Re: [PATCH 4/4] scsi: core: don't limit per-LUN queue depth for SSD

James Smart
2019-11-22T18:26:19Z

On 11/22/2019 10:14 AM, Bart Van Assche wrote:
> Thanks for having shared these numbers. I think this is very useful
> information. Do these results show the performance drop that happens
> if /sys/block/.../device/queue_depth exceeds .can_queue? What I am
> wondering about is how important these results are in the context of
> this discussion. Are there any modern SCSI devices for which a SCSI
> LLD sets scsi_host->can_queue and scsi_host->cmd_per_lun such that the
> device responds with BUSY? What surprised me is that only three SCSI
> LLDs call scsi_track_queue_full() (mptsas, bfa, esp_scsi). Does that
> mean that BUSY responses from a SCSI device or HBA are rare?

That's because most of the drivers, which had queue full ramp up/ramp
down in them and would have called scsi_track_queue_full() converted
over to the moved-queue-full-handling-in-the-mid-layer, indicated by
sht->track_queue_depth = 1.

Yes - it is still hit a lot!

-- james