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The importance of the tape edge quality

Excessive tape edge wear is a sign of trouble ahead.

A tape with worn edges may still be performing well with no error indications, but a worn edge can significantly reduce the remaining life of the tape. [Why is that?] Continued use of such a cartridge is not recommended. Our erasers include optical sensors that monitor the tape edges while erasing the tapes. A sophisticated algorithm looks for distortions in the tape edge and calculates an edge quality number.

Edge Quality Scores
16 and above Good tape edge
11 - 15 Ok to use. Monitor VeriScore® number of this cartridge
6 - 10 Ok to use, but life expectance may be diminished
0 - 5 Not recommended for use
16 +
11 - 15
6 - 10
0 - 5
Why the tape edge matters

Hold a sheet of copier paper (the cheap, thin type) and look at the face (the thickness) of the paper. On an area of the tape as narrow as the thickness of the paper a later generation LTO drive will write over 20 data tracks.

Keeping these narrow tracks aligned with the recording and read-back heads of a tape drive can be a challange.
Tape is guided in a tape drive by pushing its lower edge, commonly called the “reference edge” against guiding elements in the tape path of a tape drive. Tapes with a straight reference edge are moved through the tape path with little or no lateral movement.

Tape with an uneven but smooth tape edge. The size of the edge distortions varies from tape to tape. A large distortion can be 0.1 mm or more.

A reference edge that follows a gentle wave may cause large lateral tape movements, but the tape drive is capable of moving the heads to follow lateral tape movements that are slow, thereby maintaining the alignment between the head and the tape.

Servo Errors

Severely damaged tape edges, however, can cause rapid and erratic lateral tape movements. The tape drive may not be able to move the recording heads fast enough to follow these movements and the alignment between the head and the tape can no longer be maintained.

These errors are commonly called “servo errors”. When such a servo error occurs while writing data to tape, the tape drive will disrupt the write process immediately and attempt to write the data further down on the tape. This is time consuming and reduces the usable capacity of the tape.

Tape with uneven edge wear. The tape drive may not be able to follow the rapid lateral tape movements caused by the edge distortions.

A multitude of servo errors can cause the drive to abandon the write process entirely. A tape that causes a multitude of servo errors is therefore unusable. Less severely damaged tape edges may not cause servo errors, but even minor edge damage can be of concern. Any damage to the tape reference edge can cause disturbances between the tape and the drive’s tape path. These disturbances can increase the tape edge wear, causing further damage to the tape’s reference edge.
Damage of the tape edges is an indication that the tape’s quality has deteriorated, even if the damage is presently still too small to cause servo errors.

What causes damaged tape edges?
There are several causes for damaged tape edges. The most common are improperly maintained tape drives. The tape in all LTO drives is guided laterally by spinning flanges of the rollers. Inaccuracies of the tape edge and of the rollers can result in the flange striking the tape edge with each revolution. The rollers, which in some cases are turning faster than 10,000 rpm, can cause wear of the tape edge.
Edge wear causes more edge wear

Once tape edges start to wear the process can escalate. Uneven wear of the tape edge causes rapid and erratic lateral tape movements. These movements can further damage the tape edge.

It is important to find tapes with deteriorating edges early. Bad edges are a sign of trouble ahead.

For more information about tape edges of LTO media see:
Measurement and Origin of Tape Edge Damage in a Linear Tape Drive - A. Goldade and Bharat Bhushan
Measuring Small - Gary Collins