The process is very simple, and in fact is nothing to do with NBU.
NBU has no understanding of tape capacity, in fact, if left alone it would use the same tape 'for ever'.
NetBackup passes data to the operating system, one block at a time, to be written to the tape drive. NetBackup has no understanding of tape capacity. In theory, it would keep writing to the same tape "forever".
When the tape physically passes the logical end-of-tape, this is detected by the tape drive firmware. The tape drive firmware then sets a 'flag' in the tape driver (this would be the st driver in the case of Solaris). There is still enough physical space on the tape for the current block to be written, so this completes successfully. NetBackup then attempts to send the next block of data (via the operating system) but now the tape driver refuses, as the 'tape full' flag is set. The st driver then passes this 'tape full' message to the operating system, which passes it to NetBackup. Only when this has happened will Netbackup request the tape to be changed.
Or put more simply, the tape becoming full cannot be controlled in anyway by NBU. Additionally, the amount of data written to the tape is dependant on how 'compressible' the data is, and again, cannot be controlled by NBU.
Low amounts of data written to a tape can have various causes ...
1. Be totally normal (data is not very compressible)
2. Drive hardware fault
3. Tape driver fault
4. Tape firmware fault
Martin
