(from the Windows Administrator's Guide, page 87)

Directory-Level Backup and Restore

If you have a large NAS file system, initiating a backup on a directory
level will reduce backup and restore times and provide more flexibility
in configuring your NAS backups. By defining virtual file spaces, a file
system backup can be partitioned among several NDMP backup operations
and multiple tape drives. You can also use different backup schedules to
back up sub-trees of a file system. 

The virtual file space name cannot be identical to any file system on
the NAS node. If a file system is created on the NAS device with the
same name as a virtual file system, a name conflict will occur on the
Tivoli Storage Manager server when the new file space is backed up. See
the Administrator's Reference for more information about virtual file
space mapping commands.

Note: Virtual file space mappings are only supported for NAS nodes.

--
Mark Stapleton (stapleton AT berbee DOT com)
Office 262.521.5627