BUYER'S GUIDE

Tape Drives & Autoloaders Answer Backup Call

truly automatic, unattended backups. Autoloaders are especially powerful when coupled with a sophisticated network-based client/server backup software package. These systems allow end users to request restores of their own files from the server, resulting in near-complete automation of a process that once consumed considerable administration time.

The choice of which autoloader to purchase should be based on your capacity, bandwidth and duty cycle requirements. Capacity is the primary consideration for an automated backup system. Capacity is simply a function of the amount of data per tape multiplied by the number of tapes held in the library. Although zero downtime technologies such as Redundant Array of Inexpensive Disks (RAID) and journaling file systems are making disaster-recovery restores from tapes less frequent, adm inistrators still are being asked to back up ever-increasing amounts of data in shorter backup window s (periods of time during which backups can be run).

Bandwidth dictates how much data can be backed up within a backup window. Bandwidth and duty cycle also play significant roles, perhaps even more important than capacity, in on-demand near-line storage systems (HSM and video on demand, for example) and/or real-time data-capture systems.

Appendix A of the Manual for Your Backup Package You should begin your quest for an autoloader or library by obtaining a list of models that are fully supported by the backup, near-line HSM or data-capture software package you're using. The autoloader is much more of a commodity than the software; therefore, a good rule of thumb is to choose the software that can meet your needs first and then those of the storage hardware.

In addition, note how your controlling backup and/or HSM software makes use of multiple drives. For instance, a number of backup software packages that support multiple tape

drives will transfer data from only one drive at a ti me. Other packages, such as Legato Systems NetWorker, can perform data transfers from multiple drives simultaneously (using an optional module). If your software cannot simultaneously transfer data to and from more than one drive, then purchasing anything other than a single-drive autoloader is a waste of money.

Tape Drive Technologies A discussion of tape autoloaders and libraries without mentioning tape drive technologies would be useless. The tape drive is the controlling factor of an autoloader's most important characteristics, including data-transfer speed and tape capacity. The tape drive seek speed also has a major influence on overall random data retrieval speed and (combined with quality of media) on data reliability. Overall, the single most important criteria when choosing an autoloader is its tape-drive technology.

There are more than a dozen tape drive technologies in widespread use. Howev er, 4-mm, 8-mm and Digital Linear Tape (DLT) drive technologies dominate the midrange netw ork-server market. Your choice will depend on your application (backup or HSM), size of backup window and budget. These drives sport hardware compression on the fly and, with typical data, usually can double both storage capacity and data-transfer rates. The table below lists the flavors of, as well as the specifications for, the most common tape technologies found in this market.

Of these three leading technologies, 4-mm digital audio tape (DAT) is the most affordable. Tiny DAT tape cartridges originally were aimed at the home-audio market. Now, use of 4-mm DAT for data storage is standardized by the various Digital Data Storage (DDS) specifications. The most recent specification, DDS-3, allows up to 12 GB per tape before compression. DDS-3 is starting to make its way into the market.

DLT and 8-mm drives offer higher capacities and transfer rates, but the initial cost of these systems can be greater than that of 4-mm drives. The 8-mm tape drive industry is dominated by Exabyte Corp., which has rolled out a steady stream of nearly 10 different models of 8-mm drives over the years, each with additional performance characteristics than its predecessor.

However, backward compatibility with the previous generation is on a read-only basis, causing confusion for many IS shops. Later generations can read a previous generation's lower density 8-mm tapes, but a new drive cannot write at a lower density. Although these drives were originally criticized for their poor seek speeds, later models have greatly improved the speeds. The 8-mm drives have good capacity (up to 20 GB with the latest variant), have a low cost for media and are widely supported and understood by software packages.

In terms of its design, however, 8-mm helical scan technology is based on tightly wrapping the tape around a rotating head. This results in wear on both the tape heads and the media.