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3.4 Setting the Size of Data Blocks

You may want to adjust the block size of the data being transferred

from the host to the CTS to use the maximum capacity of the data

cartridge. If you are writing data to tape in 8200 format, block-size

optimization is particularly important. If you are writing data in

8200c, 8500, or 8500c format, block-size optimization is not as

essential, but you can achieve somewhat greater capacity by using

larger, rather than smaller, logical block sizes. (See page 1-11 for a

description of physical and logical blocks.)

Optimizing Block Size: 8200 Format

If a logical block of data in 8200 format does not fill the available

space in a physical block, the CTS pads the physical block with gap

bytes consisting of null data. If a logical block of data exceeds the

amount of available space in a physical block, the CTS splits the

logical block among two or more physical blocks and pads the final

physical block with gap bytes, if necessary.

Because gap bytes reduce the capacity of the tape, it is important to

match the logical block size to the amount of data that the CTS can

record in each physical block.

The maximum amount of data that can be recorded in a single

physical block of 8200 format data is 1,024 bytes. Only one or part of

one logical block is recorded in each physical block. Thus, the

optimum logical block size for data from the host is 1,024 bytes or

exact multiples of 1,024. Any other logical block size reduces the

capacity of the tape.

Example

If the logical block size is set to 1,536 bytes, the following occurs

during a multiple-block write operation:

1. The first physical block written to tape contains 1,024 bytes of

data from the host.

2. The second physical block written to tape contains only 512

bytes of data from the host. The CTS pads this physical block

with 512 gap bytes to equal the required 1,024 bytes.

3-12

EXB-8205 and EXB-8505

(Standard and XL)

510503

3 Implementing CTS Operations

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Exabyte EXB-8205 Betriebsanleitung Seite 74