You must place all dumps of a given fileset family (both full and incremental) onto tapes that are readable by a single tape drive. This is required because a single Tape Coordinator performs an entire restore operation, using a full dump set and any incremental dump sets as necessary. If a single Tape Coordinator cannot read all of the tapes on which the dump sets are recorded, you cannot restore all of the dumps of the fileset family.
For example, suppose the full dump of a fileset family is stored on 8-mm tape and the incremental dumps, which are done at a different time, are stored on streaming cartridge tape. When you restore a fileset from that fileset family, you must use a Tape Coordinator that uses 8-mm tapes because a restore always begins with the full dump. However, you cannot restore any of the incremental dumps because the same Tape Coordinator cannot read the streaming cartridge tapes and you cannot switch to another Tape Coordinator to continue the restore operation.
Before performing a backup, make sure the tapes are at least as large as the tape size listed in the TapeConfig file for the tape drive to be used for the operation. The Backup System fills a tape only with the amount of data listed as the capacity for the drive in the TapeConfig file. If a tape is larger than the tape size listed in that file, it simply is not filled to capacity when the backup is performed. However, if the tape is smaller than the size listed in the TapeConfig file, the backup operation fails, but only after it fills the tape and determines that it is too small for the drive.
A dump set does not have to fit entirely on a single tape; if the Backup System reaches the end of a tape while dumping a fileset from a fileset family, it puts the remaining data on another tape. The Backup Database automatically records that the fileset resides on multiple tapes.
Prior to performing a backup, you can preview the effects of your command without having the system actually perform the dump. Simply include the -noaction option with the bak dump command, specifying the remaining options as you would to really execute the dump. This lets you check a fileset family's size before actually dumping it so that you can calculate the correct number of tapes needed.