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

[Alasdair G Kergon <agk redhat com>]

On Mon, Apr 30, 2012 at 01:24:00PM -0400, Mike Snitzer wrote:
> 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.
 
But there's overlap between merge_bvec and split_io.
  - Why does stripe_merge() have:

        if (!q->merge_bvec_fn)
                return max_size;

    when it's already done the division?

    - Couldn't that be changed to avoid split_io causing a split?
        (Except, as ever, across a table reload, which prevents us
	 removing it completely.)

> 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.
 
But is that enforced in the raid code or not?

> But what I think what you're driving at is: 

(I'm not convinced the proposed patch has been tested on 32-bit+LBD,
attempting to divide by a 64-bit number etc.)

> 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?

EITHER the 32 bit split_io *must* be enforced (after we've convinced
ourselves 64 bits will never be required);
OR we keep it 64-bit and add some compat code.

Alasdair