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:
- Archive Python: A 4mm Digital Audio Tape (DAT) drive that uses the
standard Windows NT 4mm tape device driver
- Conner QIC SCSI 3200: A 4mm DAT drive that uses the CONRMQIC driver
- Sony 5200: A 4mm DAT drive that uses the 4mm Sony driver
- HP C1553a: A DAT drive from Hewlett-Packard that uses the standard
4mm driver
- Exabyte 8505: An 8mm tape drive that uses the Exabyte 8500 driver
- Exabyte 8700: An 8mm tape drive that uses the Exabyte 8500 driver
- Exabyte 210: A dual-8505 drive with an autochanger (this device
requires special autochanger support)
- Qualstar 4220: A gear-driven, high-end autochanger that has very
fast tape management and is configured as a dual 8205
- Quantum DLT 4000: A device that uses the latest optimized
dlttape.sys driver from Barratt Edwards International (UltraBac)
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.
Performance
These tape drives can be divided into three basic performance categories:
- Group 1: Low-end, low-speed drives ideally suited for
individual workstation requirements.
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.
- Group 2: Midrange drives with storage capacities of 2GB
to14GB and suitable for small LANs with either multiple backup servers or a
centralized server.
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.
- Group 3: High-speed devices for large networks that require
large amounts of data storage. These devices can hold as much as 250GB of data
and can back up with speeds approaching 100MBpm locally and 35
megabits-per-minute (Mbpm) to 100MBpm across the network, depending on the
bandwidth available.
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.
Performance Optimization
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.
Tape Formats
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.