[dm-devel] [PATCH 01/18] libmultipath: Add prioritizer context data

[Benjamin Marzinski]

On Mon, Oct 12, 2015 at 08:35:22AM +0200, Hannes Reinecke wrote:
> On 10/08/2015 09:44 PM, Benjamin Marzinski wrote:
> > Currently, running the alua prioritizer on a path causes 5 ioctls on many
> > devices.  get_target_port_group_support() returns whether alua is
> > supported. get_target_port_group() gets the TPG id. This often takes two
> > ioctls because 128 bytes is not a large enough buffer size on many
> > devices. Finally, get_asymmetric_access_state() also often takes two
> > ioctls because of the buffer size. This can get to be problematic when
> > there are thousands of paths.  The goal of this patch to to cut this down
> > to one call in the usual case.
> > 
> > In order to do this, I've added a context pointer to the prio structure,
> > similar to what exists for the checker structure, and initprio() and
> > freeprio() functions to the prioritizers. The only one that currently uses
> > these is the alua prioritizer. It caches the type of alua support, the TPG
> > id, and the necessary buffer size.  The only thing I'm worried about with
> > this patch is whether the first two values could change.  In order to deal
> > with that possibility, whenever a path gets a change event, or becomes
> > valid again after a failure, it resets the context structure values, which
> > forces all of them to get checked the next time the prioritizer is called.
> > 
> Hmm. What about reading /sys/block/sdX/device/vpg_pg83 ?
> That carries the same information, and you would need to call the
> ioctl only once ...

Sure. If you want to write a patch, that would be fine by me. But I
still think caching the result, so you don't need to rerun this makes
sense.

-Ben

> Cheers,
> 
> Hannes
> -- 
> Dr. Hannes Reinecke		               zSeries & Storage
> hare at suse.de			               +49 911 74053 688
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