Number 194 - July 1999

What About Helical Tape Versus Linear Tape?
by Roy Moglia, April 1999 Suncoast Beeper, St. Petersburg, FL
POINT: While both HELICAL scan and LINEAR drives have made significant advances over the past decade, HELICAL has the edge in terms of linear bit density, track density, overhaul (sic) [overall?] areal bit density, and data rate per head. However, no matter how interesting the engineering details may be, they are largely irrelevant to the users of tape backup. The truth is Helical and Linear drives have evolved to have nearly the same specifications and roadmaps. What really matters now is how the application of each technology affects system backup.     Two factors determine tape library capacity: tape cartridge capacity and the number of tape cartridges. Leading HELICAL cartridges are much smaller than leading half-inch LINEAR technologies, which means HELICAL-based libraries can house about twice the number of drives as the LINEAR libraries. Similarly, since performance is determined by the data rate of each drive and the number of drives in the configuration, HELICAL libraries can provide higher performance through parallel drive operation.     When it comes to tape wear, HELlCAL has a number of advantages over LINEAR. LINEAR drives record in a serpentine fashion, which means the tape is run back and forth from end to end until all tracks are recorded. HELICAL-scan drives, in contrast, record data in a single pass from one end of the tape to the other, which means much less tape wear per backup operation.	COUNTERPOlNT: The advantages of LINEAR tape data storage systems can be summarized in one word: simplicity. LINEAR tape systems, such as TRAVAN, are specifically designed for data storage applications. HELICAL-scan technologies, such as DDS (Digital Data Storage) and AIT (Advanced Intelligent Tape), have their origin in video and audio recording, and required extensive redesign to accommodate data storage needs. The design of LINEAR tape data cartridges and drives is inherently simple-in terms of drive mechanism, tape path, head design and tape formulation. HELICAL-scan technology is comparatively complex.     Tape paths are another significant difference between the two technologies. In LINEAR systems, the tape never physically leaves the cartridge and therefore is exposed to fewer environmental contaminants. HELICAL drives, on the other hand, incorporate more tape path guides-all of which must be accurately aligned, making it much more challenging to achieve high reliability at low cost. HELICAL-scan heads are much more susceptible to debris passing through the head gap region and to head gap clogging from either atmospheric debris or material worn off of the media surface.     LINEAR formats tend to be more conservative for a given capacity in the areal density, track width, and transition spacing of the data tracks as they are recorded on the media surface. Simply put, LINEAR tape allows greater physical space on the tape for each data bit that is recorded. This reduces the risk of reading/writing errors and data loss. It also means opportunities to increase storage capacity in future generations by further increasing areal density and/or decreasing track width and transition spacing.
Number 194 - July 1999