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Drive Safely

The most critical component of the autoloader is the tape drive that writes and stores to the media. Each type of tape drive has its benefits and drawbacks. Your choice will depend on how much data you have, where it's located, how long it can go offline and, of course, price. Every year, hard-drive capacities seem to increase as costs decrease--unfortunately, this isn't the case for tape drives. Tape-drive capacities are starting to increase, but prices are still high (comparable with a high-end PC).

On the low end of the small-to-medium-business spectrum, in terms of cost and capacity, are DAT (Digital Audio Tape) drives. DAT drivers are differentiated by DDS (Digital Data Storage) categories. For about $1,000, you can get a DAT drive, such as a Seagate Technology Scorpion, with an internal DDS4 drive that has a capacity of 20 GB native and typically can hold 40 GB using 2-to-1 compression. These DAT drives offer a sustained data rate of 2.4 MB per second, or about 12 to 13 GB per hour, under optimum conditions. DAT drives offer cost, capacity and performance that make them a great choice for standard backup needs. And just about every software backup program, including Computer Associates International's ARCserve and Veritas Software's Backup Exec, provide drivers for these devices, making DAT drives an affordable and efficient solution for small- or branch-office environments.

For larger-capacity and higher-performance needs, you'll need to look at the autoloader devices that use the DLT (Digital Linear Tape) standard.

DLT was developed by Quantum several years ago but is now sold by various vendors, including ADIC, Hewlett-Packard Co. and Sun Microsystems. The DLT8000 is the latest model, but you can still purchase older models, like the DLT4000 and 7000, if you need to save some money and don't need the capacity of the DLT8000. The DLT8000 offers 40 GB of native capacity at a sustained rate of 6 MB per second, or around 43 GB per hour, under perfect conditions. Although this is twice the capacity and performance of the DDS4, it costs about four times as much. Expect to pay $5,000 for a DLT external drive.

If the DLT drives are too costly, you're still in luck. The Exabyte Corp. Mammoth and the newer Mammoth-2 (or M2) offer a native capacity of 60 GB and can each hold 150 GB using a special compression algorithm. The M2 also provides a sustained transfer rate of 12 MB per second, peaking as high as 30 MB per second with compression. Priced around $4,000, the Exabyte M2 offers more capacity than the DLT8000 for about $1,000 less.

The decision between the DLT and Mammoth drives may seem to be a no-brainer, but it's not that simple. DLT drives are offered by several major vendors, and many agree that DLT is a more reliable standard. DLT drives use a technique called linear scan to write the data onto the tape. Proponents of DLT claim linear scan is more reliable than the helical-scan method used by Exabyte. The most important difference between the two methods is that linear scan writes data from the beginning of the tape to the end, then turns around and writes in the other direction just one row lower. The helical method writes data diagonally across the entire tape from start to finish.

Another significant difference involves the heads. Linear scan uses stationary heads to write and read the data, while helical scan uses mobile heads. Many argue that moving parts are more likely to cause problems than are stationary parts.

To add to your options, there's the fairly new AIT (Advanced Intelligent Tape) and the brand-new LTO (Linear Tape Open) standards (for more on LTO, see "Open Sesame," at left). Developed by Sony, AIT is available in both AIT-1, (25 to 35 GB native capacity) and AIT-II models. The newer AIT-II provides 50 GB of native capacity, runs at 6 MB per second without compression and uses the helical-scan technique. The AIT drives also rely on the same 8-mm media format used by the Mammoth.

All the drives mentioned, except DAT drives, are designed for small- and medium-sized enterprises that need to back up a lot of data in a short period of time. Enterprises with larger storage needs can invest in a DLT or AIT autoloader that can spread countless gigabytes of data across multiple tapes during a single backup process.

Capacity

When considering capacity, look beyond the tape drives and take the total capacity of the system into account. Many autoloaders, such as ADIC's AML line and Grau Data Storage Infinistore Tape Library, support different tape drives in a single loader, offering variety and increasing the total capacity of the system. Multiple tape drives are useful, too, because they let you perform several tasks simultaneously. For example, if the option is available in your software, one drive can back up while another restores a file. Imagine one autoloader unit with one DAT drive for client PC backups and one or two DLT or LTO drives for mission-critical data.

Obviously the higher the capacity, the larger the device. Some autoloaders can fill an entire room, while others tuck into a 19-inch rack. As the form factors get smaller, or at least remain the same (usually 5.25 inches), it's possible to store terabytes of information in a box about the same size as a high-end Layer 3 switch.

Software Issues

Basing any purchase on the notion that "if you buy it, it will work" is a bad idea. An autoloader will appear as a device to any OS, so the backup software must know how to talk to the unit and control the robotics. Unfortunately, Computer Associates, Veritas and others charge extra for the autoloader driver that can address your hardware. In some cases, you'll pay as much for the autoloader software option as you did for the backup software.

Also available in backup software (and hardware) is the ability to track the location of a tape using a bar-code reader or MIC (memory in cartridge). Both methods let you place a "fingerprint" on a tape so the autoloader can retrieve it from the rack quickly.

Steve Rigney is a network consultant specializing in storage management, VoIP and remote access. Send your comments on this article to him at srigney@adisfwb.com.