Re: [dm-devel] [PATCH 1/2] dm: update max_io_len to support a split_io that is not a power of 2

[Mike Snitzer <snitzer redhat com>]

On Mon, Apr 30 2012 at 12:10pm -0400,
Alasdair G Kergon <agk redhat com> wrote:

> On Sat, Apr 28, 2012 at 12:44:28AM -0400, Mike Snitzer wrote:
> > Required to support a target's use of a non power of 2 blocksize.
>  
> For which targets?

striped and thin-pool for starters.

> (merge_bvec supported?)

Yes.

> > +		boundary = ti->split_io - do_div(tmp, ti->split_io);
> 
> sector_div()?
>
> What about 32-bit arch + LBD + large split_io (from raid?)
>   - Is a 32-bit restriction on split_io unreasonable nowadays?
>     - OR reasonable on 32bit/LBD?
>     - OR fallback to old code there?

I cannot see why we'd need a split_io that is larger than 32 bits -- a
32bit split_io can support up to 2TB (2**32 * 512b sectors).  Even
on a LBD (raid) the stripe size (split_io) will not be so large.

(though yes we would need to establish a check in DM core that split_io is
limited to 32-bit -- even though the 'sector_t' is used for split_io;
and the comment inside the 'struct dm_target' would need updating).

But what I think what you're driving at is: is there a benefit/reason to
maintain the old code for some target that won't ever use non power of 2
split_io (e.g. dm-raid at the moment)?  I see no point for the duality
in the code but I'm open to the idea if you have a specific reason in
mind -- are you concerned about perf on more obscure/older hardware?