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Theory of operations
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 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.
•
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 operational software because this capability is only for diagnostic and
testing purposes.
Software data compression should never be used because the Viper drive handles it
much more efficiently.
The Viper 200 uses a derivative of ALDC-2 lossless data compression that includes
additional control codes for intelligent data compression.
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