Defining a tape use is difficult. One person might consider reading or writing a tape from end to end a single use, while another might consider reading or writing just one file a single use.

Because uses are hard to quantify, a tape's longevity is typically specified as the number of tape passes it can withstand. Here are the tape pass specifications for Tandberg Data tapes:

• 8mm MP (metal-particle) tapes: 2,000 passes
• 8mm AME (advanced metal evaporated) tapes: 20,000 passes

So what do these numbers mean?

During a read or write operation, a tape pass occurs each time a particular area of the tape crosses the recording heads. For AME tapes, this means that any given location on the tape can withstand more than 20,000 passes across the heads.

It's important to note that tape passes are not added together unless they are concentrated in the same place. If 10 passes occur in 100 separate locations on the tape, the tape has accumulated just 10 passes, not 1,000. In contrast, if you record over and over at the beginning of the tape, that area may undergo thousands of passes. In such a case, even if the rest of the tape undergoes no passes, the life of the tape is determined by the number of passes concentrated at the beginning.

Tape passes increase with start/stop motion
The number of tape passes that occur during a single read or write operation depends on the number of times the tape drive has to stop and restart tape motion.

In ideal situations, the tape drive writes or reads data without stopping the tape. This is data streaming. The tape drive can stream data to the tape when the host computer provides the data fast enough for the tape drive to write it without having to stop the tape and wait for more. Similarly, when reading data, the tape drive can stream data to the host if the host can accept the data as fast as the tape drive is sending it.

Whenever the tape drive is streaming data, just one tape pass occurs over any given area of the tape. However, if the tape drive has to stop and wait for the host, three passes occur at each location the drive stops, as follows:

• Pass 1: The tape drive receives data from the host and writes all of it to the tape. When it realizes that the host isn't keeping up, the tape drive writes gap tracks (tracks of undefined data) to aid in repositioning. Then, it stops the tape.

• Pass 2: The tape drive backs the tape up a certain distance before the end of the data and stops. This distance is required for bringing the tape up to speed and aligning the recording heads when the tape drive is ready to write data again.

• Pass 3: When more data is available, the tape drive starts moving the tape forward again. When the tape comes up to speed, the tape drive begins writing at the end of the gap tracks.

It's clear that tape passes cannot be equated to tape uses, particularly when excessive start/stop operations are involved. So how do you decide how many times to use a tape?

Set up a schedule or establish rules
It's best to set up a retirement schedule that ensures tapes are replaced well before they wear out. You should base the schedule on criteria that you can easily track. For example, depending on how heavily you use a tape, you might want to replace it on the first day of each month or on its one-year anniversary. Or, it might be easier to replace it after a specific number of backup operations.

As an alternative, you might want to track tape errors. MammothTape technology drives implement the industry-standard TapeAlert™ system, supported by many backup applications, to warn users of tape problems. If your backup application supports TapeAlert, you can track errors, such as read and write retries, and choose to retire any tape that reaches excessive values.

MammothTape drives can also maintain a tape history log that tracks the usage of each tape, including the number of passes the tape has undergone. The tape history log is accessible through TapeAlert. If the tape history log is enabled and your backup application supports TapeAlert, you can track tape passes and select a value (below the maximum) at which you will retire the tape.

As a rule-of-thumb, you should use a new or relatively unused tape to archive critical data that you plan to keep for a long period of time. Once you have written the data, store the tape as recommended.

For your routine backups that get overwritten each day or week, feel free to re-use tapes until they reach the criteria you have determined for retirement. Just keep in mind that the cost of replacing a tape before it is worn out is much lower than the cost of losing data.