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

[Christoph Hellwig <hch@lst.de>]

On Wed, Jul 16, 2025 at 02:02:41AM +0800, colyli@kernel.org wrote:
> From: Coly Li <colyli@kernel.org>
> 
> Currently in md_submit_bio() the incoming request bio is split by
> bio_split_to_limits() which makes sure the bio won't exceed
> max_hw_sectors of a specific raid level before senting into its
> .make_request method.
> 
> For raid level 4/5/6 such split method might be problematic and hurt
> large read/write perforamnce. Because limits.max_hw_sectors are not
> always aligned to limits.io_opt size, the split bio won't be full
> stripes covered on all data disks, and will introduce extra read-in I/O.
> Even the bio's bi_sector is aligned to limits.io_opt size and large
> enough, the resulted split bio is not size-friendly to corresponding
> raid456 level.

So why don't you set a sane max_hw_sectors value instead of duplicating
the splitting logic?