Each backup version contains a label for its identification. The label is automatically provided by CONTROL. It specifies the type of backup (complete or incremental backup of data or log), the backup generation, the sequence number of the last completed log segment, a version number, and a sequence number of each tape.
The version number puts the backups in an order relevant for a restore. For Backup / Save / Log Segment, the version number indicates the time when the log segment was completed; for all the other types of backup, the version number is equivalent to the generation time of the backup.
The medium label uniquely identifies the backups done since the installation of the database. It must be written down on the sticker of the tape or cassette after the backup terminates.
| Example 1: | Labels for the first backups after a first installation of the database |
Example 2:
Labels between the first two complete backups. The tape for the automatic backup of log segments became full, and the backup was only restarted after the second complete backup.
Example 3:
Labels between two complete backups. To save the log between the complete backups, the Backup / Save / Log menu function is used instead of the automatic backup of the log segments.
| DATA_A0_A | First complete backup of the database (SAVEDATA), first generation (A), first tape (A). |
| DATA_D0_A | Complete backup of the database (SAVEDATA) of the fourth generation (D). |
| DATA_A1_A | Incremental backup of the data (SAVEPAGES), the first tape (A) of it. When this backup is used for a recovery, a consistent database is restored without having to load another backup of the log. |
| LOG_A2_1 | Backup of the log after a log segment was completed. |
| DATA_B0_B | Complete backup of the database of the second generation (B), the second tape of it written simultaneously with or after the first tape. |
| LOG_B1_1 | Backup of the log of the second generation (B); for a restore, it must be loaded after the DATA_B0_A backup (and following: DATA_B0_B ...), unless there is a SAVEPAGES backup. |
| DATA_B2_A | SAVEPAGES backup of the second generation; for a restore, it can be loaded after the DATA_B0_A backup (and following: DATA_B0_B ...) instead of the log. |
For a backup started immediately (ad hoc), the label is displayed in the protocol file at the end of backup. When restoring, the label is displayed for confirmation before the restore procedure starts; it is also included in the protocol files where each save and restore action is recorded. Later, after concluding the backup, the label can be read at any time by means of the Media Manager (see also Section Backup / Media Manager).
Before you start an immediate backup to tape, you should always read the label of the backup that could already be on the tape. In this way, you can make sure that the backup is written to a tape belonging to the same generation; what is recommended.
You should always use the same tape for each type of save and generation. After some time, the sticker of a SAVEDATA backup written to the tape could look as follows:
| Example: | Sticker of the first tape of a SAVEDATA backup of the first generation (A), after using the tape six times. Of course, you must take care that the tapes and cassettes are not used more times than is recommended by the manufacturers. |
Rules for Medium Labels
1. Data backups (Backup / Save / Data or Backup / Save / Updated Pages) are identified by "data"; log backups (Backup / Save / Log Backup / Save / Log Segment) by "log". The identification of the backup is part of the backup label.
2. Each backup belongs to a backup generation. The generation of the backup can be recognized by the generation letter. All backups with the same letter belong to the same generation. The generation letter is part of the backup label. A complete backup of the database starts the next backup generation. The next letter is used.
3. Up to 26 backup generations can be defined (A, ... Z). For n generations defined, the (n+1)st "save / data" is started again with the medium label DATA_A.
4. The log segments of a database are counted sequentially starting at the time of database installation. A database without an explicitly defined log segment size is considered as a database with one log segment. Each backup is assigned the number of the last completed log segment. The first log segment has the number 1.
5. Data backups can be written to several tapes either in parallel or consecutively. Up to 32 backup media can be operated in parallel. The tape devices accessed by the backup media can be supplied with tapes several times. The sequence number of the tape is part of the backup label. Backups of the log can only be written consecutively.
Formal Description of the Medium Labels
<medium label> ::=
<save kind>_<generation>_<log>_<version>_<count>
<save kind> ::=
DATA | LOG
<generation> ::=
A .. Z
<version> ::=
0 .. 32676
<count> ::=
A..Z | 1..32
where A..Z for DATA and 1..32 for LOG