Tape Drive Technology Comparison: Sony AIT, Exabyte Mammoth, and Quantum DLT
Dale Swindler In the past decade, the explosive growth of computer disk capacity, networks, and database applications has driven a huge demand for data storage products. Due to their low cost per gigabyte stored, tape drives have quickly become the preferred method of backing up and storing data and protecting against data loss. There are currently three leading mid-range tape drive technologies: Exabyte Mammoth, Boulder, CO; Quantum DLT, Milpitas, CA; and Sony AIT, Park Ridge, NJ. When choosing to invest in one of these technologies, a buyer must have a complete understanding of the strengths and weaknesses of each drive. While DLT and Mammoth technology have seemingly dominated the marketplace, our research shows that Sony AIT technology has equaled or surpassed them in the areas of performance, capacity, and cost per gigabyte. Evolution of Tape Drive Technologies Exabyte developed its 2.5 GB, 8-mm helical scan tape drive in 1985. The mechanical subassembly was designed and manufactured by Sony, while Exabyte supplied the electronics, firmware, cosmetics, and marketing expertise. This Exabyte/Sony partnership ended when Exabyte developed the new Mammoth drive, designed and manufactured solely by Exabyte. When that partnership deteriorated, Sony continued its development in the tape technology field, and last year introduced the AIT (Advance Intelligent Tape) drive. The AIT drive, which is designed and manufactured entirely by Sony, also employs the 8-mm helical scan recording method. However, Sony's new recording format is not compatible with 8-mm drives from Exabyte. Sony's new AIT drive is the first generation of this technology with several planned future products. Quantum Corporation, the sole manufacturer of DLT (Digital Linear Tape), purchased the DLT technology from Digital Equipment Corp. in 1994. Since that time, Quantum has successfully developed and marketed several generations of DLT drive technology including its newest product, the DLT-7000. Differences in Technology Although these tape drives all strive to achieve the same result, there are some very distinct technological differences between the three. One of the first differences to note are the two methods of recording styles. Helical scan and linear serpentine are the two current recording styles in the industry. Sony AIT and Exabyte Mammoth drives use the helical scan recording style in which data tracks are written at an angle with respect to the edge of the tape. The helical pattern is achieved by wrapping the 8-mm magnetic tape partially around an angled, rapidly rotating drum. The read and write heads are precisely aligned in the drum and protrude very slightly from its smooth outer surface. As the tape moves over the angled, rotating drum, the heads create a helical data track pattern on the tape (Figure 1). In the original DLT technology, drives wrote linear serpentine tracks parallel to the edge of the tape (Figure 1). The 1/2"-wide tape moves over the head assembly, which houses the precisely aligned read and write heads. To write linear serpentine tracks, tape motion is halted while the head assembly moves incrementally up or down to precise positions with respect to the edge of the tape. In an attempt to achieve higher data capacity, the DLT-7000 incorporates a modified linear serpentine method called SPR (Symmetrical Phase Recording). The DLT-7000 head assembly actually rotates into three different positions. Blocks are written in a herringbone or SPR pattern providing a higher track density and higher data capacity than the DLT-4000 on a given length of tape (Figure 2). A third, vertical head position allows the DLT-7000 to read DLT-4000 tapes. Although this rotating head assembly provides the capability for more capacity as well as backward-read compatibility, it also had its setbacks. It is generally accepted in the tape backup marketplace that delays in the release and shipments of DLT-7000 are associated with difficulty of this implementation. Another difference between the three technologies is found in how they handle tape speed and streaming. During read/write operations, the DLT drive moves tape over the read/write heads at a relatively fast rate of 150 ips. The AIT and Mammoth drives use a much slower tape speed, less than 1 ips. It is interesting, however, that the relative speed between the heads and the tape is nearly equal in all three technologies, so it could be argued that neither offers an advantage at the critical head/tape interface. The slower moving AIT and Mammoth tape may also be less likely to undergo tape stress - especially during the start/stop motion. Besides incurring tape stress, the DLT drive's performance will suffer if the data is not supplied to the drive at a rate sufficient to keep it streaming. Non-streaming occurs when backups occur over the network, and data transfer is bottlenecked due to slow network speed relative to the transfer rate of the drive. In this scenario, the DLT drive must stop the tape, rewind well past the last file mark, wind up to 150 ips, and begin reading or writing again. All of this takes a considerable amount of time. Mammoth and Sony AIT drives can perform stop/rewind/start very quickly due to the slower tape speed. As a result these drives outperform DLT drives in applications where drive streaming is not possible. Tape loading and handling in these drives is also a delicate procedure. The tape must be pulled from the cartridge, placed over the head/drum assembly, and guided precisely across the heads and through the tape path. Improper tape handling leads to high error rates, tape damage, and even data loss. These three tape technologies differ significantly in methods of tape handling and loading. The DLT method of tape loading is most unique. When the DLT cartridge is inserted, the drive engages a leader at the beginning of the tape and pulls the tape out of the cartridge through the tape path, and onto a take-up hub inside the drive. During the read or write operation, the tape is actually spooled on and off a hub inside the DLT drive. This explains why DLT drives are physically much larger than other drives. DLT cartridges must not be dropped or roughly handled because the tape will slacken, and the leader will mis-engage when inserted into the tape drive. If mis-engagement occurs, the DLT tape cartridge is rendered useless, and the drive may require significant repair. Sony AIT and Exabyte Mammoth drives use a more common method of tape loading. When the 8-mm cartridge is inserted, drive motors engage with the cartridge hubs, which work in conjunction with tape loading guides to unload tape from the cartridge into the tape path. As the read or write operation is performed, the tape is spooled from one cartridge hub to the other through the drive's tape path. Unlike DLT, Sony AIT and Mammoth tape cartridges typically do not exhibit problems due to rough handling. Physical size of a tape drive is important when integrating these drives into tape libraries, servers, and other host systems. Tape drive size is designated by a "form factor" that indicates the width of the drive and assumes standard height and length. The Sony AIT drive is the most compact with a 3 1/2" form factor. The Exabyte Mammoth is next with a 5 1/4" half-height form factor. The Quantum DLT products are largest with a 5 1/4" full-height form factor and a nonstandard 9" length. Comparing the DLT and AIT drives when incorporated into libraries shows the significance of the size and weight differences. The average 4-drive DLT library is 44"H x 23"W x 32"D with a weight of 380 lbs. A comparable, 4-drive AIT library is 7"H x 17"W x 27"D with a weight of 56 lbs. Another critical issue is tape handling and tape speed control. Tape drives require the tape to be moved precisely through the drive's tape path and across the read/write heads during read/write operations. The relative speed between the tape and the heads must be accurately controlled. AIT and DLT drives employ a traditional servo-driven capstan/pinch roller device, which precisely controls tape speed while moving tape over the head assembly (the take-up and supply hubs simply spool and unspool the tape while keeping it under precise tension). Mammoth uses an entirely new "capstanless" design, in which the tape speed is controlled by servo-driven take-up and supply hubs. This is quite an engineering feat, as the speed of the hubs must be constantly and precisely varied as the diameter of the tape spool changes. For instance, in order to maintain a constant tape speed across the heads, the take-up hub speed must decrease steadily as the tape spool gets larger. The goal of the capstanless design is to reduce tape stress and damage caused by the capstan and pinch roller. Media Types The recording media itself is another investment to consider when choosing a tape drive. The two types of media currently used in these tape drives are MP (Metal Particle) technology, which is used in DLT-4000 and DLT-7000, and AME (Advanced Metal Evaporated) technology, which is used in Mammoth and Sony AIT. Both MP and AME tapes cost around $100 each, and they have a very similar makeup containing a base film and a recording layer of magnetic metal material. However, AME has several features that significantly improve drive reliability. The MP recording layer is composed of about 45% magnetic material mixed with a binder and other additives, while the AME media recording layer is nearly 100% magnetic material. The highly metallic surface of AME media allows higher recording densities with 50% lower tape tension than MP. This metallic surface significantly reduces drive head wear and head contamination due to tape debris. The AME media also contains a smooth, protective carbon coating on the recording surface, a lubricant, and a protective back coating. These unique features further reduce drive head wear and increase the number of passes AME can withstand without degradation. The AME media offers twice as many passes as MP media (see Table 1). Note that the term "passes" is used here rather than "uses," because one use may involve numerous tape passes over the read/write heads as the drive searches, reads, writes, and rewinds the tape. DLT-4000 and DLT-7000 use the same MP tape cartridge, and the DLT-7000 is backward-read compatible with the DLT-4000. The AME media use and compatibility vary. The Sony uses a cartridge specifically designed for the AIT unit. The Sony AIT drive accepts only AME media made for the AIT drive and will reject all other 8-mm cartridges. AME tapes written on a Sony AIT drive are write-protected from other 8-mm drives including Mammoth. Exabyte's Mammoth, on the other hand will read both AME and MP media, but will not write to the MP cartridge. The Mammoth is designed to read both media types in order to be read-compatible with existing MP 8-mm tapes. This compatibility required a special head design and necessitates special cleaning practices by the user. For example, if an MP tape is read by the Mammoth drive, the drive will not accept another tape of any kind until the drive is cleaned. Cleaning is required because the MP media binder chemistry is more prone to leave debris on the head and in the tape path. One issue that needs to be addressed is the number of times a Mammoth drive can read an MP tape without suffering permanent damage. The implications here are amplified in the case of tape libraries, in which tapes are read many times. It is perhaps more realistic for Mammoth users to transition to the AME media and avoid using MP media. Reliability Tape drive reliability is measured by MTBF (Mean Time Between Failure). MTBF is the average length of time the drive will operate without failure. In reality, drive reliability varies and cannot be accurately predicted by the manufacturer's MTBF specification. Because operational and environmental conditions play such a large role, tape drive manufacturers often add a disclaimer to the MTBF specification. Head life specifications (in hours) are subject to many of the same problems as MTBF. Until a more meaningful reliability measurement is found, the industry must depend on MTBF and head life. Table 2 shows how these three tape manufacturer's reliability specifications compare. Cleaning the drive per the manufacturer's recommendations, using high quality data tapes, and keeping the drive "streaming" will maximize reliability. Another facet of tape drive reliability is the data integrity issue. Data integrity is specified as the Bit Error Rate (BER) which gives the number of permanent errors per bits written. The Mammoth, DLT, and AIT drives all incorporate read-after-write verify error detection, a Cyclic Redundancy Check (CRC), and an Error Correction Code (ECC) algorithm to ensure an extremely high BER of 10-17 or 1 error in 100,000,000,000,000,000 bits. The Sony AIT drive is the only product that incorporates a third-level error correction code for increased data integrity. Drive cleaning is a time-consuming nuisance for most administrators. Unfortunately, it's necessary for most drives because dust, media particles, and other contaminants enter the head/tape interface area and cause error rates that slow performance, decrease capacity per tape, and eventually cause a drive failure. Tape manufacturers have traditionally addressed this issue with periodic cleaning using a cleaning cartridge that is loaded into the drive just like a data cartridge. Exabyte Mammoth and Quantum DLT products both have an LED cleaning light that flashes when the drive needs to be cleaned. Exabyte specifies that a cleaning cartridge be loaded into the Mammoth drive every 72 tape-motion hours. Quantum DLT drives have no recommended cleaning interval other than the flashing LED. Sony has taken a different approach to keeping the AIT drive's tape path and heads clean. First, the AIT drive does not rely on external fans in the server, library, or system cabinet, which force airborne dust into the drive. AIT drive cooling is achieved via an internal, variable speed fan that cools the PCBA and base plate without introducing airborne dust into the tape path. Secondly, a built-in head cleaning wheel is automatically activated by an error rate monitoring device, which ensures a clean head/tape interface and maximum performance. Finally, the AME media formulation significantly reduces tape debris. These features allow the Sony AIT drive to operate with virtually no cleaning, eliminating maintenance problems and reducing the drive's operating costs. Capacity and Performance Manufacturers measure tape drive capacity by the amount of data that can be recorded on a single tape cartridge. Greater capacity is gained by increasing the track density on a given section of tape or by increasing the physical length of the tape. These comparisons are based on the maximum tape lengths available today. Hardware data compression is also used to increase capacity, and a valid comparison must show both "native" and "compressed" values. Each manufacturer uses a different data compression algorithm, which results in different compression ratios:
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