Theory
Data Compression
34
•
Pattern repetition frequency
•
Object size (block of information to be compressed)
•
Starting pattern chosen
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 provid-
ing optimum handling for all types of data.
Intelligent Data Compression
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 compres-
sion.
•
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.
•
First: the output data stream must be decompressible following LTO rules to create the input
sequence of records and File Marks perfectly.
•
Second: an LTO compressed data stream may not contain any of the eight reserved Control
Symbols.
•
Third: while control symbols allow switching to Scheme 2, this should never be used by opera-
tional software because this capability is only for diagnostic and testing purposes.
Software data compression should never be used because the LTO-2 half-height drive's built-in intelli-
gent data compression is much more efficient than software data compression systems.
The LTO-2 half-height drive uses a derivative of ALDC-2 lossless data compression that includes addi-
tional control codes for intelligent data compression.
