The mid-layer queue depth handling is really designed/optimized around
behavior for
a JBOD. This, if it's a single-lun device, the LLDD could largely ignore
doing anything
with adjusting the queue depth.
However, for arrays, with multiple luns, the queue depth is usually a
target-level resource,
so the midlayer/block-layer's implementation falls on its face fairly
quickly. I brought this
up 2 yrs ago at storage summit. What needs to happen is the creation of
queue ramp-down
and ramp-up policies that can be selected on a per-lun basis, and have
these implemented
in the midlayer (why should the LLDD ever look at scsi command
results). What will make
this difficult is the ramp-up policies, as it can be very target
device-specific or configuration/load
centric.
In the meantime, if you look at any LLDD that is worth its salt, and it
will be implementing it's
own queue ramp-down and ramp-up algorithms internally. They will look
for QUEUE_FULLs
to ramp-down, and selecting a rate and methodology for the ramp-up. They
will use this routine
to do the queue depth changing.
-- james s
Christof Schmitt wrote:
I just came across the SCSI midlayer function scsi_track_queue_full.
If a SCSI command is returned with a status of QUEUE_FULL, then this
is mapped to ADD_TO_MLQUEUE and "device blocked". So, there is already
a mechanism in place. Is a LLD driver expected to additionally call
something like this to decrease the queue depth?
if (status_byte(scmd->result) == QUEUE_FULL)
scsi_track_queue_full(sdev, sdev->queue_depth - 1))
If a LLD does this, should it also increase the queue depth again when
no more QUEUE_FULL status are seen? To me this looks like a
duplication of the midlayer device blocking, but i assume there is a
reason in having both, scsi_track_queue_full and the device blocking.
--
Christof Schmitt
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