Re: [PATCH v5 07/12] md: support to align bio to limits

Hi,

在 2026/1/19 16:27, Christoph Hellwig 写道:
> On Mon, Jan 19, 2026 at 03:43:34PM +0800, Yu Kuai wrote:
>>>> 64k boundary is still necessary for small IO.
>>> What do you mean with "necessary for small IO"?
>> io_min and io_opt is quite similar in mdraid, IO aligned with io_opt
>> can get the best bandwidth, and IO aligned with io_min can get the
>> best iops. Currently chunk_sectors is the same as io_min, and
>> bio_split_rw() will try to align IO to io_min. I don't think we want
>> to remove this behavior.
> I'm still confused.  Let's go back to your example:
>
> 32 disks raid5 array with chunksize=64k.
>
> Let's look at writes first:
>
> Each I/O that is full aligned to 31 * 64k can be handled without a
> read-modify-write cycle, so splitting I/O at that boundary makes perfect
> sense.  Below that there really should not me much difference, i.e.
> splitting anything at the 64k boundary is not useful.  So you want the
> chunk_sectors to apply at the 31 * 64k boundary, and the io_opt as well.
> And probably io_min too. (all just looking at writes).

This sounds reasonable, however, I'm not 100% sure split at 64k boundary
is not useful, I must run some tests to confirm. This behavior exist for
quite a long time.

>
> For non-degradead reads, not much should matter.   All reads should be
> reasonably efficient, splitting 64k boundaries is going to make the
> implementation trivial, but will make your rely heavily on plugging
> below, and also means you use quite a lot of lower bios.

Correct, BTW, even if we don't split at 64k boundary in bio_split_rw(), raid5
will stil try to split at 64k boundary in chunk_aligned_read(), and this do
rely hevily on plugging below.

BTW, current plug limit really is too low for huge array, like 32+ member disks,
only 32 requests and at most 128k per request. However, I still can't find better
solution other than simply increase the limits.

>
> For degraded reads, each I/O will always read 31 * 64k.  Splitting at
> 31 * 64k makes the implementation much easier.

I don't feel this is correct, each I/O will be handled by stripes, so a
4k read to the removed disk will only need to read 4k from other disks.
Anyway, this does not matter.

>
> I guess you want different boundaries for reads and writes?

Yes, this is still a potential demand, I'll test and take a detailed look
at other personalities.

>
> Note that io_opt and io_min really just are values for the caller and
> not affect splitting decisions themselves.  Of course the underlying
> factors should be related.

Thanks for the explanation, I'll feedback soon after testing.

-- 
Thansk,
Kuai