Re: [PATCH v2 2/7] md/raid1: advertise atomic write limits and handle runtime constraints

[Abd-Alrhman Masalkhi]

Hi John,

On Tue, Jul 07, 2026 at 17:20 +0100, John Garry wrote:
> On 06/07/2026 12:35, Abd-Alrhman Masalkhi wrote:
>>> well it seems to be that everything in the driver is split over
>>> BARRIER_UNIT_SECTOR_SIZE, so is in effect a chunk size.
>>>
>>> Note that atomic_write_hw_unit_max is going to be small always compared
>>> to BARRIER_UNIT_SECTOR_SIZE.
>>>
>>> However, can you check the blk stacking code to make sure that this does
>>> as you want? As I remember, for stacking we take the atomic write limits
>>> of the first bottom device and then stack the other bottom devices and I
>>> don't think that setting atomic_write_hw_unit_max in this way has an
>>> impact - see blk_stack_atomic_writes_limits()
>>>
>> I checked blk_stack_atomic_writes_limits(), and you are right: setting
>> atomic_write_hw_unit_max here alone does not have the intended effect on
>> the final stacked device.
>> 
>> Also, since atomic_write_hw_unit_max is expected to always be smaller
>> than BARRIER_UNIT_SECTOR_SIZE, 
>
> Yes, always expected to be much smaller.
>
>> it seems that there is no need to set an
>> additional atomic write limit in raid1, or even to set
>> atomic_write_hw_unit_max at all. Is that what you mean?
>
> Even though we expect it to be much smaller, it's good practice to 
> ensure this.
>
> One method to do so - which I already mentioned - was to set the chunk 
> size to BARRIER_UNIT_SECTOR_SIZE. You were not keen on that.
>

I see, thanks for the clarification. My concern was mainly conceptual,
and I was also worried that setting the chunk size might affect other
operations, such as discard. Since that is not the case, I'll set the
chunk size to BARRIER_UNIT_SECTOR_SIZE and update it accordingly in v2.

> Another method is to set max hw sectors for the RAID1 device to 
> BARRIER_UNIT_SECTOR_SIZE.
>
> Thanks,
> John
>

-- 
Best Regards,
Abd-Alrhman