Conventional Magnetic Recording Research Articles

Amphisbaena: A Novel Persistent Buffer Management Strategy to Improve SMR Disk Performance

The explosive growth of massive data makes shingled magnetic recording (SMR) disks a promising candidate for balancing capacity and cost. SMR disks are typically configured with a persistent buffer to reduce the read–modify–write (RMW) overhead introduced by non-sequential writes. Traditional SMR zones-based persistent buffers are subject to sequential-write constraints, and frequent cleanups cause disk performance degradation. Conventional magnetic recording (CMR) zones with in-place update capabilities enable less frequent cleanups and are gradually being used to construct persistent buffers in certain SMR disks. However, existing CMR zones-based persistent buffer designs fail to accurately capture hot blocks with long update periods and are limited by an inflexible data layout, resulting in inefficient cleanups. To address the above issues, we propose a strategy called Amphisbaena. First, a two-phase data block classification method is proposed to capture frequently updated blocks. Then, a locality-aware buffer space management scheme is developed to dynamically manage blocks with different update frequencies. Finally, a latency-sensitive garbage collection policy based on the above is designed to mitigate the impact of cleanup on user requests. Experimental results show that Amphisbaena reduces latency by an average of 29.9% and the number of RMWs by an average of 37% compared to current state-of-the-art strategies.

Improved Recording Performance With Writer Design and Joint Learning-Based Neural Network for Heat-Assisted Interlaced Magnetic Recording

Heat-assisted interlaced magnetic recording (HIMR) further increases the areal density with the interlaced track layout architecture compared to conventional heat-assisted magnetic recording. However, besides the 2-D inter-symbol interference (ISI) and thermal jitters, the transition curvatures due to the circular thermal profile cause severe erasures after write and obvious nonlinear distortions of readback signals. This deteriorates the recording performance severely. Especially the degrees of transition curvatures for the low temperature written track (LT) and high temperature written track (HT) are distinct, which makes the correction of transition curvature with only the writer design impossible. Correspondingly, a comprehensive method consisting of writer design and multitrack detection algorithm with joint learning-based neural network (JLNN) is proposed to mitigate the effects of transition curvature and 2-D ISI for both LT and HT of HIMR. First, a designed writer with splitting main poles produces the spatially nonuniform write field along the crosstrack direction to correct the transition curvature of LT. Then the multitrack detection with JLNN and improved Bahl–Cocke–Jelinek–Raviv (BCJR) detector (JLNN-IB) algorithm is studied to further mitigate the effects of residual transition curvatures, thermal jitters, and 2-D ISI for both HT and LT. Specifically, the readback signals of read head array and decoded log-likelihood ratio (LLR) outputted by the low-density parity check (LDPC) decoder are fed into JLNN to obtain the equalized signal and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$a$ </tex-math></inline-formula> <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">priori</i> probability (AP) of three tracks. Then the equalized signal as well as AP of current track and corresponding soft bit estimations of adjacent tracks are embedded into the branch metrics of improved BCJR detector for data detection, and the LDPC decoder is cascaded for the error correction. The simulation indicates that at the recording density of 2.11 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , the proposed comprehensive writer design and JLNN-IB algorithm provide 9.2 and 9.7 dB signal to noise ratio (SNR) gains compared to the conventional 2-D neural network equalizer and 2-D linear equalizer under the uniform write field and circular thermal profile, respectively.

FluidSMR: Adaptive Management for Hybrid SMR Drives

Hybrid Shingled Magnetic Recording (H-SMR) drives are the most recently developed SMR drives, which allow dynamic conversion of the recording format between Conventional Magnetic Recording (CMR) and SMR on a single disk drive. We identify the unique opportunities of H-SMR drives to manage the tradeoffs between performance and capacity, including the possibility of adjusting the SMR area capacity based on storage usage and the flexibility of dynamic data swapping between the CMR area and SMR area. We design and implement FluidSMR, an adaptive management scheme for hybrid SMR Drives, to fully utilize H-SMR drives under different workloads and capacity usages. FluidSMR has a two-phase allocation scheme to support a growing usage of the H-SMR drive. The scheme can intelligently determine the sizes of the CMR and the SMR space in an H-SMR drive based on the dynamic changing of workloads. Moreover, FluidSMR uses a cache in the CMR region, managed by a proposed loop-back log policy, to reduce the overhead of updates to the SMR region. Evaluations using enterprise traces demonstrate that FluidSMR outperforms baseline schemes in various workloads by decreasing the average I/O latency and effectively reducing/controlling the performance impact of the format conversion between CMR and SMR.

MAGIC: Making IMR-Based HDD Perform Like CMR-Based HDD

The past decades have witnessed the tremendous success of Conventional Magnetic Recording (CMR)-based Hard Disk Drives (HDDs) in data storage. To eliminate the bottleneck of CMR-based HDDs in providing higher areal density, an emerging Interlaced Magnetic Recording (IMR) is capable of achieving higher areal density with limited changes to disk makeup. Nevertheless, existing approaches for IMR-based HDDs may suffer serious read and write performance degradation as compared with CMR-based HDDs. Thus, this article presents a device-level solution, namely <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">MAGIC</i> translation layer, which aims at <bold xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><u>MA</u></b> kin <bold xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><u>G</u></b> <bold xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><u>I</u></b> MR-based HDDs perform like <bold xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><u>C</u></b> MR-based HDDs in terms of comparable access performance. Specifically, not merely trying to improve the performance of raw IMR-based HDDs, this work, for the first time, moves one step forward to minimize the performance gap between IMR and CMR-based HDDs. Technically, by 1) fully utilizing two special CMR-like potentials of IMR and 2) gracefully trading the sequential access performance as space usage increases, MAGIC minimizes track rewriting overheads to achieve CMR-like performance. Our results reveal that MAGIC not only improves the write performance compared with existing designs, but also has potential to approach read and write performance of CMR-based HDD.

Design Concept of MAS Effect Dominant MAMR Head and Numerical Study

We summarize several key factors in microwave-assisted magnetic recording (MAMR) design, particularly interactions between spin-torque oscillators (STOs) and write poles, frequency matching, the negative flux-control (FC) effect, and the STO vertical field effect. To satisfy these key factors, we study several configurations with longer gap lengths of 30 nm using a simulation based on the Landau–Lifshitz–Gilbert equation with a spin torque (ST) term including both the write head and STO. This simulation compared a dual spin-injection layer (SIL) structure, a dual flux-generation layer (FGL) structure, and two types of all-in-plane STO. While the dual SIL and dual FGL structures indicate sufficient oscillation behavior to satisfy the examined key factors, the dual SIL structure demonstrates poorer signal-to-noise ratio (SNR) performance than even conventional magnetic recording (CMR). We conclude that this poor performance is due to the STO vertical field effect. We newly propose a dual FGL structure with no vertical field in the in-plane field peak, leading to much better performance than that of the dual SIL structure. While this structure requires a much longer write gap than does a CMR writer, it retains an erase width comparable with CMR writers. These results indicate that MAMR with the proposed dual FGL structure will be usable at over 2800 kFCI without TPI loss.

Multitrack Detection With 2-D Iterative Soft Estimate Aided Neural Network Equalizer for Heat-Assisted Interlaced Magnetic Recording

Heat-assisted interlaced magnetic recording (HIMR) with interlaced track layout architecture enables the further increased areal density compared to the conventional heat-assisted magnetic recording. However, the severe transition curvatures of low temperature written tracks cause noticeable nonlinear distortions of readback signal, and the 2-D intersymbol interference (ISI), media noise, and thermal jitter also bring the challenges for data recovery. Correspondingly, a multitrack detection scheme with 2-D iterative soft estimate aided neural network equalizer (2D-ISA NNE) is proposed for the HIMR system, which iteratively feeds back the soft estimate of reliable sidetracks’ information (e.g., high temperature written tracks) during the neural network equalization (NNE) of middle track (e.g., low temperature written track). Here, the Bahl–Cocke–Jelinek–Raviv (BCJR) detector and low-density parity-check (LDPC) decoder are utilized for the following data detection and error corrections. Then, the similar ISA NNE is implemented to recover the sidetracks’ data. It is found that the proposed 2D-ISA NNE algorithm mitigates the 2-D ISI and nonlinear distortion more effectively compared to the conventional 2-D linear equalizer (LE) and neural network equalizer (NNE). For HIMR at the channel bit density of 3.51 Tb/in2 and overlapping ratio of 0.46, the proposed 2D-ISA NNE algorithm significantly decreases the bit error rate gap between the low temperature and high temperature written tracks. For the low temperature written track, it provides 3.1 and 4 dB signal-to-noise ratio (SNR) gains compared to the conventional 2-D NNE and 2-D LE, respectively.

Design of an advanced soft‐output Viterbi algorithm detector for bit‐patterned media recording systems

An advanced soft-output Viterbi algorithm detector aided by extrinsic information resources is proposed for bit-patterned media recording (BPMR) systems. These BPMR systems offer significant advantages over conventional magnetic recording systems. However, the performance system is significantly affected by a two-dimensional interference. The proposed method improves the channel detection output under the assistance of a variety of extrinsic information sources. Especially, combining the proposed model with an adaptive scheme for extrinsic information to enhance system performance is examined. Furthermore, evaluating the worth of extrinsic information in the different signal-to-noise ratio (SNR) conditions is also considered in this work. The results show that the SNR gains from the proposed model are about 4 dB compared to conventional solutions at non-misregistration.

DSTL

Shingled magnetic recording (SMR) is regarded as a promising technology for resolving the areal density limitation of conventional magnetic recording hard disk drives. Among different types of SMR drives, drive-managed SMR (DM-SMR) requires no changes on the host software and is widely used in today’s consumer market. DM-SMR employs a shingled translation layer (STL) to hide its inherent sequential-write constraint from the host software and emulate the SMR drive as a block device via maintaining logical to physical block address mapping entries. However, because most existing STL designs do not simultaneously consider the access pattern and the data update frequency of incoming workloads, those mapping entries maintained within the STL cannot be effectively managed, thus inducing unnecessary performance overhead. To resolve the inefficiency of existing STL designs, this article proposes a demand-based STL (DSTL) to simultaneously consider the access pattern and update frequency of incoming data streams to enhance the access performance of DM-SMR. The proposed design was evaluated by a series of experiments, and the results show that the proposed DSTL can outperform other SMR management approach by up to 86.69% in terms of read/write performance.

Perpendicular Interlaced Magnetic Recording

Perpendicular magnetic recording is the current generation hard disk drive technology, which enables the continued and significant growth of large storage systems in data centers. There are currently two common architectures for the layout of tracks in perpendicular hard disk drives: conventional magnetic recording (CMR) and shingled magnetic recording (SMR). In CMR, any track can be written at any time and neighboring tracks do not intentionally overlap. In SMR, the tracks are written sequentially in bands with the tracks intentionally overlapped like shingles on a roof. In this article, interlaced magnetic recording (IMR) architecture is applied with a perpendicular magnetic recording system. With Perpendicular Interlaced Magentic Recording (PIMR), we observed 2%–3% increase in areal density for single writer PIMR and 7%–20% increase in areal density for two writer PIMR over CMR whereas with SMR, we observed 19% increase in areal density over CMR.

Data Recovery for Multilayer Magnetic Recording

Multilayer magnetic recording (MLMR) is a next-generation technology that has the potential to enhance the longevity of magnetic recording technology by storing data on multiple layers that are stacked on top of one another. The challenges associated with the development of MLMR fall into two main categories: the challenges of writing to multiple layers and the challenges of reading back from multiple layers. As there is already a fair amount of existing work addressing the former, in this article we target the latter by building and testing detection schemes that are able to separate the signals from multiple layers that are mixed together during the readback. In this initial work, we consider a two-layer system. The resolution of the bottom layer is worse than that of the top due to the increased separation of the bottom layer from the reader. We assume a geometry of two single-width tracks on the top-layer straddling a double-width track on the bottom layer and with intersymbol interference (ISI) response on the bottom layer having half the amplitude and double the length of that on the top-layer. We examine the performance of two candidate detection algorithms, a Viterbi-based algorithm operating on a trellis, and a least-squared (LS) algorithm that attempts to invert the effect of the channel’s interference. We evaluate the error rate performance of these detection schemes using a common channel model that sums the ISI and intertrack interference (ITI) components from each of the layers. The main result from this work is that 14.5% density gains over the conventional perpendicular magnetic recording (PMR) and 7.6% gains over two-dimensional magnetic recording (TDMR) could be possible for the novel MLMR system.

Optimization of the 3-D-MAMR Media Stack

Microwave-assisted magnetic recording (MAMR) is an advanced magnetic recording scheme that is expected to extend the areal density beyond that achieved by conventional perpendicular magnetic recording (PMR). This is done by introducing a high-frequency (HF) oscillating magnetic field during the writing, reducing the write field from the head needed to switch the grains. Grains with larger anisotropy, greater thermal stability, and reduced grain size can thereby be used in the media. 3-D-MAMR uses this same principle not only to assist the writing but also to select a particular layer in a multi-layer medium stack for writing, thereby realizing 3-D magnetic recording. Optimization of a 3-D-MAMR system is complicated by the large number of variables involved and interactions between the recording layers. It is generally difficult to write on a target layer without some degree of unwanted writing on a different layer in a multi-layer stack. In this paper, we perform an optimization of a 3-D-MAMR media stack using the response-surface methodology (RSM), a method used in the design of experiments (DoEs). This results in a media design in which the signal-to-noise ratio (SNR) performance in the target layer and the number of written errors in the other layer are jointly optimized.

Adjacent Track Interference in Heat-Assisted Magnetic Recording: Impact and Implications

Some aspects of the challenges to high track density heat-assisted magnetic recording (HAMR) are addressed with a focus on track interferences (adjacent as well as non-nearest neighbor erasure). Comparisons with corresponding conventional perpendicular magnetic recording (PMR) parameters are also made. Adjacent and non-nearest neighbor track erasure data from functional HAMR and PMR drives and simulation data of signal-to-noise ratio loss in HAMR are also presented.

High field enhancement between transducer and resonant antenna for application in bit patterned heat-assisted magnetic recording.

Heat-Assisted Magnetic Recording (HAMR) is a well-known technology that uses the concept of a plasmonic antenna to shrink the optical focused spot to sizes of tens of nanometers, in order to overcome the limits of conventional magnetic recording and to reach areal density >1 Tb/in2. In this paper, we propose a novel concept that allows to increase a power transfer to the recording media deposited on the patterned substrate that takes a full advantage of the resonant antenna conditions to achieve further intensity enhancement in the gap and consequently high-power transfer to the recording media. Apart from the high field enhancement, the proposed concept ensures uniform field distribution in the gap that translates into uniform and higher temperature in the recording media, which is highly desired in the HAMR technology.

Micromagnetic simulator for complex granular systems based on Voronoi tessellation

Abstract A micromagnetic code based on Voronoi tessellation and the non-uniform fast Fourier transform (NUFFT) method is presented. The code is capable of efficiently and accurately simulating magnetization dynamics in large and structurally complex granular systems, such as multilayer granular media used for perpendicular magnetic recording, bit patterned media, granular nanowires, and read heads. In these systems the granular microstructure and distributions in grain and interface properties play an important role in device performance. The presented Voronoi simulator allows comprehensive studies to be performed as it accounts for the detailed granular microstructure and distributions that characterize true systems. Simulation time is greatly reduced by a non-uniform fast Fourier transform algorithm and implementation on graphics processing units (GPUs). Simulations of conventional magnetic recording, heat-assisted magnetization reversal, domain wall dynamics in granular nanowires, and particulate tape recording are presented.

Areal Density Comparison Between Conventional, Shingled, and Interlaced Heat-Assisted Magnetic Recording With Multiple Sensor Magnetic Recording

Heat-assisted magnetic recording (HAMR) is the next-generation hard disk drive technology which enables continued and significant areal density growth. There are currently three write architectures for the layout of tracks in hard disk drives: conventional magnetic recording (CMR), shingled magnetic recording (SMR), and interlaced magnetic recording (IMR). The read-back architecture multiple-sensor magnetic recording (MSMR) can be combined with the three different write architectures to increase the areal density by using two or more readers to read-back the same track. In this paper, we compare the areal density capability of HAMR CMR, HAMR SMR, and HAMR IMR combined with read-back with one reader, MSMR with two readers, and MSMR with three readers.

Joint Equalization of Asynchronous Reads in Array-Reader-Based Interlaced Magnetic Recording

This paper proposes a joint read equalization scheme for interlaced magnetic recording (IMR) employing an array of readers, where multi-reader captured waveforms from asynchronously recorded tracks are jointly processed for single or multiple track data retrievals. In the array-reader-based IMR (ARIMR), the inter-track interference (ITI) collected from one reader is not necessarily synchronized to the other readers, especially in the case of a large cross-track separation (CTS) between the readers due to the linear density differentiation between the neighboring tracks. For the efficient mitigation of such asynchronous ITIs, an effective resampling-based joint equalization scheme is proposed in this paper, where multiple readbacks are digitally synchronized for designated target tracks, and joint processing of them suppresses the ITI in the equalized output. Furthermore, the proposed effective synchronization can be applied for joint multi-track retrievals when the CTS of the array reader is closed to the track pitch. For performance evaluations of the proposed joint read equalization scheme, numerical ARIMR channel simulations by the micro-pixelated magnetic channel model are conducted, and a bit error rate performance is evaluated under various configurations. In addition, waveforms captured from a dual-reader hard disk drive are evaluated with the proposed joint read equalization schemes for practical performance measurements of ARIMR. In both evaluations, unfavorable CTS conditions are investigated to account for the dominant skew effect in ARIMR, where the proposed joint equalization scheme enhances the off-track capability performance, especially with the customized configurations of interlaced recording. In overall, ARIMR with joint read equalization shows the potential of an areal density capability gain and a doubling of the data transfer rate over the conventional perpendicular magnetic recording and legacy IMR.

High Track Pitch Density for HAMR Recording: 1M TPI

Differences between heat-assisted magnetic recording (HAMR) and conventional perpendicular magnetic recording at narrow track widths are compared. We can see that HAMR appears to have similar adjacent track interference, but increased readback signal from the adjacent tracks (side reading). This background interference signal appears to be a consequence of the media architecture, and can be decreased if the media soft underlayer spacing to the hard layer, and transducer is reduced. The impact is also reduced for very high linear densities, due to a smaller fraction of unbiased flux from the neighbor tracks reaching the test track. The use of two reader multiple sensor magnetic recording at high track density appears to address some of this issue, and we can demonstrate viable track writing for HAMR beyond 1M tracks per inch using this readback technique.

Impact of Magnetic Medium Grain Height in Heat-Assisted Magnetic Recording

Compared to conventional perpendicular magnetic recording, heat-assisted magnetic recording occurs at temperatures more than twice as high in degrees Kelvin. The ability of head field to align the medium magnetization over the short duration in recording a bit is limited by the ratio of magnetic potential energy to the thermal energy. In this letter, we present a systematic micromagnetic modeling study of the transition jitter resulting from thermal agitation during the recording process. As we continue to decrease grain size in order to increase recording areal density, the height of grains in the recording medium needs to increase accordingly to keep the number of magnetization-misaligned grains to a minimum.

Adaptive Asynchronous Inter-Track Interference Cancellation for Interlaced Magnetic Recording with Random Frequency Offsets

In this paper, an adaptive inter-track interference (ITI) mitigating scheme is proposed for interlaced magnetic recording (IMR) with random frequency offsets. Recently, proposed IMR has a potential areal density capability gain over conventional perpendicular magnetic recording, as it involves writing tracks with differentiated linear densities in an interlaced manner, while suppressing the ITI efficiently during the reading operation via asynchronous ITI cancellation (A-ITIC). The A-ITIC estimates and subtracts the ITI contributions from its side tracks in the effectively synchronized domain via interpolation-filter-based relative timing adjustments. However, when the tracks are recorded with non-negligible random frequency offsets, the ITI response is averaged out, providing a weak estimate, and the A-ITIC-based bit error rate performance gain disappears. In order to compensate for these dynamic timing misalignments, an adaptive ITI mitigation scheme is proposed called an adaptive A-ITIC, where the ITI response is continuously updated via effective timing adjustments along the sector. Numerical evaluations using a micro-pixel-based magnetic channel model show that the legacy A-ITIC performs poorly for IMR with non-negligible random frequency offsets, whereas the proposed adaptive scheme compensates for most of the performance degradation by effectively tracing the relative timing shifts via adaptation measures.

Heat-Assisted Interlaced Magnetic Recording

Heat-assisted magnetic recording (HAMR) is the next generation hard disk drive technology that enables continued and significant areal density growth. There are currently two common architectures for the layout of tracks in hard disk drives: conventional magnetic recording (CMR) and shingled magnetic recording (SMR). In CMR, any track can be written at any time and neighboring tracks do not intentionally overlap. In SMR, the tracks are written sequentially in bands with the tracks intentionally overlapping like shingles on a roof. In this paper, we introduce a novel track layout, interlaced magnetic recording (IMR), and apply it to a HAMR recording system. With heat-assisted IMR, we observed a 31% increase in areal density over HAMR CMR whereas in a HAMR SMR architecture, we observed a 27% increase in areal density over HAMR CMR.