The behavior that you are seeing is actually can found in the TSM 5.3
Administrator's guide, chapter 10. Managing Storage Pools and volumes under
"How the Server Selects Volumes with Collocation Disabled"
According to the manual:

1. A previously used sequential volume with available space (a volume with
the most amount of data is selected first)
2. An empty volume

I have not monitored what would happen after it finishes writing to an
empty volume... but I suspect it will grab next volume with the most amount
of data (fill status) and cycle repeats.
That being said, if one is trying to minimize the # of tapes to be sent
offsite it will require some manual work whether move data or start
reclamation before tapes are ejected for offsite.

Sung Y. Lee
E-mail  sunglee AT us.ibm DOT com

"ADSM: Dist Stor Manager" <ADSM-L AT VM.MARIST DOT EDU> wrote on 02/08/2006
11:08:52 AM:

> Actually, I think I remember this coming up once before, and it's WAD.
> (That's Working as Designed, not Working as Desirable.)
>
> But I don't know where to find any doc for it.
>
>
> -----Original Message-----
> From: ADSM: Dist Stor Manager [mailto:ADSM-L AT VM.MARIST DOT EDU] On Behalf 
> Of
> William Boyer
> Sent: Tuesday, February 07, 2006 10:55 PM
> To: ADSM-L AT VM.MARIST DOT EDU
> Subject: Processes calling for scratch instead of an existing FILLING
> tape
>
>
> I've noticed that processes will fill a tape and then instead of calling
> for another FILLING tape in the storage pool, it will mount
> a scratch tape. I just saw this again this morning...I had a BA STG
> running from onsite tape to offsite tape. There were 4 tapes in
> the offsite pool...2 filling.. 1 full..and a filling tape being written
> to. The current tape filled up and instead of mounting one
> of the other 2 filling tapes, a new scratch tape was called for. And one
> of the filling tapes was only 1.4% used!
>
> I search the TSM site and the archives, but I must not have the correct
> search words, 'cause I can't find any hits. My TSM server is
> 5.3.2.1 on AIX 5.3 ML2.
>
> Bill Boyer
> "Growing old is mandatory, growing up is optional" - ??