Data Capacity With Data Compression - Sun Microsystems SPARCstorage Library Installation And User Manual

Data Capacity with Data Compression

Hardware Data Compression
Software Data Compression
The data capacity of drives with data compression is dependent on the
redundancy of the stored data. Sun specifies capacity figures that assume a
compression ratio of two to one (2:1). This ratio is typical of data stored on a
computer system. The compressibility of data can vary. Text and binary files
tend to compress at about a three to one (3:1) ratio. Image files are typically not
redundant and do not compress well. Typical overall compression ratios are
near 2:1.
Data compression can be done by the drive's internal hardware-based data
compression. This is specified by the format/device identifier as shown in
Table D-1. One disadvantage of this approach is that this data format is not
readable by the earlier 8500 or 8200 drives because they do not incorporate
hardware data compression.
Note – Never use two compression methods on the same data. The use of a
second compression method rarely compresses the data further; double
compressed data can actually expand in size.
Data compression can be done via software. The compress and restore
commands are used for writing and reading, respectively. The Solstice Backup
package incorporates the capability to do software-based data compression.
Software data compression results in a better compression ratio than the
drive's internal data compression but takes significantly longer.
Note – Never use two compression methods on the same data. The use of a
second compression method rarely compresses the data further; double
compressed data can actually expand in size.
General 8mm Tape Drive Information
D
D-3