A Bacula output file consists of Blocks of data. Each block contains a block header followed by records. Each record consists of a record header followed by the record data. The first record on a tape will always be the Volume Label Record.
No Record Header will be split across Bacula blocks. However, Record Data may be split across any number of Bacula blocks. Obviously this will not be the case for the Volume Label which will always be smaller than the Bacula Block size.
To simplify reading tapes, the Start of Session (SOS) and End of Session (EOS) records are never split across blocks. If this is about to happen, Bacula will write a short block before writing the session record (actually, the SOS record should always be the first record in a block, excepting perhaps the Volume label).
Due to hardware limitations, the last block written to the tape may not be fully written. If your drive permits backspace record, Bacula will backup over the last record written on the tape, re-read it and verify that it was correctly written.
When a new tape is mounted Bacula will write the full contents of the partially written block to the new tape ensuring that there is no loss of data. When reading a tape, Bacula will discard any block that is not totally written, thus ensuring that there is no duplication of data. In addition, since Bacula blocks are sequentially numbered within a Job, it is easy to ensure that no block is missing or duplicated.
Kern Sibbald 2010-08-30