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

[John Garry <john.g.garry@oracle.com>]

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

And the same is done for md raid0/dm stripe; I suppose that the idea is 
to encourage larger than chunk size writes to see the performance 
advantage in striping.

 > > 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.
>

Right, and so it seems counterproductive to have chunk size much bigger 
than bottom device io_opt

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