For a write operation on a tape an entire block of

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Unformatted text preview: r each record, then more than half of the tape could be unused (occupied with IRGs), and the tape I/O operation would also be very slow due to very frequent interruption in tape movement during I/O operation. To overcome these problems of inefficient usage of tape storage space and inefficient I/O operation, records are usually grouped in blocks of two or more, separated by an inter-block gap {IBG). The process of grouping two or more records together to form a block of data is known as blocking and the number of records put together in each block is called blocking factor. The blocking factor generally depends on the record length. The rule of thumb for deciding blocking factor is that the block should be at least ten times as long as the IBG to reduce wastage of tape storage space. Blocking also helps in faster tape I/O operation because it permits additional records to be transmitted with each "read" or "write" operation. For each tape "read" operation, all data records between adjacent IBGs are read at a time into the main memory of the computer system for processing. The next "read" operation transmits the next block of records to the main memory. For a "write" operation on a tape, an entire block of records are transmitted from the main memory to the tape drive. Then a block of data and an IBG are written on the tape. Figure 8.4 illustrates the concept of blocking. Figure 8.4 (a) shows an unblocked tape (one which does not use blocking), Figure 8.4 (b) shows a tape which uses a blocking factor of two, and Figure 8.4 (c) shows a tape which uses a blocking factor of three. Notice how the tape usage increases as the blocking factor increases. However, the blocking factor should not be too large otherwise in case of data loss due to some parts of the tape becoming bad, all the records in a block will become irretrievable. A group of records form a block, and a set of blocks constitute a file. A single tape is normally used t...
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This document was uploaded on 04/07/2014.

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