Re: [PATCH v5 07/12] md: support to align bio to limits

["Yu Kuai" <yukuai@fnnas.com>]

在 2026/1/19 15:27, Christoph Hellwig 写道:

> On Mon, Jan 19, 2026 at 03:21:14PM +0800, Yu Kuai wrote:
>> Hi，
>>
>> 在 2026/1/19 14:47, Christoph Hellwig 写道:
>>> On Sun, Jan 18, 2026 at 07:40:23PM +0800, Yu Kuai wrote:
>>>> No, the chunk_sectors and io_opt are different, and align io to io_opt
>>>> is not a general idea, for now this is the only requirement in mdraid.
>>> The chunk size was added for (hardware) devices that require I/O split at
>>> a fixed granularity for performance reasons.  Which seems to e exactly
>>> what you want here.
>>>
>>> This has nothing to do with max_sectors.
>> For example, 32 disks raid5 array with chunksize=64k, currently the queue
>> limits are:
>>
>> chunk_sectors = 64k
>> io_min = 64k
>> io_opt = 64 * 31k
>> max_sectors = 1M
>>
>> It's correct to split I/O at 64k boundary to avoid performance issues, however
>> split at 64 *31k boundary is what we want to get best bandwidth.
>>
>> So, if we simply changes chunk_sectors to 64 * 31k, it will be incorrect, because
>> 64k boundary is still necessary for small IO.
> What do you mean with "necessary for small IO"?

io_min and io_opt is quite similar in mdraid, IO aligned with io_opt can get the best
bandwidth, and IO aligned with io_min can get the best iops. Currently chunk_sectors
is the same as io_min, and bio_split_rw() will try to align IO to io_min. I don't think
we want to remove this behavior.

>
-- 
Thansk,
Kuai