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Thresholds During a Write Operation
In start/stop mode, the motion threshold controls the starting and
stopping of tape motion. In streaming mode, the reconnect threshold
controls the rate of disconnects and reconnects between the tape drive
and the initiator. The values of both of these thresholds can be
changed with a MODE SELECT command.
Motion Threshold
In a start/stop write operation, the initiator-to-buffer transfer speed is
slower than the buffer-to-tape transfer speed. In this mode of
operation, the motion threshold value represents the minimum
amount of data that must be in the tape drive’s 1-MByte DRAM data
buffer before tape motion will start. When the initiator sends data to
the tape drive, the tape drive waits until the buffer reaches the motion
threshold to begin writing. Then it writes data to tape until the buffer
is empty.
The buffer allows the tape drive to operate with fewer starts and
stops, reducing wear on the tape and the tape drive.
Reconnect Threshold
In a streaming write operation, the initiator-to-buffer transfer rate is
equal to or greater than the buffer-to-tape transfer rate. In this mode
of operation, the tape drive writes data to tape continually. When the
buffer becomes full, the tape drive disconnects from the initiator, and
the initiator is free to perform other tasks.
For the tape drive to continue streaming, the initiator must begin
sending data again before the buffer is empty. The reconnect
threshold value represents the minimum amount of free space that
must be in the tape drive’s 1-MByte buffer before the tape drive will
reconnect to the initiator to accept additional data. When the
reconnect threshold value is exceeded, the tape drive reconnects to
the initiator and data transfer resumes until the buffer is full.
May 1994 EXB-8205 and EXB-8505 3-5
(Standard and XL)
3 Functional Description