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Date: Thu, 17 Jul 2025 06:52:10 +0200
From: Christoph Hellwig <hch@lst.de>
To: Yu Kuai <yukuai@kernel.org>
Cc: Coly Li <i@coly.li>, Christoph Hellwig <hch@lst.de>,
	Coly Li <colyli@kernel.org>, 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>, Keith Busch <kbusch@kernel.org>
Subject: Re: [RFC PATCH] md: split bio by io_opt size in md_submit_bio()
Message-ID: <20250717045210.GA27227@lst.de>
References: <437E98DD-7D64-49BF-9F2C-04CB0A142A88@coly.li> <20250716114121.GA32207@lst.de> <D12A8BDA-5C2B-4FA7-9C92-731BD321A611@coly.li> <20250716114533.GA32631@lst.de> <danlsghtalte7sku3vlfxkgngujgwzspanfayaxy4jfnk54jbf@yfvmr5plavmp> <20250716121449.GB2043@lst.de> <DE36C995-4014-44DC-A998-1C4FF9AFD7F9@coly.li> <20250716121745.GA2700@lst.de> <109C6212-FE63-4FD2-ACC3-F64C44C7D227@coly.li> <284433c9-c11d-401f-8015-41faa9d0fde1@kernel.org>
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On Thu, Jul 17, 2025 at 12:29:27AM +0800, Yu Kuai wrote:
>> If opt_io_size is (chunk_size * data_disks), setting new max_hw_sectors as rounddown(current max_hw_sectors, opt_io_size) is good idea.
>
> I think round down max_hw_sectors to io_opt(chunk_size * data_disks) will 
> really
> make things much easier, perhaps Christoph means this way. All you need to 
> do is to
> handle not aligned bio and split that part, and for aligned bio fall back 
> to use
> bio_split_to_limits().

If the raid5 code can handle multiple stripes per I/O, than you
just need to round it down, yes.  I assumed it could only handle
a single full or partial stripe at a time, but I guess I was wrong.