User manual QUANTUM LTO-4

[. . . ] Quantum LTO-4 Half-Height Tape Drive User's Guide, P/N 20020140-001, November 2007. Quantum Corporation provides this publication "as is" without warranty of any kind, either express or implied, including but not limited to the implied warranties of merchantability or fitness for a particular purpose. Quantum Corporation may revise this publication from time to time without notice. COPYRIGHT STATEMENT Copyright 2007 by Quantum Corporation. [. . . ] The write operation will not continue until the correct servo tracking is re-established. Data Compression Typical data streams of text, graphics, software code, or other forms of data contain repeated information of some sort, whether it is at the text level where you can readily recognize regular repetitions of a single word, or at the binary level where the repetitions are in bits or bytes. Although most data is unique and random, the binary level data exhibits patterns of various sizes that repeat with varying degrees of regularity. Storage efficiency is increased if the redundancies or repetition in the data are removed before the data is recorded to tape. Data compression technology significantly reduces or eliminates redundancies in data before 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 represented by codewords or symbols that allow the same LTO-4 Half-Height Tape Drive User's Guide 27 Chapter 4 Theory Data Compression data to be recorded in a fewer number of bits. These codewords or symbols 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 4 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 LTO-4 Half-Height Tape Drive User's Guide 28 Chapter 4 Theory Data Compression 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 4 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 (uncompressed) size, then the record is written in its native form. [. . . ] See Mounting the Internal Tape Drive on page 9. 3 Connect a serial-attached SCSI (SAS) interface cable to the drive. See Connecting the Internal Drive Interface and DC Power Cables on page 10. 4 Connect a serial cable, if installing the tape drive in a tape library system. [. . . ]