What does SmartDisk "Stream count (0 defunct)" mean? (4033174)

In the NetVault Backup GUI, under Device Management > right click SmartDisk > select STATUS

What does the "Stream count (0 defunct)" really mean?

- Stream count = the number of active streams currently writing or reading to or from SmartDisk

- Defunct stream = the number of streams that have finished sending or receiving data to/from SmartDisk, but are in a 30 second count down awaiting complete closure.

Explanation:

SmartDisk and the NVBU Media Manager have rules to control and monitor the number of concurrent streams SmartDisk receives.
By default, SmartDisk is set to receive a maximum of 8 streams, but this can be configurable using the following command:
smartdisk config --stream-limit xx

If you run 8 jobs at the same time to SmartDisk, whether they are backups or restore, the Stream Count will be 8.

Now be aware that there is a 10 second gap between streams before they are assigned, which means backups will always have to wait 10 seconds before getting a stream from SmartDisk.
This is to ensure SmartDisk doesn't suddenly get assigned too many streams in cases where there are many media requests and multiple NVBU servers using the same NVSD instance.

When a stream is closed, SmartDisk has a rule of keeping this stream reserved for the same job for 30 seconds, this is similar to the NVBU Media session locks for tapes.
This means that even if all data in a stream have been sent, SmartDisk won't close that stream immediately to assign it to other jobs,
instead it will still count this stream as defunct for 30 seconds after the stream closes, in case it needs to be reused by the same job.
This mechanism is implemented to accommodate plugins which perform multistreams (such as database plugins for Oracle, Exchange... ),
In the event such plugins need to start another stream shortly after the previous stream completed, this ensures it will get access to the same reserved stream and other jobs' media requests won't acquire it, exhausting the maximum stream limit and hanging the already active job.