Chapter 4 Theory

Data Compression

The transfer rate depends on factors such as:

Compression ratio

Drive buffer size

Host computer input/output (I/O) speed

Effective disc speeds of the host computer

Record lengths that the host computer transmits

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.

Intelligent Data

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

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.

LTO-3 Tape Drive User’s Guide

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Quantum LTO-3 manual Intelligent Data, Compression