Re: About scsi device queue depth

Martin K. Petersen
2021-01-13T06:09:15Z

James,

> A co-operative device can help you find the optimal by returning
> QUEUE_FULL when it's fully occupied so we have a mechanism to track
> the queue full returns and change the depth interactively.

Many disk drives and SSDs only return QF/TSF when they are out of actual
command slots (so typically 255). There is often a watertight barrier
between the transport-specific command processing and the media
management code. Very sad.

So while the exponential backoff approach works great for disk arrays,
it's hit or miss with drives. And it's really hard to come up with a
good heuristic since the optimal queue depth is highly workload and
device-specific.

We had a few folks looking at managing write queue depth based on
completion latency a couple of years ago but it didn't come to fruition.

--
Martin K. Petersen Oracle Linux Engineering