Exabyte EXB-8505 8mm Thresholds During a Write Operation, Motion Threshold, Reconnect Threshold

3 Functional Description

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)