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Date: Tue, 17 Jun 2014 13:46:46 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] block/iscsi: use 16 byte CDBs only when
 necessary
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To: Peter Lieven <pl@kamp.de>, Michael Tokarev <mjt@tls.msk.ru>, ronnie sahlberg <ronniesahlberg@gmail.com>
Cc: Kevin Wolf <kwolf@redhat.com>, qemu-devel <qemu-devel@nongnu.org>, Stefan Hajnoczi <stefanha@redhat.com>

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.

> 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.

Paolo