBs only when necessa


From: Peter Lieven
Subject: Re: [Qemu-devel] [PATCH] block/iscsi: use 16 byte CDBs only when necessary
Date: Tue, 17 Jun 2014 13:50:21 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.5.0

On 17.06.2014 13:46, Paolo Bonzini wrote:
Il 17/06/2014 13:37, Peter Lieven ha scritto:
On 17.06.2014 13:15, Paolo Bonzini wrote:
Il 17/06/2014 08:14, Peter Lieven ha scritto:


BTW, while debugging a case with a bigger storage supplier I found
that open-iscsi seems to do exactly this undeterministic behaviour.
I have a 3TB LUN. If I access < 2TB sectors it uses READ10/WRITE10 and
if I go beyond 2TB it changes to READ16/WRITE16.

Isn't that exactly what your latest patch does for >64K sector writes? :)

Not exactly, we choose the default by checking the LUN size. 10 Byte for
< 2TB and 16 Byte otherwise.

Yeah, I meant introducing the non-determinism.

Ah okay :-)


My latest patch makes an exception if a request is bigger than 64K
sectors and
switches to 16 Byte requests. These would otherwise end in an I/O error.

It could also be split at the block layer, like we do for unmap. I think there's 
also a maximum transfer size somewhere in the VPD, we could to READ16/WRITE16 if 
it is >64K sectors.

That would be an idea. I will have a look.

Peter


Paolo


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