How does NetBackup choose which active tape to use
NetBackup 6.5.5, RHEL4.7.
I'm trying to figure out how NetBackup will choose which tape to write to. I've looked at article HOWTO32854 (About selecting media in robots on UNIX/Linux), and the section in the admin guide II "About selecting media in robots on UNIX/Linux" (both the same content, it appears), but they appear to be incomplete to me. Specifically I'm looking to find out which of several active, unmounted tapes currently in the robot get chosen.
The docs say that first a mounted tape is looked for, then the media catalog is checked - but all it says is that if a suitable volume is found, it is used. What if several suitable volumes are found? Now does NBU decide? It doesn't appear to be least recently used, nor numeric order.
What I see is that I have 5-7 active tapes is a specific volume pool at a certain retention level. A backup starts, calling for the pool and retention level matching the active tapes, and NBU seems to be shunning certain media. One of the tapes has been sitting idle for a month, while other tapes get written to and new tapes are brought into the pool (the new tapes are for images spanning media, so I understand whey new tapes are brought in instead of using active tapes).
Can anyone offer any insight?