Exabyte EXB-8505 8mm manual Logical Blocks, Logical Block Compression, Logical Block Packing

2 Recording Format

Logical Blocks

A logical block contains user data that is transferred from the host to the tape drive. Logical blocks can have fixed or variable lengths, which can be intermixed on the tape. The tape drive supports uncompressed logical block sizes from 1 to 240 KBytes.

For information about setting the logical block size, refer to the EXB-8205 and EXB-8505 SCSI Reference.

Logical Block Compression

When writing data in 8500c and 8200c formats, the tape drive compresses logical blocks of user data before placing them in the physical blocks. Thus, each physical block contains a compressed representation of the original user data. Assuming that the compression algorithm allows user data to be compressed an average of two times, each physical block written by the tape drive in compressed format can contain, on average, the compressed equivalent of 2,048 bytes of user data.

Note: The actual compression ratio achieved by the tape drive in 8500c and 8200c formats depends on the type of data to be compressed.

For more information about compression, refer to Chapter 3.

Logical Block Packing

To optimize tape capacity when writing tapes in 8200c, 8500, or 8500c formats, the tape drive packs logical blocks of user data into physical blocks. Each 1,024-byte physical block on tape can contain multiple logical blocks (for example, two 512-byte uncompressed logical blocks can be written in one physical block). To prevent losing data capacity when small logical block sizes are used, the tape drive can begin writing a logical block in one physical block and end in a subsequent physical block.

Note: In 8200 format, logical blocks are not packed. Only one or part of one logical block can be written in each physical block.

Figure 2-9 shows three examples of how logical blocks can be written into one 1,024-byte physical block.

May 1994	EXB-8205 and EXB-8505	2-13
	(Standard and XL)