Re: [PATCH 1/2] block: ignore underlying non-stack devices io_opt

[Yu Kuai <yukuai1@huaweicloud.com>]

Hi,

在 2025/08/18 16:00, Christoph Hellwig 写道:
> On Mon, Aug 18, 2025 at 02:31:20PM +0800, Yu Kuai wrote:
>> Hi,
>>
>> 在 2025/08/18 14:18, Christoph Hellwig 写道:
>>> On Mon, Aug 18, 2025 at 02:14:06PM +0800, Yu Kuai wrote:
>>>> Please take a look at the first patch, nothing special, the new flag
>>>> will be passed to the top device.
>>>
>>> But passing it on will be incorrect in many cases, e.g. for any
>>> write caching solution.  And that is a much more common use case
>>> than stacking different raid level using block layer stacking.
>>
>> I don't quite understand why it's incorrect for write caching solution,
>> can you please explain in details? AFAIK, the behaviour is only changed
>> for the first mdraid device is the stacking chain.
> 
> The way I read the patch, the flag is inherited if any underlying
> device sets it.
> 
> Now if you stack something that buffers most I/O the md raid limits
> aren't really that relevant, and you'd rather expose the limits
> for the writeback or read caching.

Why? We just set the flag for mdraid disks first, and then inherit to
top devices that is stacked by mdraid, so md raid limits should always
be relevant. I still don't understand the problem that you said :(

Thanks,
Kuai

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