Databases Reference
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stored in a repository for future reference. This eliminates the need to
rerun compression analysis by allowing the user to call up and display the
results of a previous analysis. These results can then be compared with a
current analysis in order to select the optimum compression method.
Another product, Data Packer from BMC Software, can reduce DASD
requirements for DB2 tables an average of 30 to 80%. Additional hardware
compression techniques can result in another 10% reduction.
Data Packer offers a choice of compression techniques to choose from,
including
• Hardware compression, which exploits the hardware compression fa-
cility of IBM products or, when hardware is not available, provides
software emulation;
• Custom dictionary for hardware, which builds a dictionary based on
the unique characteristics of the actual data, resulting in higher com-
pression — up to 90% — while reducing overhead;
• Dynamic adaptive, which analyzes data and chooses either the Static
Huffman or Extended Character technique for compression;
• Extended character, which recognizes repeating strings of characters
and reduces them to a smaller symbolic character;
• Static Huffman, which utilizes static compression tables that are built
using representative samples of typical business data;
• Custom Huffman, which builds custom compression tables using rep-
resentative samples of actual data;
• Repeating character, which identifies and eliminates repeating charac-
ters to achieve higher compression.
With Data Packer, there is no need to drop tables to add or change com-
pression. This eliminates the downtime associated with DB2 compression.
Data that has already been compressed is never recompressed, and there
is no attempt to expand uncompressed data.
BMC Software also offers an analysis tool that lets the user determine
the most effective compression technique by trying multiple techniques
for one table in one pass. It can perform trial compression on an uncom-
pressed table, verify compression results by confirming the actual amount
of compression, and generate customized EDITPROCs that create optimal
compression for different types of data.
IBM Solutions
IBM has been offering DASD/DB2 compression since the early 1990s. The
company even offers a data compression program for VM/ESA, enabling
VM users to compress and expand CMS minidisk file data automatically
and transparently. This capability can help reduce DASD utilization, load
on the I/O subsystem, and staff time spent managing minidisk space and
monitoring “disk-full” conditions.
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