How This DLT Era Innovation Drives LTO-10 Performance

By Pete Paisley

In the world of tape storage, where data densities have increased exponentially over decades, performance improvements are built on new and old technologies alike. One of the most interesting aspects of IBM’s LTO-10 tape drive is a sophisticated head tilt design—technically known as the servo format angle—that enables the increased capacity and performance amongst ever tightening tolerances like track pitch, track count, and SNR on the same ½” tape format LTO has used since its debut with LTO-1.

This approach to optimizing tape track alignment isn’t new to the industry. In fact, its conceptual roots trace back to innovations in Digital Linear Tape (DLT) technology from the 1990s, when engineers first grappled with how to maximize track density on linear tape formats. Understanding the evolution from DLT’s pioneering work to LTO-10’s current implementation reveals how incremental engineering refinements compound to create new storage capabilities.

The Fundamental Challenge

Magnetic tape recording faces a geometric constraint that has challenged engineers since the technology’s inception: how to pack more data tracks onto a fixed tape width without the read/write heads interfering with adjacent tracks. LTO-10 pushes this challenge to new extremes, cramming 14,784 data tracks across a tape ribbon just 12.65mm wide—an increase of 5,824 tracks over LTO-9’s 8,960 tracks.

The tracks themselves don’t run perfectly perpendicular to the tape’s edge. Instead, they’re written at a slight angle—the servo format angle—relative to the servo bands that guide head positioning. This angular approach is fundamental to achieving the track densities that make modern LTO capacities possible.

And having a head that can use a variable angle based on certain conditions can solve issues related to “TDS”, or Tape Dimensional Stability, (some refer to as Transverse Dimensional Stability). Tape media will change in width due to environmental considerations like temperature, humidity, and even the overall condition of the tape itself, and these dimensional changes must be compensated for by the drive to avoid mistracking or “misregistration” causing read errors.

DLT’s Angular Innovation

The conceptual foundation for angled track recording was established with DLT technology, as documented in US Patent 5371638A filed in 1993. George Saliba was the inventor on this and other related patents filed by Quantum for the DLT7000 generation drive. George, (who was affectionately called “Mr DLT” by many of us in the business), recognized the benefits of tilting the head stack at a slight angle relative to the tape’s direction of travel.

This particular DLT patent describes a “skewed azimuth recording” technique where the head assembly is deliberately positioned at an angle to the tape path. This geometric change had significant implications: when tracks are angled, the effective spacing between them—as measured perpendicular to the tape edge—can be reduced without the physical head assembly overlapping adjacent tracks.

The patent specifically addresses how this angular approach enables higher track densities while maintaining adequate separation between tracks to prevent crosstalk and interference. By writing tracks at an angle, DLT could pack more data onto the same tape width than perpendicular recording would allow, helping the format compete effectively in the backup market throughout the 1990s and early 2000s.

We asked Mr. Saliba for comments on the differences between the DLT angled head implementation vs. LTO-10, and he commented that:

“The DLT 7000 had three new recording technologies, shingle recording (track trimming), track pitch adjustment by rotating the head, and azimuth recording (tracks written at two different data +|- angles). I can’t be sure without details from IBM, but it looks like LTO10 implemented the first two technologies. The patent I filed for DLT70000 describes these three technologies plus a hybrid servo that used embedded servo in the data tracks. LTO today uses a dedicated servo similar to old HDD technology.”

George added that these changes to DLT7000 increased the track density capabilities by 4x or more.

Further Work at Quantum to Solve the Tape Dimensional Stability Challenge

Engineers at Quantum have continued to carry out important work on tilted head designs that addresses performance challenges over the years. Quantum engineer John S. Judge patented a tilted head design that more specifically addressed TDS and adjusted the angle of the head dynamically in 1998. Patent US3141174A was filed by Quantum and inventors John S. Judge and Robert A. Johnson in 1998, it describes a closed loop servo system that adjusts for track pitch changes in a DLT drive.

And Quantum filed patents by engineer Joe K. Jurneke specifically addressing TDS compensation systems in LTO drives in 2020 under patent US11367459B2. This patent seems to describe exactly how the current LTO-10 tilted head design works. The tape head in this patent can tilt to “adapt to potential changes in tape width due to changes in temperature, humidity, tension, cartridge creep and/or aging…” as cited in the patent.

In generations prior to LTO-10, drives used tensioning solutions for TDS. LTO-9 would increase tension to “manage tape width and therefore track position, during writes” according to a Quantum tech brief from 2023. This is why LTO-9 had a unique calibration process and would sometimes see very long unload times, as the drive had to “perform a process to re-wind the media into the cartridge back to BOT using a pre-programmed tension profile that optimizes the media for archival storage” according to the same Quantum Tech brief.

LTO’s Servo Format Angle Evolution

With LTO-10, the LTO format adopted and significantly refined the angular recording concept first introduced in DLT, but with a more sophisticated implementation suited to its higher density requirements. LTO uses dedicated servo bands—pre-written patterns that run the length of the tape—to maintain precise head positioning. LTO-9 employs five servo bands that separate and define four data bands, with the head assembly spanning two servo bands on either side of a data band.

The data tracks are written at a specific angle relative to these servo bands, and this servo format angle becomes a critical specification that defines track density. The angle must be precisely maintained across the entire length of the tape—1076 meters—while accounting for variations in tape tension, temperature, humidity, and mechanical tolerances.

LTO-10’s transition from LTO-9 involved careful optimization of this servo format angle to accommodate 39% more tracks in the same physical space. LTO-10 maintains the same four data bands and five servo bands structure as LTO-9, but packs significantly more tracks into each band. This required not just changing the servo format angle, but also refining the entire track geometry and shingling approach.

Shingling and Angular Recording

The servo format angle works in concert with another density-enhancing technique in LTO-10: shingled magnetic recording (SMR). The LTO-10 read-write head contains 32 elements, or channels, each with a write component sandwiched between two read components. The write tracks are partially overlapped like roof shingles, leaving a narrower, clearly readable track in the middle of each write track.

This shingling approach is only practical because of the angular track geometry. If tracks were perfectly perpendicular to the tape edge, the overlapping write operations would be more likely to interfere with adjacent tracks. The servo format angle provides additional geometric separation that makes aggressive shingling possible without compromising data integrity. This innovation was also borrowed from similar techniques used in DLT patents filed by George Saliba and Quantum. Many readers may also recognize this technique from hard drive technologies, as Seagate started shipping device-managed SMR hard drives in September 2013. There’s other precedents, including going back to 1980’s VHS VCRs.

LTO-10 pushes shingling further than LTO-9, the increased track count required more aggressive overlap patterns. The servo format angle had to be optimized to accommodate this more aggressive shingling while maintaining the servo system’s ability to accurately position the head over the narrower read tracks.

Engineering Trade-offs and Precision

The head tilt approach introduces significant engineering challenges. The LTO-10 drive must maintain the exact servo format angle throughout the entire tape length while the tape speeds underneath the head at up to 15 meters per second. The servo system continuously reads positioning information from the servo bands and adjusts head position to follow the angled track path, compensating for tape dimensional changes, tension variations, and mechanical tolerances.

The comparison between DLT and LTO implementations highlights how the same fundamental concept—angled recording—evolved to meet different requirements. DLT’s simpler servo system and lower density targets led to one implementation of head tilt, while LTO’s sophisticated servo architecture and extreme density requirements drove continuous refinement of the servo format angle with each generation.

However, this optimization comes with costs. LTO-10 lost backward compatibility with LTO-9. This limitation surely stems from the servo format angle changes—the head assembly and servo system optimized for LTO-10’s track geometry cannot read the different track angles used in the prior generation.

The Path Forward

As LTO continues to evolve, the servo format angle remains a key parameter that engineers can optimize to increase capacity. The progression from DLT’s pioneering work in the 1990s to LTO-10’s current implementation demonstrates how fundamental engineering principles can be refined over decades to push the boundaries of magnetic recording.

Yet the approach faces physical limits. LTO-10’s 14,784 tracks represent an extraordinary achievement in precision engineering, but each generation makes the next more challenging.

For more about the path forward, we reference the INSIC International Magnetic Tape Storage Technology Roadmap 2024. It’s expected that the next generation will likely implement a doubling of channels from 32 to 64, significant continued increase in TPI, tape length, and may include a big jump in the current 400MB/s maximum streaming drive data rate due to the increase in channels. Something that could be very meaningful to the format. Stay tuned as we continue to develop our content around LTO drive technology.

In an era where data growth seems limitless, the precision engineering embodied in something as seemingly simple as a head tilt angle reminds us that revolutionary advances often come from perfecting the fundamentals. The servo format angle in LTO-10 represents decades of incremental refinement, each generation building on insights from predecessors like DLT to push magnetic tape storage to densities that would have seemed impossible just years ago.

Pete Paisley is the host of the LTO Show, the premier podcast for leaders in the LTO tape storage hardware community. Please reach out with story ideas or comments, we’ll respond to each directly. pete@ltoshow.com

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