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

[Christoph Hellwig <hch@infradead.org>]

On Mon, Aug 18, 2025 at 04:57:30PM +0800, Yu Kuai wrote:
> Ok, let's ignore the case there are other drivers in the stack chains,
> just in this case: mdraid on the top of another mdraid, which we already
> have. And in order not to introduce regression, we can do this inside
> mdraid.

Whatever you want to do in that case you can do by looking at
BLK_FEAT_RAID_PARTIAL_STRIPES_EXPENSIVE.

But I'm not even sure what you want to do.  Assume you have to raid5
stacked on top of each other, using the same chunk size, but a different
non-multiple number of stripe units.  The only thing you could do is to
multiply the values, but I doubt anything above will take the resulting
number serious.  And depending on the stripe size of the lower raid5
the upper one might not even be capable of feeding it large enough I/O.