Re: DM MULTIPATH: Allow dm to send larger request if underlying device set to larger max_sectors value

[Mike Snitzer <snitzer@redhat.com>]

On Mon, Jul 09 2012 at  9:16am -0400,
Mike Snitzer <snitzer@redhat.com> wrote:

> On Mon, Jul 09 2012 at  9:00am -0400,
> Mike Snitzer <snitzer@redhat.com> wrote:
> 
> > On Sun, Jul 08 2012 at  1:59pm -0400,
> > Chauhan, Vijay <Vijay.Chauhan@netapp.com> wrote:
> > 
> > > Even though underlying paths are set with larger value for max_sectors, dm 
> > > sets 1024(i.e 512KB) for max_sectors as default. max_sectors for dm 
> > > device can be reset through sysfs but any time map is updated, max_sectors
> > > is again set back to default. This patch gets the minimum of max_sectors from 
> > > physical paths and sets it to dm device.
> > 
> > There shouldn't be any need for additional DM overrides for max_sectors.
> > 
> > DM will stack the limits for all underlying devices each table reload
> > (via dm_calculate_queue_limits).  And max_sectors is properly stacked in
> > the block layer's bdev_stack_limits (called by dm_set_device_limits).
> > 
> > So is something resetting max_sectors with sysfs?  multipathd?
> 
> BLK_DEF_MAX_SECTORS = 1024
> blk_set_stacking_limits: lim->max_sectors = BLK_DEF_MAX_SECTORS
> 
> But that just establishes the default, the stacking done by
> blk_stack_limits will reduce 'max_sectors' accordingly based on the
> underlying paths' max_sectors.
> 
> I can clearly see that max_sectors is reduced according to the
> underlying device(s):

Ah, but you were saying max_hw_sectors and max_sectors may be larger
than 1024 and that you'd like to have the multipth device's max-sectors
reflect the larger values (not be capped by the block layer's
BLK_DEF_MAX_SECTORS).

Very interesting case that we haven't seen raised before.  This will
require block layer changes (DM will then get the change for free).

Mike