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Definition

The parameter tape_size always defines the physical tape size (tape length * write density). The physical tape size is the total volume (in MB or GB) of the data that can actually be written to a volume without compression. Consequently, different amounts of data can be stored on the same tape size, depending on whether you back up with or without compression. Since BRBACKUP or BRARCHIVE takes this information into account, you do not need to change the physical tape size if you use software compression or hardware compression (for tape devices with hardware compression).

Figure 1: Effect of Hardware Compression on Tape Volume Size

Pay attention to the following:

  • If you require several tapes for your database backup, BRBACKUP uses the value of parameter tape_size to distribute the files for backup to the volumes. During serial backups, the program makes sure that the physical end of tape is not reached. During parallel backups, the program also optimizes the file distribution.

  • A single file should not be bigger than the tape size (possibly after compression).

  • If parameter tape_size is configured too small, the backup program requires more volumes than are actually necessary.

  • When parameter tape_size is too large, cpio reaches the physical end of tape. See cpio Continuation Tape. In this case, you must correct the parameter value.

  • When using tape devices with hardware compression, always leave a reserve of around 10% of tape capacity for any errors in the calculation of the compression rate. The compression rate is estimated based on an analogous software compression and is therefore not exactly the same as the actual compression rate of a hardware compression.

    Caution

    The estimation errors can be larger after a database installation, a tablespace extension, or a reorganization. If the physical end of tape is reached in such a situation, reduce the value of tape_size by an additional 10 to 20% of tape capacity.

Example

Typical values for tape_size for tape devices with hardware compression:

Tape Type

tape_size

60 m DAT tape / 4 mm DDS-1

1200M

90 m DAT tape / 4 mm DDS-1

1800M

120 m DAT tape / 4 mm DDS-2

3800M

112 m Videotape / 8 mm

2000M

112 m Videotape / 8 mm/ high density

4500M

3590 IBM tape

10000M

DLT 2000 - 10/20 GB

10000M

DLT 2000XT - 15/30 GB

15000M

DLT 4000 - 20/40 GB

20000M

DLT 7000 - 35/70 GB

35000M

Make sure that you set the parameters required for using hardware compression.

Example

Typical values for tape_size for tape devices without hardware compression:

Tape Type

tape_size

60 m DAT tape / 4 mm DDS-1

1200M

90 m DAT tape/ 4 mm DDS-1

1800M

120 m DAT tape/ 4 mm DDS-2

3800M

112 m Video tape/ 8 mm

2000M

112 m Video tape/ 8 mm / high density

4500M

3490 IBM tape

700M