Choosing the best type of high-end tape backup

We differentiate DLT, DAT, and 8 mm so you can pick the right backup solution

By Ron Levine

January  1998
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With networks continually expanding, the backup process is getting much more complex. Luckily, the backup options available today keep improving. In this article we focus on three: DLT, DAT, and 8 mm. We present an overview so you can pick the most appropriate solution. (2,300 words, including sidebar, "Combining tape and backup software.")

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Your network has grown. You've added database servers, intranet servers, and Web servers. At one time, you could shut down the system for a few hours and back it up, but now you have worldwide users logged on almost 24 hours a day. The volume of data has grown, but the time available for a backup "window" has shrunk. To make things worse, management is improving its bottom line by shrinking expenses, and you've been asked to economize.

With all these conflicting constraints, how can you back up your data?

Thanks to improved tape technologies, there are several high-end tape options available that can handle offline backup chores and data archiving at phenomenal speeds; each tape solution has its own unique advantages and disadvantages.


Top of the line: DLT
If you have to migrate extremely large amounts of data for backup or archiving in a short amount of time, digital linear tape (DLT) is the clear-cut choice. Quantum Corp.'s DLT technology, available from many VARs, offers the fastest throughput rates of all the tape alternatives.

The DLT data transfer rate is 5 megabytes per second in native mode, and 10 megabytes per second in compressed mode. Is volume a problem? Not really. DLT offers a single-cartridge data capacity of 35 gigabytes, which can double to 70 gigabytes in compressed mode. DLT offers a range of interfaces (like SCSI-2 or Fast and Wide) and excellent reliability, with a head life expectancy of 30,000 hours and mean time between failure (MTBF) of 200,000 hours for the drive. Users like DLT due to its reliability and ease of installation.

DLT offers an additional advantage to a busy network backup environment in mixed-platform surroundings. Since Quantum aims its product at mainframes, midrange systems, and Unix workstations as well as network servers, DLT drives and media are designed for handling heavy volume, with the media going through hundreds of passes per day. If you have a midrange or mainframe computer center in your organization, you may find that DLT is already backing it up. In this case, moving your network to DLT backup may be the next natural step.

If DLT is so good, what's the downside? Cost! DLT is the most expensive tape technology. List price for a DLT 7000 drive typically runs about $7,000 and a 35-gigabyte cartridge costs $99. And, when you're calculating your total cost, don't forget to add in the associated software packages required to handle backup and archiving operations. DLT is the highest performance tape technology available, but it doesn't come cheap. (Note that street prices for drives and media are often as much as 20 percent lower than vendors' list prices.)

Gene Nagle at Overland Data Inc., a San Diego, CA-based VAR specializing in DLT libraries, says, "Sun servers are extremely popular at sites running data-intensive applications where unpredictable growth of capacity and performance requirements are the rule." For example, in the oil and gas industry several large oil companies use Overland Data's LibraryXpress storage units on their Solaris-based systems. The DLT libraries provide high-duty cycle service for data logging and archiving. At one large Sun server user site in Houston, the Remote Sensing Department is using two Overland Data LibraryXpress storage devices, each with 36 cartridge slots, to archive enormous amounts of geophysical data being sent from satellites. One DLT 7000 library is equipped with two drives, the other has four drives.

The volume of data at such sites tends to grow rapidly, and the DLT library must be capable of growing with it, states Nagle. "Overland Data's rack-mounted LibraryXpress can grow from a base of one or two DLT drives and 10 cartridges all the way to 138 cartridges and 16 tape drives in moderate-sized increments. Through the use of a single global control module, the entire stack always operates as one library."

DLT is the leader in performance for the tape backup or archiving task if you're faced with a high-data volume or a narrow backup window. But what if the cost of a DLT solution is too high? Or if your volume is less, or your backup window wider, allowing you the flexibility to choose a less expensive alternative? In that situation, digital audio tape (DAT DDS-3) becomes a viable option.

DAT: The next best thing
DAT DDS-3 costs only a fraction of a DLT solution, with a typical drive listed at around $1,900, and media available at $200 per box of five- to 12-gigabyte cartridges. But with lower cost comes an associated sacrifice of volume and speed.

DAT transfer rates are 1.2 megabytes per second in native mode and 2.4 megabytes per second when data is compressed. The cartridges also hold less data, storing 12 gigabytes of uncompressed or 24 gigabytes of compressed data, though this volume limitation can be overcome by using DAT libraries and cartridge autoloaders. (Typical list price for a DAT DDS-3 drive and autoloader is approximately $4,000.)

Hewlett-Packard (HP), for instance, offers users of Sun systems tape storage solutions like the HP SureStore DAT24 Library, with an individual media capacity of 24 gigabytes (compressed); this can be expanded with the addition of a six-tape autoloader which can swap cartridges within 15 seconds and restore any file from tape within 40 seconds. While the performance of DAT tape is considerably lower than that of a DLT solution, there is a corresponding reduction in backup and archiving cost.

DAT DDS-3, offers SCSI-2 interfacing only, making it less flexible than DLT. However, reliability is as good as DLT, with 200,000 hours MTBF for the drive and a 30,000 hour head life. There are also some minor advantages, like a smaller footprint (3.5-inch form factor) verses a 5.25-inch, full-size DLT drive. Also, DDS-3 is fully backward-compatible with older DDS media, so "stepping up" to DDS-3 is an easy, natural upgrade for an organization with an investment in earlier DAT tape media libraries.

Where is a DAT DDS-3 tape solution most appropriate? Primarily in small- to medium-size networks where DLT would be overkill. In these smaller networks, with lower volumes of data, and usually a larger backup window available, DDS-3 is more than adequate. Moreover, if RAID devices are already in use on the network, DDS-3 makes a good, inexpensive, "safety backup," offering an extra level of data protection.

On to 8 mm
While DLT falls at the highest end of the tape backup and archiving range, costing more but offering the greatest storage capacity and throughput, and DAT DDS-3 at the lower end of high-speed tape solutions, costing much less but with reduced capacity and throughput, 8-mm tape products fall in the upper-middle realm.

With 8-mm tape technology, the backup and archiving solution costs less than with DLT -- a typical drive goes for $4,900, and media costs $90 per cartridge. As usual, a reduction in cost from DLT results in a sacrifice of volume and speed, but not much. The Exabyte Mammoth and Sony AIT 8-mm tape products both offer transfer rates of three megabytes per second in native mode, and six megabytes per second in compressed mode. Exabyte offers 20 gigabytes of storage capacity (40 gigabytes in compressed mode), while Sony offers 25 gigabytes (50 gigabytes compressed). With these transfer rates and capacities, 8-mm technology comes very close to the power of DLT, at reduced cost. Moreover, the reliability is just as good, with a head life rating of 30,000 hours and 50,000 hours for Exabyte and Sony drives, respectively, and both products matching DLT's 200,000 hour MTBF for the drive. There are, however, significant differences between the 8-mm products available.

The Exabyte drive, for instance, offers a 5.25-inch, half-height footprint, while the Sony drive's footprint is a more compact 3.5 inches. Exabyte offers a fibre channel interface, and an interface to all the versions of SCSI including serial SCSI and SCSI-3, while Sony only interfaces to SCSI -2 and Fast and Wide. Exabyte emphasizes improvements like capstan-less design, and improved tape-loading and tape transport mechanisms, while Sony emphasizes its Advanced Intelligent Tape (AIT) technology.

This AIT technology not only offers improvements in head and media technology, but has features like an embedded 16 kilobit memory chip in the data cartridge, a feature lacking in DLT, Exabyte 8 mm, and the DAT DDS-3 competition. This chip, which Sony calls its Memory in Cassette (MIC) system, holds the system log and other user-defined data. This data is immediately available and always at hand after a cartridge is mounted, and makes rewinding the tape to read the log unnecessary, thereby speeding up data access and retrieval times for the second and subsequent retrievals.

What is the best environment for 8-mm tape? Like DLT, the target market is the higher end, including imaging, transaction processing, large database applications, workstation and networks, and video server applications.

There is, however, one major disadvantage in choosing the 8-mm backup and archiving technology: the divergence in standards between Exabyte and Sony. Exabyte was first in the market, and its devices will read all previous 8-mm tape formats. Sony came later, and in creating AIT, chose to give up backward compatibility. In effect, AIT itself cannot work with previously installed 8-mm products. As a result, choosing 8-mm technology also means choosing a vendor, and determining what standard you will follow.

But help with this incompatibility dilemma is on the way. Boulder, CO-based Breece Hill Technologies, a leading developer, manufacturer, and marketer of tape library systems, is about to introduce an automated tape library based on Sony's Advanced Intelligent Tape (AIT) 8-mm technology that provides full backward compatibility with the installed base of Exabyte 8-mm drives. Shipping of the company's FireFox AIT 8-mm library is scheduled to begin at the end of the first quarter of 1998, says Lee Elizer, vice president of marketing at Breece Hill.

The Sun-compatible FireFox libraries will be available in four configurations (10, 20, 40, and 60 cartridges) supporting up to four AIT drives for a maximum library capacity of three terabytes and data throughput in excess of 86 gigabytes per hour. "The new library design provides owners of Exabyte drives with access to the higher capacity and intelligence of AIT technology media, while maintaining read-write functionality with their existing base of Exabyte 8-mm tapes. The new FireFox AIT libraries can accommodate mixed interchange of AIT and Exabyte 8-mm tape in a single library enclosure," Elizer says.

Given the variety of high-end tape backup products available for Sun-based systems, which should you choose? If funding is not a problem, and you need the fastest throughput and largest media capacity possible for large volumes of data, go with DLT. If you have some flexibility in terms of speed and volume, 8 mm offers an excellent alternative at a slightly lower cost, but you'll have to choose between Exabyte or Sony. If financial considerations require a much lower cost alternative, or you have the luxury of a wider backup window with a low-to-medium volume of data, DAT DDS-3 may provide all the backup power you need at a price you can well afford.


About the author
Ron Levine is a freelance writer based in Carpinteria, CA. He specializes in networking, storage device, and emerging technology applications. His most recent feature for SunWorld was "Stretching the LAN," (August 1997). Reach Ron at

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Combining tape and backup software

To protect against data loss or corruption so that data recovery procedures can quickly bring back critical data in the event of a disaster, you can't beat the time proven reliability of tape storage. The tape storage operation requires an appropriate backup and archiving software package to transfer data to tape, both online and offline. Backup software must be compatible with the type of tape technology employed. It must also be compatible with the hardware platform (like SPARC), operating system (Solaris), and database (e.g., Oracle) running at the site.

Orent Graphics Art, an Omaha, NE-based company specializes in high-end scanning. The company scans in transparencies and then "magically" transforms them (by digitally revising images, changing color, and applying other graphics techniques). The end result is a digital file which is sent to the printer for production. Orent produces weekly ads and magazine inserts for large, well-known national and international publishing houses and retail stores, and it relies on a DLT solution as its Sun-based network's data safety net.

Ken Young, engineering manager at Orent Graphics says, "Our clients expect and receive a two to four day turnaround on their projects. There isn't any extra time to deal with lost or corrupted data. A typical image processing job generates about seven files, each of which are 40 megabytes to 100 megabytes in length. As a result, we always have multigigabytes of data perpetually online. This is data the company depends on for survival, and therefore it must be safely backed up."

Although the company employs a fault-tolerant storage array (RAID) on its two Sun SPARC 20 servers, it knows that RAID is not itself a total data backup tool; therefore, a DLT 4000 tape library is included in the company's "lights out" processing environment. The library and software (installed by Direct Connect Systems, Atlanta, GA) provides offline data backup as an additional safety net. The tape backup is integrated into the total automated data management scheme.

Hierarchical storage management (HSM) software, which operates invisible to the user, is a common option that permits many Sun system users to protect their data by migrating files between hard disk, optical devices, and tape storage -- all online. An HSM program places the most recently used files on magnetic disk, less frequently used files on near-online storage devices such as optical and tape libraries, and rarely used files are backed up to offline tape media. The software always "knows" where the file is located and how to get it. An HSM software package can intermix diverse devices (e.g., magnetic, optical, and tape drives) into an efficiently functioning storage subsystem -- all automatically, without user intervention.

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