Re: [PATCH] block: add support for shared tag maps

[Mike Anderson <andmike@us.ibm.com>]

Doug Maxey <dwm@enoyolf.org> wrote:
> 
> Ravi,
> 
> While working on a patch to add shared tags to qla4xxx was looking at
> the shost->can_queue settings, I see the value is set pretty high:
> 
> qla4xxx_probe()
> ...
> 	host->can_queue = REQUEST_QUEUE_DEPTH + 128;
> 
> where REQUEST_QUEUE_DEPTH works out to be 1024.
> 
> My question:
> what is the relationship between the can_queue and the
> setting in
> qla4xxx_slave_configure()
> 	if (sdev->tagged_supported)
> 		scsi_activate_tcq(sdev, 32);
> 	else
> 		scsi_deactivate_tcq(sdev, 32);
> 
> Does this imply that the firmware can ultimately track more requests
> than we can possibly stuff in it?  Where do the other 1012 requests get
> queued, in the block layer?

Maybe I misreading your question. host->can_queue is the per host instance
(adapter) limit and scsi_activate_tcq will set the per dev (lun) limit.

host->can_queue being large is a good thing (if it is backed by real
resources). In theory can_queue should be scaled to support a hosts
${max_number_of_devices} * ${max_queue_depth}.

-andmike
--
Michael Anderson
andmike@us.ibm.com