Quantum LTO-3 manual Intelligent Data, Compression

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 37