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

[Peter Lieven <pl@kamp.de>]

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.
>
>> 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.

There is a max_xfer_len in the block limits VPD. But at least in my case its 0 (=no limit). So for <2TB LUNs without
this patch the limit is 64K.

Peter

>
> Paolo


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Peter Lieven

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