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LTO: A better format for mid-range tape
Abstract and Figures
In the last two years, Linear Tape-Open® (LTO®) tape drives have become clear leaders in the mid-range tape marketplace. Tape drives designed to the LTO Ultrium® format were the first of the super drives to be shipped to mid-range tape customers. This paper describes some of the eclectic features designed into the LTO format. The technical emphasis is on aspects of the logical format that are new or different from preceding drives, though some aspects of the LTO roadmap and physical format are also discussed. The logical format comprises all of the data manipulations and organization involved in writing customer data to tape. This includes data compression to compact the data, appending of error-correction codes (ECCs) to protect the data, run-length-limited encoding of the ECC-encoded data, prepending headers to the encoded data to make it self-identifying on read-back, and storing of information about the data and the way it is stored in a cartridge memory module. Physical format aspects that are discussed include encoding data into the servo pattern and write shingling. Also discussed are the format-enabled aspects of drive functionality that have been improved over previous tape drive products, including enabling backward writing, elimination of problematic failure mechanisms, dynamic rewrite of defective data, handling servo errors without stopping tape, and enabling robust reading. Contrasts are made with previous products and competing products based on other format choices. Also discussed throughout is the way in which an eclectic format can be created by cooperation among three format-development companies.
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... Data writes to LTO tapes are done through a technology called Shingled Magnetic Recording (SMR) [8]. The idea behind SMR is to partially overlap previous wraps on new writes so that the written "bits" occupy the minimum wrap (or track) size possible, thus leading to increased storage density. ...
For a long time, tapes have had a single logical partition that was efficiently operated by dedicated software in batch mode. Nowadays, after the introduction of the LTO 5 standard, tapes support logical partitioning and, with the arrival of the Linear Tape File System (LTFS), their contents are exposed through the file system interface as regular files and directories. As a consequence, tape medium can now be accessed by concurrent processes that may create files in parallel. This creates two potential problems: interleaved data blocks and expensive partition switches. This paper presents the design and implementation of Unified, an I/O scheduler for LTFS that addresses these problems. We observe the effectiveness of delayed writes on decreased file fragmentation and the reduction of partition switches with the use of buffering and of redundant file copies. We also find out that software-based read prefetching, commonly used to manage disk devices, does not improve read times on tapes, but rather introduces potential overhead. With the techniques described in this paper, the Unified scheduler allows tape operations to be performed close to the raw hardware speed.
... In modern tape systems, servo bands are prewritten onto the tape media during tape manufacturing. In current LTO drives, five dedicated servo bands are recorded onto the medium embedding four data bands [19], [20]. Each dedicated servo band contains servo frames, which are designed to extract essential servo parameters, such as tape velocity, lateral head position, and longitudinal tape position. ...
In a tape system, the core of the reel-to-reel and track-following servomechanisms is the servo channel, which processes the servo signal generated by a servo reader reading a timing-based servo pattern. The servo channel estimates essential servo parameters, including tape velocity, head lateral position, and longitudinal tape position. The achievable precision of the various estimates determines how densely data can be placed on tape. In this paper, the architecture and the design of a highperformance synchronous servo channel (SSC) are presented. The channel performance is evaluated by means of analytical bounds on the estimation error, simulations, and experimental results, which demonstrate that the proposed servo channel is instrumental to achieve head positioning with nanometer accuracy and reliable detection of longitudinal position information in tape systems. Methods for the estimation of servo parameters with enhanced precision and of dynamic tape-to-head skew by dual SSCs operating on adjacent servo bands are also presented and their performance evaluated.
In a tape system, the core of the reel-to-reel and track-following servomechanisms is the servo channel, which processes the servo signal read back by a servo reader reading a timing-based servo pattern. The servo channel estimates essential servo parameters, such as tape velocity, head lateral position and longitudinal tape position. The achievable precision of the various estimates determines how densely data can be placed on tape. In this paper, the architecture and the design of the building blocks of a novel high-performance synchronous servo channel are presented. Simulation and experimental results indicate that the proposed servo channel allows nanometer head positioning in tape systems.
Tape drive operation with flangeless rollers in the tape path has been introduced in tape drives to extend the lifetime of drive and medium and to achieve nanometer positioning accuracy in track following by mitigating high-frequency lateral tape motion. However, the use of flangeless rollers requires dynamic-skew compensation to keep the head perpendicular to the direction of tape motion in the presence of large lateral tape excursions. Furthermore, a large uncertainty on the initial values of the tape-to-head skew and of the head lateral position must be resolved prior to track following for write/read operation. In this paper, we consider a control system for a two-degree-of-freedom head actuator using a dual synchronous servo channel to provide lateral position and tape-to-head skew feedback. The initial uncertainty on the tape-to-head skew and on the data band being spanned by the head at the beginning of drive operation is resolved by testing hypotheses related to different tape-to-head skew angles with an ad hoc metric obtained by the dual servo channel. Simulation results are presented to illustrate the performance of the proposed method.
We present a planar thin-film servo write head for writing timing-based servo patterns. The planar head operates at low current ( ~ 100 mA) with nanosecond switching times. The planar head can handle dc current, enabling trailing-edge writing. Magnetic force microscopy measurements of the transitions written by the planar head and a conventional commercial servo write head on longitudinal metal particle and nonoriented barium ferrite media were made to assess the quality of the written transitions. The transition response width PW50 is found to be independent of the tape velocity during write for the planar head. For the conventional head, PW50 is larger and increases with the tape write velocity. The reduced PW50 results in larger readback signal amplitude and hence an improved track-follow performance. The faster switching capability of the planar head enables formatting at higher tape velocity.
In a tape drive, the head assembly hosting servo readers and data writers/readers must initially be moved to a target position, so that information can be written to or read from the desired tracks. In tape systems with flangeless rollers, as recently introduced in IBM’s tape drives for improved performance and extended drive and tape lifetime, the initial positioning of the servo readers over the servo bands, from which the position-error signal for steady-state track-following operation is obtained, has to be fast. This requirement is dictated by the nonnegligible lateral tape motion experienced in tape paths with flangeless rollers. In this paper, a method is proposed to utilize both the servo readers and the data readers in a drive to detect the presence of a valid servo signal. If the servo readers are not positioned over a servo band, the distance between servo readers and servo band can thereby be continuously estimated by identifying the data reader detecting the presence of a servo signal. This information is then provided to a control element that controls the joint operation of a coarse stepper motor and a voice-coil motor based fine actuator with large stroke in a feedback or feed-forward configuration to achieve fast servo reader positioning. In the feedback-based approach, all data channels are continuously monitored to detect the presence of a valid servo signal and a pseudo position-error signal is generated and fed back to the control unit to drive the actuator towards the target position. In the feed-forward approach, a trajectory for the actuator is determined as soon as the servo band is in the capture range of the fine actuator and a control signal is generated so that the servo reader rapidly lands over the servo band with optimum control effort.
The storage market is presently dominated by hard disk drives. However, magnetic tape has maintained its cost-per-gigabyte ad- vantage with respect to hard disk drives and remains the primary choice for backup and mass data storage applications 1,2. Tape research can be divided into four main fields 1: a head technology, b recording media technology, c tape transport technology, and d recording channel electronics technology. In this paper, we focus on tape transport technology and review the present understanding of lateral tape motion and its effect on track density. Currently used instrumentation to measure lateral tape motion is described. In addition, the primary sources of lateral tape motion are discussed. Finally, the concept of active friction guiding and the design of a dual stage actuator tape head are discussed. 2 Recording Density
Timing-Based Servo (TBS) is a unique servo technology developed
specifically for linear tape drives. In TBS systems, recorded servo
patterns consist of transitions with two different azimuthal slopes, and
head position is derived from the relative timing of pulses generated by
a narrow head reading the pattern. Position signals are nearly perfectly
linear over multiple track widths, and highly immune to errors caused by
head wear, head instability, debris, and media defects. Multitrack TBS
servo patterns are written in a single pass using a novel multigap
horizontal thin film servo writing head. The design of the pattern and
its dimensions are optimized to provide sampling rate, noise level, and
error rate suitable for the intended application. An all-digital TBS
servo channel provides a speed-invariant position signal. Pattern
recognition algorithms detect servo signal errors, providing a highly
robust servo signal. Test results show approximately I pm linearity and
0.3 μm noise level over a width of 400 μm width with 18 kHz
sampling rate. The TBS pattern allows flexibility for encoding
additional information without affecting the position signal. By
shifting transitions as little as 0.1 μm from their nominal pattern
positions, a low error rate serial bitstream can be encoded in the servo
track. This technique allows tape longitudinal position to be encoded
with a resolution of about 2 mm, allowing efficient and precise tape
transport control based on the servo signal alone
For the last 50 years, tape has persisted as the media of choice when inexpensive data storage is required and speed is not critical. The cost of tape storage normalized per unit capacity (dollars per gigabyte) decreased steadily over this time, driven primarily by advances in areal density and reduction of tape thickness. This paper reports the next advance in tape storage—a demonstration of a tenfold increase in capacity over current-generation Linear Tape-Open® (LTO®) systems. One terabyte (1 TB, or 1000 GB) of uncompressed data was written on half-inch tape using the LTO form factor. This technical breakthrough involves significant advances in nearly every aspect of the recording process: heads, media, channel electronics, and recording platform.
Error Correction Code
- W G Bliss
- G L Dix
- N N Heise
- D N Moen
- G C Thomas
W. G. Bliss, G. L. Dix, N. N. Heise, D. N. Moen, and
G. C. Thomas, "Error Correction Code," IBM Tech.
Disclosure Bull. 23, No. 10, 4629 -4632 (March 1981).