Re: [PATCH 5/6] scsi: core: don't limit per-LUN queue depth for SSD when HBA needs

["Martin K. Petersen" <martin.petersen@oracle.com>]

Ming,

> NVMe doesn't have such per-request-queue(namespace) queue depth, so it
> is reasonable to ignore the limit for SCSI SSD too.

It is really not. A free host tag does not guarantee that the target
device can queue the command.

The assumption that SSDs are somehow special because they are "fast" is
not valid. Given the common hardware queue depth for a SAS device of
~128 it is often trivial to drive a device into a congestion
scenario. We see it all the time for non-rotational devices, SSDs and
arrays alike. The SSD heuristic is simply not going to fly.

Don't get me wrong, I am very sympathetic to obliterating device_busy in
the hot path. I just don't think it is as easy as just ignoring the
counter and hope for the best. Dynamic queue depth management is an
integral part of the SCSI protocol, not something we can just decide to
bypass because a device claims to be of a certain media type or speed.

I would prefer not to touch drivers that rely on cmd_per_lun / untagged
operation and focus exclusively on the ones that use .track_queue_depth.
For those we could consider an adaptive queue depth management scheme.
Something like not maintaining device_busy until we actually get a
QUEUE_FULL condition. And then rely on the existing queue depth ramp up
heuristics to determine when to disable the busy counter again. Maybe
with an additional watermark or time limit to avoid flip-flopping.

If that approach turns out to work, we should convert all remaining
non-legacy drivers to .track_queue_depth so we only have two driver
queuing flavors to worry about.

-- 
Martin K. Petersen	Oracle Linux Engineering