Chapter 4 Theory

Data Compression

 

 

Data compression algorithms can be tailored to provide maximum

 

 

compression for specific types of data. Because varying types of data are

 

 

encountered in normal day-to-day operating circumstances, however, an

 

 

effective data compression method for a tape drive must serve various

 

 

data types. Additionally, the data compression method must adapt to

 

 

different data types, automatically providing optimum handling for all

 

 

types of data.

 

The tape’s compressed capacity is maximized through the use of

 

Intelligent Data

Compression

intelligent data compression. The intelligent data compression hardware

 

 

determines the compressibility of each record. If the size of the record is

 

 

larger after a compression attempt than the native (uncompressed) size,

 

 

then the record is written in its native form.

 

 

The intelligent data compression utilizes two compression schemes:

 

 

Scheme-1 is a LZ1 based compression scheme using a history buffer

 

 

to achieve data compression.

 

 

Scheme-2 is a pass-through compression scheme designed to pass

 

 

uncompressible data through with minimal expansion.

 

 

There are three specific requirements for compliance with the LTO

 

 

specification.

 

 

• The output data stream must be decompressible following LTO rules

 

 

to create the input sequence of records and File Marks perfectly.

 

 

• An LTO compressed data stream may not contain any of the eight

 

 

reserved Control Symbols.

 

 

• While control symbols allow switching to Scheme 2, this should

 

 

never be used by operational software because this capability is only

 

 

for diagnostic and testing purposes.

 

 

Software data compression should never be used because the LTO-2

 

 

Half-Height Tape Drive's built-in intelligent data compression is much

 

 

more efficient than software data compression systems.

 

 

The LTO-2 Half-Height Tape Drive uses a derivative of ALDC-2 lossless

 

 

data compression that includes additional control codes for intelligent

 

 

data compression.

LTO-2 Half-Height Tape Drive User’s Guide

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Quantum LTO-2 manual Intelligent Data, Compression