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Data Compression Monitoring
The tape drive constantly monitors the compression ratio to
determine whether compressing the data will actually decrease the
size of the data set. When a logical block expands (as it might, for
example, when it has already been compressed by the initiator), the
tape drive automatically switches to uncompressed format. It remains
in this format until it encounters a compressible logical block. Then, it
switches back to the compressed format.
Streaming and Start/Stop Modes
The data buffer enables the tape drive to operate as either a streaming
tape device or as a start/stop tape device. When the tape drive
transfers data to the 1-MByte data buffer, the data buffer’s formatter
performs the following tasks:
Formats logical user data blocks into physical blocks
Appends tag, address, and index information to each data block
The mode of operation (streaming or start/stop) depends on the rate
that data can be transferred between the initiator and the tape drive,
as follows:
The tape drive operates in streaming mode if the initiator can
sustain a minimum of:
•
1 MByte per second for 8500c format (EXB-8505 and
EXB-8505XL only)
•
500 Bytes per second for 8500 format (EXB-8505 and
EXB-8505XL only)
•
500 KBytes per second for 8200c format
•
262.5 KBytes per second for 8200 format
Note: These values assume a 2:1 compression ratio for 8500c and
8200c formats.
The tape drive operates in start/stop mode if the initiator cannot
sustain these minimum transfer rates; starting and stopping occur
automatically.
3-4 EXB-8205 and EXB-8505 510504
(Standard and XL)
3 Functional Description