In order to increase tape cartridge capacity while maintaining the current cartridge form factor, either the area of tape that is available to store data must be increased and/or the areal density must be increased. One approach to increase the area available for storing data is to improve the format efficiency, e.g. through the use of more efficient coding schemes.
An even simpler approach is to increase the available surface area of the tape, however, there are some limitations. For example, there is a strong incentive to preserve the half inch tape width used in linear tape drives due to the large investment in equipment for manufacturing tape in this format. Similarly, it is desirable to preserve the cartridge form factor due to the large installed base of automated tape libraries that are compatible with the current formats.
On the other hand, the tape length can be increased without changing the cartridge form factor if the tape thickness is simultaneously decreased. Unfortunately, this renders the tape more fragile and necessitates advances in the tape transport system and tape path design.
Over the next 10 years the INSIC tape roadmap predicts a 32× increase in cartridge capacity, of which tape length scaling is expected to contribute a roughly 50% increase in capacity (i.e. 1.5×) with an additional 15% capacity increase (1.15×) due to improvements in format efficiency. Hence, the majority of the expected capacity increases will have to be achieved by scaling the linear and track density.
While continued incremental increases in linear density are expected in the future, there is a much larger potential for increasing track density by reducing the width of the individual data tracks. In order to exploit this potential one has to minimize tape lateral motion and compensate for any remaining disturbances by adjusting the position of the write/read heads dynamically to follow the disturbances.
For very high track densities, positioning control down to the nanometer scale will be required. Such precise control necessitates significant improvements in the performance of all elements of the track follow servo system, including the servo pattern, the servo channel, the head actuator and the track-follow servo controller [1]-[5]. In addition, continued improvements in tape media dimensional stability (TDS), e.g with respect to changes in environmental conditions, or the implementation of a TDS control scheme will be required to maintain all of the write/read elements on track under the varying environmental conditions in which tape drives operate.
A third challenge associated with track density scaling arises from the need to reduce the width of the data read element in proportion to the width of the data track. This results in a reduction in signal to noise ratio (SNR) of the read-back signal that must be compensated for by a combination of improved media technology, improved read transducers and improvements in the read channel [6].
Another significant scaling challenge results from track edge effects such as side erasing and track edge curvature. As track widths are reduced these effects become large relative to the track width and must be minimized through innovative design of the write transducers.
In the area of media technology, the SNR performance of particulate media can be improved by reducing the volume of the particles, which reduces media noise. However, this has the consequence of reducing the thermal stability of the particles, which must be compensated for by increasing the anisotropy energy density of the particles. This in turn requires the development of write transducers capable of producing sufficiently large magnetic fields to write these particles.
A second approach is to make the media smoother in order to reduce the magnetic spacing, i.e. the distance between the read/write transducers and the magnetic coating. However, increasing the tape smoothness tends to increase tape-head friction, which in turn reduces the “runability” of the media and leads to high frequency velocity variations that can degrade the performance of the data detection process.
Continued scaling of the data rate of tape systems also poses significant technical challenges.Historically, the data rates of tape systems have been increased through a combination of enhanced linear density and higher tape speed, and by increasing the number of write/read transducers that operate in parallel.
IBM’s latest enterprise drive, the TS1140, writes and reads 32 data tracks in parallel. In the future, linear density increases are expected to be more modest than in the past. There are also limits to the maximum tape speed that can be achieved without increasing the disturbances to the track follow system to unacceptable levels. Hence the number of parallel channels will likely be scaled more aggressively in the future.
However, increasing the number of channels while maintaining a constant head span to minimize TDS effects leads to a reduced spacing between transducers. This is particularly challenging for write transducers due to fabrication issues and due to the possibility of cross-talk between adjacent writers at small transducer pitch. Avoiding such cross-talk effects requires careful design of the write transducers. In addition, adding more transducers in the head results in an increase in the number of electrical connections that must be routed over the long flex cable connecting the head to the electronics card of the drive.
Increasing the data rate of an individual channel by increasing linear density and/or tape speed is also quite challenging, due to the impedance of the flex cable. Some of these challenges can be addressed by moving some of the front-end electronics off the drive card and directly adjacent to the head on the flex cable, using flip-chip technology.
References
[1] Scaling Tape-Recording Areal Densities to 100 Gbit/in2,
A. Argumedo, D. Berman, R. Biskeborn, G. Cherubini, R. Cideciyan, E. Eleftheriou, W. Häberle, D. Hellman, W. Imaino, J. Jelitto, K. Judd, P.-O. Jubert, M.A. Lantz, G.M. McClelland, T. Mittelholzer,S. Narayan, S. Ölçer,
IBM J. Res. Develop. (Storage Technologies and Systems) 52(4/5) (2008).
[2] Characterization of Timing Based Servo Signals,
G. Cherubini, R.D. Cideciyan, E. Eleftheriou, P.V. Koeppe,
Digest of Technical Papers IEEE Int'l Magnetics Conf. "INTERMAG 2008," Madrid, Spain, May 5-8, 2008, pp. 600-601.
[3] Synchronous Servo Channel Design for Tape Drive Systems,
G. Cherubini, E. Eleftheriou, J. Jelitto, R. Hutchins,
Proc. 17th Annual ASME Information Storage and Processing Systems Conf. "ISPS 2007," Santa Clara, CA, June 2007, pp. 160-162.
[4] Nanoscale Track-follow Performance for Flexible Tape Media,
M. A. Lantz, A. Pantazi, G. Cherubini and J. Jelitto,
Proceedings of the 18th IFAC World Congress, Milano (Italy) August 28 - September 2, 2011.
[5] Servo-Pattern Design and Track-Following Control for Nanometer Head Positioning on Flexible Tape Media,
M. Lantz, G. Cherubini, A. Pantazi, J. Jelitto,
IEEE Transactions on Control Systems Technology, Volume: 20, Issue: 2, March 2012, pp. 369-381.
[6] Adaptive Noise-Predictive Maximum-Likelihood (NPML) Data Detection for Magnetic Tape Storage Systems,
E. Eleftheriou, S. Ölçer, R.A. Hutchins,
IBM J. Res. Develop. 54(2) (2010) Paper 7.