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Re: [Question] qemu-img convert block alignment
From: |
Zhenyu Ye |
Subject: |
Re: [Question] qemu-img convert block alignment |
Date: |
Wed, 4 Aug 2021 15:18:59 +0800 |
User-agent: |
Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.12.1 |
On 2021/8/3 23:03, Eric Blake wrote:
> On Fri, Apr 02, 2021 at 11:52:25AM +0800, Zhenyu Ye wrote:
>> Hi all,
>>
>> commit 8dcd3c9b91 ("qemu-img: align result of is_allocated_sectors")
>> introduces block alignment when doing qemu-img convert. However, the
>> alignment is:
>>
>> s.alignment = MAX(pow2floor(s.min_sparse),
>> DIV_ROUND_UP(out_bs->bl.request_alignment,
>> BDRV_SECTOR_SIZE));
>>
>> (where the default s.min_sparse is 8)
>> When the target device's bl.request_alignment is smaller than 4K, this
>> will cause additional write-zero overhead and makes the size of target
>> file larger.
>>
>> Is this as expected? Should we change the MAX() to MIN()?
>
> Yes it is expected, and no we shouldn't change it. Even when a target
> advertises a bl.request_alignment of 512, our goal is to avoid needing
> read-modify-write cycles when that target is really on top of a 4k
> sector disk. Writing extra 0s out to the 4k boundaries does not
> change the fact that allocation is in 4k chunks anyways, regardless of
> whether the disk supports smaller 512-byte reads.
>
Thanks for your reply.
Zhenyu