linux-kernel - Re: [PATCH v3 5/5] block: use chunk_sectors when evaluating stacked atomic write limits

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Message-ID: <yq1a55edu8i.fsf@ca-mkp.ca.oracle.com>
Date: Tue, 08 Jul 2025 21:39:11 -0400
From: "Martin K. Petersen" <martin.petersen@...cle.com>
To: John Garry <john.g.garry@...cle.com>
Cc: Mikulas Patocka <mpatocka@...hat.com>, <axboe@...nel.dk>, <agk@...hat.com>,
        <snitzer@...nel.org>, <song@...nel.org>, <yukuai3@...wei.com>,
        <hch@....de>, <nilay@...ux.ibm.com>, <dm-devel@...ts.linux.dev>,
        <linux-kernel@...r.kernel.org>, <linux-raid@...r.kernel.org>,
        <linux-block@...r.kernel.org>, <ojaswin@...ux.ibm.com>,
        <martin.petersen@...cle.com>
Subject: Re: [PATCH v3 5/5] block: use chunk_sectors when evaluating stacked
 atomic write limits


John,

> For io_min/opt, maybe reduce to a factor of the stripe size / width
> (and which fits in a unsigned int).
>
> I am not sure if it is even sane to have such huge values in io_min
> and the bottom disk io_min should be used directly instead.

The intent for io_min was to convey the physical_block_size in the case
of an individual drive. And for it to be set to the stripe chunk size in
stacking scenarios that would otherwise involve read-modify-write (i.e.
RAID5 and RAID6).

io_opt was meant to communicate the stripe width. Reporting very large
values for io_opt is generally counterproductive since we can't write
multiple gigabytes in a single operation anyway.

logical <= physical <= io_min <= io_opt <= max_sectors <= max_hw_sectors

-- 
Martin K. Petersen