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|Can you determine the quality of a cartridge?|
|Unfortunately, it's difficult to determine the quality|
| of a tape cartridge. Most users don't have the resources required for an ongoing, full-scale quality assurance program, even though the benefits of such a program are obvious. The only method available to the general user is to monitor the read/write errors of the cartridge. Read/write errors can be caused by the cartridge as well as by the tape drive.
As a cartridge becomes used, an increase in the number of read/write errors could identify a deteriorating cartridge. LTO drives keep internal track of read/write errors; some drives gather fairly detailed statistics. But usually, only the drive manufacturers have ready access to these detailed statistics. Some of the read/write error statistics assembled by the drives can be accessed through the interface (using the log sense command). User software can track cartridge quality, and can identify a potentially deteriorating cartridge, hopefully long before disaster strikes.
In addition to using the log sense command, users may also obtain error statistics by accessing the cartridge memory chip inside the LTO cartridge. A special reader that can talk to cartridge memory chips is needed. LTO drives use such readers to update the cartridge memory every time the cartridge is loaded into the drive. But they aren't widely available as separate units. For more information on separate unit cartridge memory readers, visit http://www.mountainengineering.com/cm.html.
Once you decide which drive to purchase, the next big decision is: Which media should you use? LTO media quality, which can vary, is extremely important to successful backups and restores.
We randomly purchased two cartridges each from four LTO media manufacturers: Fuji, Imation, Maxell and TDK. All cartridges were manufactured in 2003 and are 200GB Generation 2 cartridges. Our tests proved that not all LTO tapes are of the same quality. All LTO cartridges usually work in any LTO tape drive, but the tapes--which are all manufactured to the same specification--vary considerably. This leads to the question: How do you evaluate LTO tape cartridges?
Just a few of the many parameters that determine the overall quality of a tape cartridge are: the reliability of the cartridge mechanism; the edge quality of the tape; the consistency of the magnetic layer on the tape; and the accuracy of the servo tracks. We will show why the quality of the tape edge is so important to successful backups and restores and also describe what our tests of leading manufacturers' LTO media revealed.
Tape edge bumps
Tape edges are produced during the manufacturers' slitting process. Slitting machines slice the original tape, which is produced on wide rolls, into the traditional half-inch tape. Each roll produces many half-inch tapes. However, slitting really ought to be called tearing; in reality, the tape actually tears in front of the slitter. The knife tips in the slitters never touch the tape itself.
It's the same phenomenon that occurs when cutting a long piece of paper in a straight line using scissors. Other than pushing the scissors forward, hardly any movement is required because the paper is tearing ahead of the blades. Not even the most meticulous manufacturing processes have been able to avoid producing rough edges. Viewed microscopically, these torn tape edges appear spectacularly uneven.
To make things worse, data tracks on an LTO cartridge are narrow: roughly 20 micrometers on a Generation 2 cartridge (about one-fifth of the thickness of a sheet of copier paper). As tape is moved over the read-write head in a horizontal direction, it inevitably moves up and down because of its rough edges. These vertical movements can grossly disrupt the crucial alignment between the data tracks and the read elements. It's a constant challenge for a tape drive to properly align the narrow data tracks with the read elements in the read-write head.
This was first published in March 2004