Drive preference in a mixed-media library sharing environment

TSM supports multiple media types in a single library definition. It handles these through media type information reported by the physical library,
combined with the drive read and write format lists provided by the drives. Unfortunately, in this configuration, TSM will choose the most-capable drive
to place a volume in, rather than the least-capable drive. For instance, if a library has both a LTO5 and LTO6 drive available, and a process or session
needs a LTO5 volume loaded, TSM will choose the LTO6 drive rather than the LTO5 drive. If a subsequent LTO6 volume mount request comes in, it will have
to wait until the LTO6 drive is available. If TSM had placed the LTO5 volume in the LTO5 drive, then the LTO6 drive would be available for LTO6 requests.

The documented workaround for this for 3592 libraries[1] is to have one TSM library definition per media type. An administrator would then use the
media-type-specific library definition when creating device classes, and check media into the media-specific library definition. While this solves part
of the problem, it does not allow the administrator to take advantage of the backwards compatibility available with tape drives. For instance, with
the above example, if a second LTO5 request comes in, this workaround would force the request to wait until the LTO5 drive became available, rather than
allowing it to be mounted in the LTO6 drive (but only after the LTO5 drive became available).

The enhancement I would propose is this - volume mount requests should be loaded into the least-capable available drive rather than the most-capable drive.
Capability would be measured by comparing how far down the read or write format list TSM would have to search to match the request media. Taking the above
example, loading a LTO5 volume into a LTO6 drive would require going three levels of the format list (ULTRIUM6C, ULTRIUM6, ULTRIUM5C), but only one for the
LTO5 drive (ULTRIUM5C).

This problem is compounded by the fact that the "Mixing LTO drives and media in a library"[2] document does not actually mention the workaround mentioned in
the 3592 document. If everything else in this RFE is rejected, this document should be enhanced to include the workaround described in the 3592 document.

[1] http://www-01.ibm.com/support/knowledgecenter/SSGSG7_6.3.0/com.ibm.itsm.srv.doc/t_devclass_3592_generation_mix_ulw.html
[2] http://www-01.ibm.com/support/knowledgecenter/SSGSG7_6.3.0/com.ibm.itsm.srv.doc/t_devclass_lto_generations_mix_ulw.html