User's Manual

44
DAT 72/DDS-4 Product Manual
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.
Considering these factors, Seagate engineers concluded:
The most effective data compression method must compress as much data as
possible while assuring that
•
The transfer rate of the host computer is not impeded.
•
Adaptation is made to different types of data.
•
Data integrity is maintained.