9.1 Choosing a Tape Drive
Consider the following issues when choosing a tape drive:
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.
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.
Tape drives are commonly available in ATAPI/IDE, SCSI, and parallel interfaces.
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.
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.
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.
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.
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.