Re: [PATCH 14/17] scsi_dh_alua: Use workqueue for RTPG

[Hannes Reinecke <hare@suse.de>]

On 05/11/2015 03:49 PM, Christoph Hellwig wrote:
> On Mon, May 04, 2015 at 02:42:20PM +0200, Hannes Reinecke wrote:
>> The current ALUA device_handler has two drawbacks:
>> - We're sending a 'SET TARGET PORT GROUP' command to every LUN,
>>   disregarding the fact that several LUNs might be in a port group
>>   and will be automatically switched whenever _any_ LUN within
>>   that port group receives the command.
>> - Whenever a LUN is in 'transitioning' mode we cannot block I/O
>>   to that LUN, instead the controller has to abort the command.
>>   This leads to increased traffic across the wire and heavy load
>>   on the controller during switchover.
>>
>> With this patch the RTPG handling is moved to a workqueue, which
>> is being run once per port group. This reduces the number of
>> 'REPORT TARGET PORT GROUP' and 'SET TARGET PORT GROUPS' which
>> will be send to the controller. It also allows us to block
>> I/O to any LUN / port group found to be in 'transitioning' ALUA
>> mode, as the workqueue item will be requeued until the controller
>> moves out of transitioning.
> 
> I'm having a hard time understanding the workqueue use here.
> What is the benefit of that one worker function to do everything?
> It seems having a work struct in struct alua_queue_data to just
> run STPG, and a different one to run RPTG in the port group structure
> would be more sensible instead of interwinding them.
> 
Hmm. It _might_ be possible. However, we need to serialize
STPG and RTPG against each other; not sure if that's still possible
when using different workqueue items.

> Also why do you need the sigle threaded workqueue?  That seems
> like a possible limiting factor in a large enough system having
> to deal with a cable disconnect cutting off multiple port groups,
> or just during bootup.
> 
Okay, will be switching to a normal workqueue.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		               zSeries & Storage
hare@suse.de			               +49 911 74053 688
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
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