DSM/Tape Catalog User's Guide

Configuring DSM/TC
DSM/Tape Catalog User’s Guide — 520233-008
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Data Compression

To set compression off by default:
MC> ALTER MEDIADEFS, COMPRESSION OFF
Using DEFINEs to Override the System Compression Default
After you set the system compression default, the drive always writes using the default
setting. You can override the default specification for a specific job by using a
TAPECATALOG DEFINE with the COMPRESSION attribute.
The COMPRESSION attribute lets you override the system default specification on a
job-by-job basis. The COMPRESSION attribute has two settings:

COMPRESSION IDRC

COMPRESSION OFF
After using a TAPECATALOG DEFINE to temporarily override the system compression
default specification, the system returns to the default specification for the next job. If
you do not specify compression in the DEFINE, the system uses the default
specification set by the ALTER MEDIADEFS command. If no default specification was
set using the ALTER MEDIADEFS command, the default specification is OFF.
Examples
The first two examples show TAPECATALOG DEFINEs specified for backup
operations:

This example for a labeled tape specifies a TAPECATALOG DEFINE named
“=BACK” with compression on (IDRC). The BACKUP command copies all the files
in the $DATA volume to tape. The LISTALL option lists the names of all files
backed up, and the NOUNLOAD option directs BACKUP to rewind the final tape
and leave it online when the BACKUP operation is completed:
1>ADD DEFINE =BACK, CLASS TAPECATALOG, LABELS BACKUP, &
USE OUT, COMPRESSION IDRC, FILEID WEEKLY_BACKUP
2>BACKUP =BACK, $DATA.*.*, LISTALL, NOUNLOAD

This example for an unlabeled tape specifies a TAPECATALOG DEFINE named
“=UNLABEL” with compression off. For unlabeled tape, you must specify a device
in the DEFINE (DEVICE $TAPE). Because the example uses an unlabeled tape,
the LABELS attribute has the value OMITTED:
1>ADD DEFINE =UNLABEL, CLASS TAPECATALOG, &
LABELS OMITTED, CATALOG OFF, DEVICE $TAPE, &
COMPRESSION OFF
2>BACKUP =UNLABEL, $DATA.*.*, LISTALL, NOUNLOAD