Data Compression Considerations

In an effective data-compression method, several factors are important:

•The amount of compression, which is measured by the compression ratio. This ratio compares the amount of uncompressed data to the amount of compressed data. It 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 relative 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:

•Data pattern

•Compression algorithm

•Pattern repetition length

•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 providing optimum handling for all types of data.

Intelligent Data Compression

The compressed capacity of the tape 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 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.

Do not use software data compression because the built-in intelligent data compression of the PowerVault LTO-4-120 Half-Height tape drive is much more efficient than software data compression.

The PowerVault LTO-4-120 Half-Height tape drive uses a derivative of ALDC-2 lossless data compression that includes additional control codes for intelligent data compression.

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