Re: [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  2:36pm -0400,
Alasdair G Kergon <agk@redhat.com> wrote:

> 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?

No idea, need Jon to weigh in here.  I'm hopeful we can impose 32bit
within dm-raid and coordinate with Neil on getting the appropriate MD
code (chunk_sectors) to reflect the reality that 32 bit is adequate.

> > 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.)

Right, it wasn't tested on 32bit.  It'll fail to build due to split_io
being sector_t.
 
> > 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.

Yeap, I'm hopeful we can go with the former.

Jon, what do you think?