Re: [PATCH 01/18] libmultipath: Add prioritizer context data

[Hannes Reinecke <hare@suse.de>]

On 10/22/2015 12:16 AM, Benjamin Marzinski wrote:
> 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.
> 
Well ... the information is already cached in sysfs; I doubt read()
is that much of an overhead.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		               zSeries & Storage
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