BUYER'S GUIDE

Tape Drives & Autoloaders Answer Backup Call

But DLT can brag about more than its speed-and-feed numbers. Unlike 8 mm, DLT has a low tape tension and stationary heads. These features translate to a much longer tape media life in terms of the number of head passes before the media or head wears out. Depending on the specific models and media compared, DLT media can survive more than 300 times more head passes than 8-mm driv es. If you plan on using an autoloader strictly for backup purposes (where data is usually written once during nonoperating hours and then rarely, if ever, read), the advantage of DLT's low wear-and-tear design is diminished. But if the plan for your autoloader involves use of HSM, other data migration schemes or near-line storage, DLT might be the best choice. In these environments, tape may pass under drive heads 80 percent or more of the time, all day long. In this case, DLT's vastly superior media head-pass lifetime might help you sleep better at night.

Do we sound biased toward DLT? The bias comes from real-world experience. Network Computing's Madison, Wis., lab is at the University of Wisconsin-Madison Computer-Aided Engineering (CAE) Center. Whenever I am not writing for Network Computing, I work for the CAE Center and am involved with its daily operations. A few months ago, the CAE Center file-server pool, which consists of a few dozen Novell NetWare and Unix servers facilitating approximately 5,000 users, switched from top-generation 8 mm to top-generation DLT. Your mileage may vary, but in the CAE Center's case, the switch to DLT has made a noticeable and positive impact.

All Shapes and Sizes Aside from the tape drive, the other major pieces of an autoloader are the structure and robotics. Although many folks use the terms stacker, autoloader and library interchangeably, they are different. A stacker can only insert and remove tapes into a drive in a predetermined, sequential order. Stackers are suitable solely for performing automated backups where the amount of material being backed up cannot fit onto one tape.

An autoloader has the additional capability of being able to insert any tape on demand, making it suitable for automated backup and recovery systems and near-line storage.

Finally, libraries often are referenced simply as very large autoloaders containing multiple tape drives that allow for simultaneous read/ write operations from different tapes. Multiple drives increase avail ability (if the data is on different tapes, you don't need to wait for a backup operation to complete before a restore is performed) and throughput (back up multiple volumes at the same time).

As for t he robotics, engineering ingenuity has produced many different mechanisms, including carousel, stack, drive moves to the tape and tape transferred to the drive. What counts, however, is the average time it takes to load a tape and the reliability of the mechanics. Reliability is better measured by the average number of tape exchange cycles between failures than just the number of power-on hours between failures.

As administrators find themselves needing to back up ever-increasing amounts within fixed backup windows, tape arrays are becoming more popular. A tape array occurs when a library with multiple drives is integrated with a RAID controller. Similar to their disk-based RAID cousins, tape arrays can provide increased bandwidth by striping the data to multiple tape drives. They also increase fault-tolerance by making one tape drive write parity information. If one tape in the backup set fails, perhaps because of worn media, it can be reconstructed from the parity information found on its relatives. Howe ver, tape arrays have added administrative hassles (each tape must be bundled with the others in the same stripe), so you should have some very demanding situations

in mind, especially now that new generation standalone 8-mm and DLT drives can write more than 5 MB per second (with compression).

Operational and Media Management Backup and HSM software typically handle the management of the autoloader. This is similar to how an operating system controls the management of a disk drive. However, some autoloaders are bundled with software or sophisticated menu-driven LCD front panels that allow the administrator to monitor robotic and drive activities. The software also lets managers perform simple configuration and diagnostic tasks.

On some autoloaders, media is inserted and removed one cartridge at a time through a slot. On others, a magazine allows multiple cartridges to be quickly and easily inserted/removed as a group. Some open like a refrigerator and allow tapes to be shuffled at will. M any others are some combination of the above.

The backup or HSM software must be able to discover and positively identify exactly which tape has been placed into which slot. This identification process normally is accomplished by inserting the tape into the drive, at which time the backup or HSM management software reads a specific label or ID header written at the beginning of each tape.

A far faster method, especially when many tapes are being added, involves externally labeling every tape with a bar-code pattern, which is then quickly read inside the autoloader by a bar-code scanner (often available as an option).

Todd Tannenbaum can be reached at ttannenb@nwc.com.