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Date: Mon, 25 Nov 2013 16:56:20 +0100
From: Peter Lieven <pl@kamp.de>
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Subject: Re: [Qemu-devel] [PATCH 1.8 3/6] qemu-img: add option to specify
 alternate iobuffer size
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: kwolf@redhat.com, qemu-devel@nongnu.org, stefanha@redhat.com

On 25.11.2013 16:48, Paolo Bonzini wrote:
> Il 25/11/2013 16:24, Peter Lieven ha scritto:
>> On 25.11.2013 16:14, Paolo Bonzini wrote:
>>> Il 25/11/2013 16:07, Peter Lieven ha scritto:
>>>>>> since the convert process is basically a sync operation it might
>>>>>> be benificial in some case to change the hardcoded I/O buffer
>>>>>> size to an alternate (greater) value.
>>>>> Do you really need the extra knob?  You can just add to BlockLimits the
>>>>> optimal transfer length, and use it unconditionally.
>>>> If you say patch 5 and 3 are ok. What could be done is to remove
>>>> this knob and increase the iobuf_size to cluster_size if cluster_size
>>>> is greater.
>>> Yes, that makes sense because it avoids unnecessary COW.
>>>
>>>> I do not want to increase the default iobuf size to anything
>>>> greater than 2MB. I do not know why this was choosen, but maybe
>>>> there was a reason for it.
>>> I think it is fine to increase it to the cluster_size or even to the
>>> optimal transfer length (new BlockLimits field).
>> okay scsi speaking:
>>   bs->bl.optimal_transfer_length = max(iscsilun->bl.opt_xfer_len,
>> iscsilun->bl.opt_unmap_gran) ?
> I was thinking more of
>
>    bs->bl.optimal_transfer_length = lun2qemu(iscsilun->bl.opt_xfer_len);
>
>   ...
>   iobuf_size =
>        max(bs->bl.optimal_transfer_length, cluster_sectors)*512;
>   iobuf_size = min(16MB, max(2MB, iobuf_size));
okay. same result ;-)

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