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

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Message-ID: <cad33609-9a92-444b-9ff5-5f5a68598866@oracle.com>
Date: Mon, 21 Jul 2025 15:09:24 +0100
From: John Garry <john.g.garry@...cle.com>
To: "Martin K. Petersen" <martin.petersen@...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
Subject: Re: [PATCH v3 5/5] block: use chunk_sectors when evaluating stacked
 atomic write limits

On 09/07/2025 02:39, Martin K. Petersen wrote:
> 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

Does pbs need to be a power-of-2?

The block queue limits and splitting code seems to rely on that, but it 
is not policed AFAICS.

The stacking code seems to just want it to be a multiple of lbs, which 
itself must be a power-of-2.

Thanks,
John