HP 3000 Manuals HP 3000 Manuals
Compressing Data
Backup performance can be improved through the use of host data
compression. By specifying the COMPRESS parameter, the host system
filters the data through a compression algorithm prior to writing it to
the backup device. This operation has two effects:
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1. The device's throughput is increased by transferring more data
than the device's maximum sustained rate.
2. Fewer media changes are required by putting more data on each
piece of media.
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Two levels of compression are supported by the COMPRESS parameter; HIGH
and LOW.
NOTE No matter whether you specify HIGH or LOW, the actual level of data
compression is dependent upon the type of data being compressed.
For example, program files will not compress as much as data files.
You must determine which of the compression algorithms best suits your
needs. In some cases the low-compression algorithm may perform almost as
well as the high-compression algorithm.
It is advisable to try both algorithms on your data in order to evaluate
which algorithm best suits your needs. Some of the factors to consider
are:
* Elapsed time of backup.
* Amount of CPU time required.[REV BEG]
* Amount of media required.[REV END]
With the high-compression algorithm running, backup takes most of the
processing capabilities of the machine. With the low-compression
algorithm running, other activity can coexist with backup.
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Use the high-compression algorithm if backup time is dedicated or if the
difference in the amount of media used for backup is substantial enough
to warrant the complete consumption of the machine.[REV END]
When compressed format files are restored, RESTORE automatically
recognizes the compress format and restores the files properly.
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To compress data when storing files to tape, use the FILE command, then
use the COMPRESS parameter in your STORE command. For example,
FILE T;DEV=TAPE
STORE @.OPERATOR.SYS;*T;COMPRESS=HIGH
To compress data when storing files to magneto-optical disks, use the
MOSET and COMPRESS parameters in your STORE command. For example,
STORE @.@.@;;MOSET=(101);NAME=DAILY.D24SEPT.TIGER;COMPRESS=HIGH
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