Product manual

Theory of operations 45
recording the information to tape. This increases the amount of data that can be
stored on a finite medium and increases the overall storage efficiency of the system.
With data compression, the redundant information in a data stream is identified and
then represented by codewords or symbols, which allow the same data to be
recorded in a fewer number of bits. These symbols or codewords point back to the
original data string, using fewer characters to represent the strings. Because these
smaller symbols are substituted for the longer strings of data, more data can be
stored in the same physical space.
Some important benefits result from data compression in tape drives:
• The same amount of information can be stored on a smaller length of tape.
• More data can be stored on a given length of tape.
• Performance can more closely parallel to that of high-transfer-rate computers.
• More information can be transferred in the same time interval.
Data compression considerations
In an effective data-compression method, several factors are important:
• The amount of compression (measured by the compression ratio, which is a
ratio that compares the amount of uncompressed data to the amount of
compressed data and is obtained by dividing the size of the uncompressed data
by the size of the compressed data)
• The speed with which data is compressed and decompressed in relation to the
host transfer rate
• The types of data to be compressed
• The data integrity of the compressed data
The amount of compression possible in a data stream depends on factors such as
the data pattern, the compression algorithm, the pattern repetition length, the pattern
repetition frequency, the object size (block of information to be compressed) and the
starting pattern chosen.
The transfer rate depends on factors such as the compression ratio, the drive buffer
size, the host computer input/output (I/O) speed, the effective disc speeds of the
host computer and the record lengths that the host computer transmits.
Data compression algorithms can be tailored to provide maximum compression on
specific types of data. But because varying types of data are encountered in normal
day-to-day operating circumstances, 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 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