Re: Improper io_opt setting for md raid5

[Keith Busch <kbusch@kernel.org>]

On Tue, Jul 29, 2025 at 08:13:31AM +0200, Hannes Reinecke wrote:
> > > Note that chunk_sectors limit is the *stripe* size, not per drive stride.
> > > Beware of the wording here to avoid confusion (this is all already super
> > > confusing !).
> > 
> > This is something we're not in the same page :( For example, 8 disks
> > raid5, with default chunk size. Then the above calculation is:
> > 
> > 64k * 7 = 448k
> > 
> > The chunksize I said is 64k...
> 
> Hmm. I always thought that the 'chunksize' is the limit which I/O must
> not cross to avoid being split.
> So for RAID 4/5/6 I would have thought this to be the stride size,
> as MD must split larger I/O onto two disks.
> Sure, one could argue that the stripe size is the chunk size, but then
> MD will have to split that I/O...

Yah, I think that makes sense. At least the way nvme uses "chunk_size",
it was assumed to mean the boundary for when the backend handling will
split your request for different isolated media.