geom <-> cam disk

[Andriy Gapon]

Preamble.  I am trying to understand in detail how things work at GEOM <-> "CAM
disk" boundary.  I am looking at scsi_da and ata_da which seem to be twins in
this respect.

I got an impression that the bioq_disksort calls in the strategy methods and the
related queues are completely useless in the GEOM single-threaded world.
There is only one thread, g_down, that can call a strategy method, the method
enqueues a bio, then calls a schedule function and through xpt_schedule the call
flow continues to a start method which dequeues the bio and off it goes.
I currently can see how a bio queue can accumulate more than one bio.

What am I missing? :-)
I will be very glad to learn more about this layer if anyone is willing to
educate me.
Thank you in advance.

P.S. I wrote a very simple to DTrace script to my "theory" experimentally and my
testing with various workloads didn't disprove the theory so far (which doesn't
mean that it is correct, of course).

The script:
fbt::bioq_disksort:entry
/args[0]->queue.tqh_first == 0/
{
        @["empty"] = count();
}

fbt::bioq_disksort:entry
/args[0]->queue.tqh_first != 0/
{
        @["non-empty"] = count();
}

It works on all bioq_disksort calls, but I stressing only ada disks at the moment.
-- 
Andriy Gapon