DSM/Tape Catalog User's Guide
Configuring DSM/TC
DSM/Tape Catalog User’s Guide — 520233-008
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Data Compression
The tape drive configured default compression setting (set using the SCF ALTER
TAPE command)
For drives that support a hardware override, the hardware setting takes precedence
over any software setting. The default hardware setting is to use the software setting.
The tape drive is reset to the default hardware setting prior to each tape media use.
Therefore, if you want a hardware override, you must manually apply it before each
use.
For example, if you use the DLT density button to select 2.6 or 6.0, compression is
turned off. Any higher setting enables compression at the specified level.
For drives that do not support compression or sof
tware control of compression, the
default system or tape drive compression setting is ignored, and attempts to control
compression are either ignored (for example, the DEFINE setting) or result in an error
(for example, file system error 2 in response to a SETMODE 162 call).
Setting the System Compression Default Using MEDIACOM
To set the system default specification for compression, use the MEDIACOM ALTER
MEDIADEFS command.
The compression attribute of the ALTER MEDIADEFS command has two settings:
COMPRESSION IDRC and COMPRESSION OFF (the default). If you specify IDRC,
any tape drive that has compression capability (supports compression and contains the
appropriate hardware) compresses data when writing. If a tape drive does not have
compression capability, specifying IDRC has no effect.
Example
To check and set the system compression default specification:
1. Display the current system setting for compression:
MC> INFO MEDIADEFS
Search List (\SKY, \SEA, \STAR)
File Catalog \SKY.FILE_CATALOG_SKY
Volume Catalog \SKY.MASTER_TAPE_CATALOG
Pool Name TAPES_FOR_SKY
Retention 90 DAYS
Abend Retention 0 DAYS
Support Level COMPATIBLE
Automated OFF
Compression OFF
Compression OFF means that, by default, compression is not attempted during a
write to a scratch tape volume.
2. Set compression on or off:
To set compression to be on by default:
MC> ALTER MEDIADEFS, COMPRESSION IDRC