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Date: Wed, 16 Jul 2025 13:37:37 +0200
From: Christoph Hellwig <hch@lst.de>
To: colyli@kernel.org
Cc: linux-raid@vger.kernel.org, linux-block@vger.kernel.org,
	Yu Kuai <yukuai3@huawei.com>, Xiao Ni <xni@redhat.com>,
	Hannes Reinecke <hare@suse.de>, Martin Wilck <mwilck@suse.com>,
	Christoph Hellwig <hch@lst.de>, Keith Busch <kbusch@kernel.org>
Subject: Re: [RFC PATCH] md: split bio by io_opt size in md_submit_bio()
Message-ID: <20250716113737.GA31369@lst.de>
References: <20250715180241.29731-1-colyli@kernel.org>
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In-Reply-To: <20250715180241.29731-1-colyli@kernel.org>
User-Agent: Mutt/1.5.17 (2007-11-01)

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?