Quantum LTO-2 manual Intelligent Data, Compression

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 33