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

Ewan D. Milne
2019-11-25T18:29:07Z

On Fri, 2019-11-22 at 10:14 -0800, Bart Van Assche wrote:
>
> Hi Ming,
>
> 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?
>

Some FC HBAs end up returning busy from ->queuecommand() but I think
this is more commonly due to there being and issue with the rport rather
than the device.

-Ewan