Quantum LTO-2 User Manual

Chapter 4  Theory

Data Compression

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

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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.

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

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.