In the course of the software reviews, I used a broad range of SCSI-based tape devices. Some of these devices are appropriate for workstation use, whereas others are better suited for enterprise-scale backup servers. The specific devices I used included:
DAT refers to a basic 4mm tape design, but these DAT tapes are not digital audio tapes, despite the name. They adhere to Digital Data Storage (DDS) standards, which currently include DDS-1 and DDS-2 specifications (DDS-3 is scheduled for release this spring). The primary difference between them is capacity (90-meter maximum for DDS-1 and 120-meter maximum for DDS-2). In addition, DDS-2 supports better compression schemes than DDS-1 and thus has additional storage capabilities.
QIC standards are for quarter-inch tapes and represent an inexpensive drive design. Currently, the Conner 3200 is one of the few available. This drive does not support compression in Windows NT and is viable only for workstation backups.
Digital Linear Tape (DLT) represents a major advance in tape construction and longevity. These tapes can withstand 500,000 passes through a tape drive and have a shelf life of 10 years, which far exceeds 4mm and 8mm designs. In addition, these drives offer large amounts of storage (10GB to 40GB), use a dual-channel read/write head, and support Digital Lempel-Ziv data compression. These features allow excellent data streaming (where data-in rate speed matches the drive speed, so fewer stop/start interrupts occur). Quantum currently makes all DLT engines.
Because of their high costs, 8mm tape devices are typically only in high-end LAN environments. However, this situation is changing because Exabyte has introduced a packaged system (the 8700) that includes an 8505 tape drive, a SCSI card (Adaptec 1505), all necessary cabling, and a CD with Seagate's Backup Exec for less than $3000. The advantage of 8mm drives is capacity: With reasonable compression, a single tape can hold about 10GB of data.
These tape drives can be divided into three basic performance categories:
This group includes the Conner 3200, the Archive Python, and the Sony 5200. In my experience, none of these drives supports hardware compression well in NT. The Python offers it, but without decent compression or speed. The Conner 3200 and the Sony 5200 work best for files that don't compress well, such as photo CD images and small C++ files. The average capacity of these units is 2GB, with a backup speed of 15MB-per-minute (MBpm) to 30MBpm, depending on your PC's speed and configuration. In my opinion, these drives are only for workstations. The prices are much less than $1000.
The drives in this group are the Exabyte 8505/8700 and the HP C1533a. They both offer 4GB to 10GB of real storage and have backup rates of 30MBpm to 45MBpm, depending on PC speed and data type. An interesting side effect is that these drives don't stream tapes, so disruptive events don't bother the drives' performance much. In other words, using two of these devices doesn't significantly slow either device. Group 2 pricing ranges from less than $1000 to $4000. All the listed devices can easily handle most stand-alone PCs and small LANs.
This group consists of high-end devices that offer large tape capacities or include an autochanger. The Exabyte 210, Qualstar 4220, and Quantum DLT 4000 are all in this group. The DLT4000 is rapidly becoming the standard for large database backups.
It is always important to keep the backup devices as separate from high-speed drives as you can. Use separate controllers if at all possible. This approach isolates system I/Os and optimizes speed and efficiency. For example, in my test environment, I have the backup units on the external bus of a multiplexing SCSI controller (an Adaptec 3940). The NT swap file is on a fast/wide SCSI drive controlled by an Adaptec 2940, and the NT applications are on a different drive that this same controller handles.
Reducing the Human Factor
One of the most expensive (and error prone) aspects of backups is the necessity for human intervention. Changing tapes can be a hassle. That's where an autochanger comes in. An autochanger typically can handle the weekly or monthly backup requirements of most networks, assuming you use standard rotation schemes. When you couple an autochanger with a fast, high-capacity tape drive, the resulting device can sustain very high throughputs (e.g., 10 to 20GB per hour).
Another automation technology to consider is bar code reading. Several tape devices, including the Exabyte 210, have built-in readers that let them scan a tape label to determine the tape format and usage. When combined with an autochanger, this capability lets the backup software review which tapes are available for the current backup application. Some backup software, such as NetWorker, also provides utility functions to generate bar code labels.
Often, users express concern about tape formats. Because the software products I reviewed operate in the native, 32-bit NT environment, you might expect most of them to support the Microsoft tape format In reality, there has been little acceptance for the Microsoft format outside Microsoft's own products and Seagate's Backup Exec. For example, Seagate's Backup Director uses an ANSI standard, and Legato's NetWorker uses Legato's OpenTape format. As it stands today, there is no clear winner when it comes to tape format.