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

Martin K. Petersen
2019-11-22T02:25:45Z

Ewan,

> Delaying the queue re-run vs. a ramp down might negatively affect
> performance. I'm not sure how accurate the ramp is at discovering the
> optimum though.

The optimum - as well as the actual limit - might change over time in a
multi-initiator setup as other hosts grab resources on the device.

I do think that the only way forward here, if we want to avoid counting
outstanding commands for performance reasons, is to ensure that the
BUSY/QUEUE_FULL/TASK_SET_FULL handling is relatively fast path and not
something deep in the bowels of error handling. Which it actually
isn't. But I do think we'll need to take a closer look.

--
Martin K. Petersen Oracle Linux Engineering