Exabyte EXB-8505 8mm manual Logical Block CRC

3 Functional Description

If data compression is turned on, the data goes from the SCSI bus to the compression integrated circuit where it is compressed and then decompressed. The tape drive performs a Compression Integrity Check™ by comparing the decompressed data to the original data.

If the decompressed data does not match the original data, the tape drive indicates that a compression error has occurred and does not write the data to tape.

If the decompressed data matches the original data or if data compression was turned off, the tape drive appends two bytes of CRC data to each logical block. Then, it transfers the data to its data buffer.

Note: The tape drive compresses data written to tape at an average ratio of 2:1. However, the actual compression ratio achieved depends on the type of data to be compressed.

③Once the motion threshold is exceeded in the data buffer, tape motion begins, ECC and physical-block CRC bytes are integrated with each physical block, and data is written to tape.

④The tape drive performs a read-after-write check on the written data to ensure that the data on tape was written accurately.

⑤If necessary, the tape drive rewrites the data.

Logical Block CRC

The tape drive adds two bytes of cyclic redundancy check (CRC) data to every logical block written in compressed format. These bytes add an extra check to ensure that the user data is compressed and decompressed accurately.

Note: These logical block CRC bytes are in addition to the two bytes of physical block CRC data that the tape drive adds to every physical block on tape.

Adding logical block CRC bytes to each logical block reduces the data capacity of the tape by two bytes for every logical block. For example, if you are writing 1,024-byte logical blocks, the data capacity of the tape will be reduced by 0.2% (that is, 2 ÷ 1,024 × 100%).

May 1994	EXB-8205 and EXB-8505	3-3
	(Standard and XL)