Re: [PATCH 2/2] md: split bio by io_opt size in md_submit_bio()

[Christoph Hellwig <hch@infradead.org>]

On Sun, Aug 17, 2025 at 11:26:45PM +0800, colyli@kernel.org wrote:
> This patch introduces bio_split_by_io_opt() to solve the above issue,
> 1, If the incoming bio is not limits.io_opt aligned, split the non-
>   aligned head part. Then the next one will be aligned.
> 2, If the imcoming bio is limits.io_opt aligned, and split is necessary,
>   then try to split a by multiple of limits.io_opt but not exceed
>   limits.max_hw_sectors.
> 
> Then for large bio, the sligned split part will be full-stripes covered
> to all data disks, no extra read-in I/Os when rmw_level is 0. And for
> rmw_level > 0 condistions, the limits.io_opt aligned bios are welcomed
> for performace as well.
> 
> This patch only tests on 8 disks raid5 array with 64KiB chunk size.
> By this patch, 64KiB chunk size for a 8 disks raid5 array, sequential
> write performance increases from 900MiB/s to 1.1GiB/s by fio bs=10M.
> If fio bs=488K (exact limits.io_opt size) the peak sequential write
> throughput can reach 1.51GiB/s.

All this code duplication seems like a bad idea.  What is the problem
with setting max_hw_sectors to a stripe size aligned value and then
letting bio_split_by_io_opt do the work?