9.1 Choosing a Tape Drive
Consider the following issues when choosing a
tape
drive:
- Capacity
-
The single most important consideration. Get a drive that can back up
all data on one tape, allowing for some growth. If your data set
exceeds the capacity of one tape, you may find that drive is no
longer usable (if no one is available to change tapes during an
overnight backup) or that it has suddenly become very expensive to
use (because you must buy twice as many tapes). In such a case, the
only alternative to replacing the drive is to use a backup scheme
that mixes full and incremental or differential partial backups,
which is riskier for your data.
Tape drive manufacturers arbitrarily rate their drives at double
actual capacity, assuming that you will use software or hardware
compression that effectively doubles the space available. The actual
compression ratio you experience depends on the data mix (e.g.,
documents and spreadsheets compress well; executables, images, and
archives much less so), the backup software you use, and sometimes on
the speed of the computer where the drive resides. We find that
real-world data sets typically compress at 1.5:1 to 1.7:1, so plan
accordingly.
- Speed
-
This may or may not be a critical factor, depending on your own
environment and practices. If you have a limited backup window
available, speed may be as important as capacity. If you can simply
start a backup when you finish work for the day and allow it to run
overnight, speed may be a minor factor.
Actual throughput depends on the drive mechanism, the interface, the
speed of the computer in which the drive is installed, and the data
set being backed up, but will likely be lower than the drive
manufacturer advertises. Compression may also have a significant
impact on throughput, for better or worse. For example, our Seagate
TapeStor TR4 ATAPI drive is rated at 30 MB/min native and 60 MB/min
compressed, but we actually get 20-22 MB/min native and 35-38 MB/min
when using compression. Our OnStream DI30 ATAPI drive is rated at 60
MB/min native and 120 MB/min compressed. We actually get 45-50 MB/min
native, but only 15-17 MB/min when using the bundled Echo software
with compression turned on. Our Seagate Travan NS20 SCSI drive is
rated at 60 MB/min native and 120 MB/min compressed, but we actually
get about 100 MB/min. These figures are for backing up local volumes.
Backing up data across a network (10BaseT or 100BaseT) commonly cuts
throughput by half or more due to operating system overhead,
filesystem overhead, and network latency.
- Media cost
-
Travan and OnStream ADR drives are constructed with loose tolerances
and are accordingly inexpensive, but require expensive tapes built to
close tolerances. DDS drives, conversely, are expensive because they
are built to tight tolerances, which allows them to use inexpensive,
loose-tolerance tapes. A typical tape rotation may require from four
to fifty or more tapes. Tapes must be replaced periodically (on the
schedule recommended by the drive and/or tape
manufacturer—trying to stretch the lifetime of tapes is a
foolish economy). Tape drives have a realistic service life of
perhaps two to three years with heavy use, and four or five years
with moderate use (by which time the drive is likely no longer
adequate for your needs anyway). Expect to spend from as much to
several times as much as the cost of the drive to buy tapes over the
drive's life.
- Interface
-
Tape drives are commonly available in ATAPI/IDE, SCSI, and parallel
interfaces.
- ATAPI
-
ADR and Travan drives are available with ATAPI interfaces. ATAPI
drives are typically less expensive than those using other
interfaces, can use the ubiquitous IDE interface present on any
modern motherboard, provide reasonably high throughput, and are easy
to install. Choose an ATAPI drive for convenience, ease of
installation, or when cost is an overriding issue.
- SCSI
-
ADR, Travan, and DDS drives are available with SCSI interfaces. SCSI
drives typically sell for at least a $50 to $100 premium over similar
ATAPI models, require adding a $100+ SCSI interface card if the PC is
not already so equipped, and are more complicated to install and
configure than ATAPI models. SCSI drives typically provide much
higher throughput and much lower CPU utilization than ATAPI models.
The largest and most feature-laden drives are available only in SCSI.
Most SCSI-only models are designed for use on servers, and are
therefore better built and more reliable than ATAPI drives designed
for the mass market. Choose a SCSI drive for highest capacity,
performance, durability, and reliability. SCSI is the only option if
there are no available ATAPI connections or if you require capacity
and/or features available only in a SCSI model.
- Parallel
-
These typically have half or less the
throughput of ATAPI/IDE, but may be a reasonable choice if you must
use one drive to back up local data on multiple standalone PCs that
do not have recent USB ports. On a small network, it is usually
better to map a drive on the server where a tape drive resides for
each local workstation volume, and back up centrally to an ATAPI or
SCSI drive. If you are considering a parallel drive because you have
several standalone PCs that must be backed up, consider instead
connecting those PCs with a simple network and using an internal
server-based tape drive.
- USB
-
These typically have two-thirds the throughput of ATAPI/IDE, and are
a better choice than parallel port drives for backing up local data
on multiple standalone PCs if all of those PCs have reasonably recent
USB ports (Intel SE440BX motherboards or later). USB 1.1 tape drives
have a theoretical maximum throughput of about 90 MB/min and a
typical actual throughput of 60 MB/min or less. USB 2.0 models are
limited by the speed of the drive mechanism rather than the speed of
the interface.
- FireWire
-
There are a few external tape drives available that use the FireWire
interface, none of which we have tested. FireWire tape drives offer
Plug-N-Play compatibility similar to USB models, but are much faster
than USB 1.1 drives. The relatively high cost of FireWire tape drives
and the fact that few PCs have FireWire interfaces make them a poor
choice for most people. The exception is if you have desktop PCs and
particularly notebook PCs that have a FireWire interface installed.
In that case, a FireWire tape drive may be the fastest, easiest tool
for backing up.
- Cross-drive compatibility
-
Here's a dirty little secret that drive
manufacturers don't talk much about. You might
reasonably assume that a tape you created in one drive
would be readable in a similar drive, but that's not
always the case. In particular, we have found that some Travan TR-4
drives produce tapes that cannot be read by another drive, even one
of the identical make and model. We seldom encounter that problem on
DDS, Travan NS8/NS20, and OnStream ADR drives, although we have had
infrequent reports of such compatibility problems occurring with
them.
Unless you use a tape drive to transfer large quantities of data
between computers, this may seem a minor issue. It can be critical,
however, if your computer is stolen or damaged by flood or fire. Even
if your backup tapes are safely locked away, you may find that a
replacement drive of the same model is unable to read them. If your
data is important enough to warrant extreme precautions, buy two
identical tape drives and verify that a tape written in either drive
is readable by the other. Repeat this verification periodically,
because drives do start marching to their own drummer as they age.
Alternatively, consider backing up key data frequently to CD-R and
verifying that the CD is readable.
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