Fundamental Differences Between SDLT and LTO Tape Drive Servo Systems.

During the second half of 1999, tape users will hear much about the merits of two competing tape formats, Super Digital Linear Tape (SDLT) and Linear Tape Open (LTO). A significant difference in the implementation of these two formats can be found in the track-following servo system, the mechanism responsible for enabling the extremely high capacities desired by tape customers. While the optical tracking technique being implemented on SDLT is still in a development phase, this article will attempt to contrast the merits of the two track-following servo systems used by SDLT and LTO tape formats. Knowing more about the differences between the two formats will help tape customers to make more informed and educated choices when upgrading current systems or investing in new ones.

In linear tape recording, data is written on many adjacent tracks that run parallel to the edge of the tape. Using Digital Linear Tape (DLT) technology, it has been possible to push this technology to record 208 tracks across a one-half-inch tape, allowing each tape to store up to 35GB. However, to achieve next generation tape capacities of 100GB and greater, significantly more tracks must be written on the media. Tape engineers recognized the need for a track-following servo, a mechanism that allows a tape head to dynamically follow a tape track, because track positions fluctuate due to slight media variations, mechanical tolerances, and other environmental factors.

The track-following servo is crucial to realizing the full capacity of tape storage solutions and achieving reliable data storage. A servo is similar to a car's cruise control, which allows the car to automatically maintain a constant speed. The car's servo compares the actual velocity against the desired speed and regulates the accelerator accordingly. Similarly, a tape drive using a track-following servo will constantly move the head, the component that actually performs the read and write function, and align it to the desired track.

Three years ago, IBM introduced the Magstar 3590 tape drive, a mainframe storage product that uses tape with pre-written reference tracks to guide the track-following servo. Returning to the automobile metaphor, reference tracks are like lane markers on a road. They provide the driver with a continuous reference as to where the car should be positioned. Similarly, on a tape drive, the head has special sensors that monitor the reference tracks. If the sensors detect the head moving off the track, the servo will automatically move the head to regain position. Additional recording elements on the head are located at precise distances from the servo sensors, allowing accurate writing and reading of data tracks.

Leveraging the track-following servo method deployed in the Magstar 3590, in November 1997, IBM, Hewlett Packard, and Seagate announced a new linear tape format for mid-range tape product customers called LTO. Together, these companies defined an aggressive technology road map for LTO and made it available to any drive or media manufacturer through a licensing process, supporting the belief that multiple competing manufacturers will assure long-term stability, better product availability, and more competitive pricing. LTO-compliant drives are expected from IBM, Fujitsu, Hewlett Packard, and Seagate, and five media manufacturers have purchased licenses to make the LTO-format media.

Concurrently, Quantum Corporation has designated SDLT, based on its DLT technology, as the format of choice for its next significant tape product. Further, Quantum has indicated that they will implement a new optical servo technique called Laser Guided Magnetic Recording (LGMR), which provides the servo with position information by shining a laser on a reflective strip manufactured on the back of the tape. Proponents of LGMR claim that this technique eliminates the cost and inconvenience of writing servo reference tracks on the media and allows more tape surface area to be available for data storage.

Although the LGMR method may save inconvenience, media manufacturers can easily eliminate additional costly steps in the LTO method by writing the servo tracks as they load the tape into their cartridges. While servo reference tracks occupy seven percent of the tape area, the loss of space is amply compensated by the data integrity they provide. Additionally, with planned improvements in track spacing and bit density along a track, the LTO format will allow future drives to offer capacities up to 800GB.

However, the most crucial difference between tape servo systems is the issue of reliability. Using the LGMR optical tracking method is like driving a car on a road without visible lanes while watching a median over 1.8 miles away. This calculation is derived by scaling up the track width, assumed to be approximately 27[micro]m, to a standard lane width of 14 feet. The consequences of a head moving off track under these circumstances can be disastrous--in less than a second, 10MB of data could be obliterated. The track-following servo method used on LTO-compliant drives offers a more robust approach by locating reference tracks close to the recorded data rather than on the other side of the media.

As customers continue to demand greater and greater storage capacities, it is important to have a proven, reliable tape servo system that protects data integrity, facilitates future capacity growth, and allows later generation drives to read earlier generation tapes. Having multiple world-class manufacturers of tape drives and media all committed to the LTO format will create competition in the tape industry that will ultimately benefit customers. With at least four major suppliers competing for the same customer base, this will result in better product availability, improved product quality, and more competitive pricing, but, perhaps, most importantly, customers can rest assured that they are making a wise decision about their tape format. Their investment will be protected for many product generations to come.

It is expected that several LTO products will be available by the end of 1999. More information about LTO technology is available at lto-technology.com. Randy Glissman is the business development manager of enterprise storage products at Fujitsu Computer Products of America (Longmont, CO).