Aerodynamically induced power loss in hard disk drives

In this paper, the effects of rotational speed, form factor and enclosure conditions on power dissipation in hard disk drives are presented. The aerodynamically dissipated power losses by 3.5, 2.5 and 1 in. hard disks are experimentally measured using a vacuum chamber and compared to theoretical estimations. Experiments in open air without enclosure agree well with theoretical predictions; a 3.5-in. disk satisfies the turbulent model but 1 and 2.5-in. disks match the laminar one, which is inversely proportional to the half power of Reynolds number. Experiments using a single 3.5-in. disk in enclosure show that aerodynamic power loss is proportional to the second power of rotational speed and the fourth power of disk radius, which agrees with the laminar theory rather than turbulent one. It is also shown that the aerodynamic power loss is reduced as the axial gap and radial clearance of enclosure decrease.     

An experimental study on the flow behavior near the read-and-write arm in a hard disk drive model with a shroud opening

Complex flow inside a hard disk drive (HDD) was investigated using a simplified 3.5″ model for clarifying the mechanism causing flow-induced vibration. In contrast to the authors’ related study in the past, our model had a non-axisymmetric geometry equipped with a shroud opening and a read/write arm (RWA). The model is designed to serve as a benchmark to study HDD flows both in experiments and in numerical simulations. The complex flow behavior in the disk-to-disk space was investigated with the RWA inserted into the inter-disk space. Flow measurements were carried out with a test rig which consisted of transparent disks, RWA and covers. The measurements were performed at the disk Reynolds number Re _d  = 1.2 × 10⁵ which corresponds to the rotation speed of 7,700 rpm of a real 3.5″ HDD. Two sets of the flow measurements were performed—the first Reynolds stress components measured along four different lines with the RWA inserted at a shallow angle (experiment I), and the other mean and rms velocity statistics along several selected lines with two different RWA insertion angles (experiment II). The mean velocity and velocity variance were obtained at a spatial resolution of 30 μm along eight different lines perpendicular to the disk surfaces. The high spatial resolution of the results was achieved using a laser Doppler velocity profile sensor with a physical resolution in micrometers and a velocity uncertainty of 0.2 %. In the experiment I, the mean velocity and velocity variance statistics were mostly consistent with the common findings in other studies using axisymmetric models except for the flow behavior in the radial direction at the shroud opening. The secondary flow behavior was likely caused by the shroud opening which was not included in most of the models in the past. In experiment II, the mean velocity and velocity variance were successfully measured through examination of the flow above and below the RWA in the space between the rotating disks. The resulting velocity statistics exhibit turbulent Couette-like flow in the narrow 1 mm space between the disks and the RWA.     

The optimal helium fraction for air–helium gas mixture HDDs

Helium-filled drives have recently been commercialized to enable a high recording density. However, because the use of helium increases production costs, binary gas mixtures such as air–helium have been investigated. In this paper, the dominant performance metrics of hard disk drives (HDDs) are the windage losses, the flow induced vibration (FIV), the lubricant transfer and lubricant depletion. These were investigated for air–helium gas mixtures as a function of the helium fraction. The frictional torque was empirically derived in both the laminar and turbulent regimes. The windage loss and the FIV of a helium-filled drive were found to be similar to that using an air–helium gas mixture with a helium fraction of 0.75. On the other hand, the quantity of accumulated lubricant and the maximum lubricant depletion in a helium fraction of 0.75 were superior to those in a helium fraction of 1.0. Further investigation of performance metrics should be carried out. However the performance metrics considered here showed that a helium fraction of 0.75 was favorable to a helium fraction of 1.0.     

Resource scheduling in a high-performance multimedia server

Supporting continuous media data-such as video and audio-imposes stringent demands on the retrieval performance of a multimedia server. In this paper, we propose and evaluate a set of data placement and retrieval algorithms to exploit the full capacity of the disks in a multimedia server. The data placement algorithm declusters every object over all of the disks in the server-using a time-based declustering unit-with the aim of balancing the disk load. As for runtime retrieval, the quintessence of the algorithm is to give each disk advance notification of the blocks that have to be fetched in the impending time periods, so that the disk can optimize its service schedule accordingly. Moreover, in processing a block request for a replicated object, the server will dynamically channel the retrieval operation to the most lightly loaded disk that holds a copy of the required block. We have implemented a multimedia server based on these algorithms. Performance tests reveal that the server achieves very high disk efficiency. Specifically, each disk is able to support up to 25 MPEG-1 streams. Moreover, experiments suggest that the aggregate retrieval capacity of the server scales almost linearly with the number of disks     

A video caching policy for providing differentiated service grades and maximizing system revenue in hierarchical video servers

A video server normally targets at providing abundant bandwidth access and massive storage in supporting large-scale video archival applications. Its performance is sensitive to the deployment of the stored contents. In this paper, we propose a video caching policy for a video server, based on the knowledge of video profiles, namely: access rate, video size and bandwidth, tolerable rejection probability, and rental price. We consider the video server as having a hierarchical architecture which consists of a set of high-speed disk drives located in the front end for caching a subset of videos, and another set of high-capacity tertiary devices located in the back end for archiving the entire video collection. The front-end disks particularly, are organized together by employing a proposed data striping scheme, termed the adaptive striping (AS), which is flexible on heterogeneous disk integration. The proposed policy determines what video set should be cached, and how to arrange them in the front-end disks with two objectives in mind: (1) offering differentiated service grades conforming to the video profiles as well as (2) maximizing the overall system revenue. We simulate the system with various configurations, and the results affirm our effective approach.     

Issues in the design of a storage server for video-on-demand

We examine issues related to the design of a storage server for video-on-demand (VOD) applications. The storage medium considered is magnetic disks or arrays of disks. We investigate disk scheduling policies, buffer management policies and I/O bus protocol issues. We derive the number of sessions that can be supported from a single disk or an array of disks and determine the amount of buffering required to support a given number of users. Furthermore, we propose a scheduling mechanism for disk accesses that significantly lowers the buffer-size requirements in the case of disk arrays. The buffer size required under the proposed scheme is independent of the number of disks in the array. This property allows for striping video content over a large number of disks to achieve higher concurrency in access to a particular video object. This enables the server to satisfy hundreds of independent requests to the same video object or to hundreds of different objects while storing only one copy of each video object. The reliability implications of striping content over a large number of disks are addressed and two solutions are proposed. Finally, we examine various policies for dealing with disk thermal calibration and the placement of videos on disks and disk arrays.     

Heat transfer in cylindrical shrouded cavities

The flow field, temperature field and the heat transfer rates in cylindrical shrouded cavities with rotation, recirculation and coolant through-flow have been analyzed numerically. Two cavity configurations are considered. In the first configuration, a heated cylindrical shroud is enclosed by a stationary insulated stator disc and a rotating insulated rotor disc. The coolant air enters the cavity by a central opening in the rotor and exits through an annular gap at the rim of the rotor. The second configuration studies the heat transfer from an air cooled gas turbine disc using the model of a plane disc rotating close to an insulated shrouded stator. The coolant enters centrally through the stator disc and exits radially through a gap between the shroud and the rotor. The flow field and heat transfer rates are computed for several values of coolant flow rate, the rotor swirl speed, the cavity aspect ratio and the exit gap width in the two cavity configurations. The swirl of the rotor changes immensely the flow pattern, recirculating zones and isotherms inside such cavities. In general, increasing coolant flow rate, decreasing swirl and decreasing aspect ratio enhances the heat transfer from the shroud in the first cavity configuration. For the second cavity configuration, the heat transfer rates increase with increasing coolant flow rate, increasing swirl of the rotor, increasing size of the cavity and decreasing exit gap width between the stator and the rotor.     

Disk resonance due to electro-magnetic excitation

Disk vibration is a substantial contributor to the Track-Mis-Registration error budget for the high rpm disk drive applications (10k, 15k rpm) because of high air turbulence. Measures are taken to reduce disk vibration such as to add stator between disks, build shroud around the disk stack to direct the airflow, or use thicker disks etc. In addition to air turbulence excitation, disk is also subjective to excitation from electro-magnetic forces, occurring only when the disk mode frequency lined-up with excitation frequency. In this paper, the focus is not on how to reduce disk vibration, but on how to avoid disk resonance from a spindle motor design point of view. Basic Campbell plot is presented as a reference useful to all spindle motor designers. Case studies are discussed including root cause analysis and solutions.     

Arrangement of multi-dimensional scalable video data for heterogeneous clients

A scalable video server extracts data corresponding to the resolution requested by its client from the total data containing the information encoding a full resolution video. Depending on the requested resolution, the extracted data may not be contiguously placed on a disk or a disk array. For this reason, the traverse distance, which indicates the difference between the first read position and the last read position, can be much larger than the amount of the requested data. This causes additional rotational latency in a disk and thus degrades disk performance. Furthermore, scalable video data more seriously deteriorates the independency of disks in a disk array. That is, even a small read request can be split into multiple disk requests across disks of a disk array, because the requested data are scattered across multiple disks. To address these problems, we propose new data arrangement schemes for scalable video data. In these new schemes, we first deal with the arrangement of multi-dimensional scalable video data, which can be employed regardless of the number of scalability dimensions. Second, we improve disk performance by reducing average disk cost, which is based on both the traverse distance of each disk and the independency of disks. Third, we improve overall performance of disk devices through considering the entire request pattern, when large numbers of clients concurrently demand heterogeneous resolutions of videos from a server. We also propose fast arrangement algorithms to reduce the computation time required for searching an effective arrangement so that they can be easily applied to practical server system.     

一种异构磁盘系统的数据分配方法

随着各种存储系统应用和技术的不断发展 ,出现了异构磁盘系统 .如果采用同构系统的数据分配和放置方法如 striping来管理异构系统 ,并不能针对异构系统中各个磁盘的性能差异来进行数据分配 ,降低了异构系统的性能 .提出了一种新的异构系统数据分配方法 .这种方法能够根据异构系统中磁盘的差异有效的进行数据分配 .它通过把系统中的磁盘划分成带宽相同的逻辑盘 ,并使用在同构系统中很成熟的 striping方法来对这些盘进行管理 ,实验证明这种方法能够有效的提高异构系统的吞吐率     

Local heat transfer characteristics in a single rotating disk and co-rotating disks

The present study investigates convective heat/mass transfer and flow characteristics inside rotating disks. The rotating disks are simulated on the commonly used 3.5 hard disk drives (HDD). The experiments are conducted for the various hub heights of 5, 10 and 15 mm in a single rotating disk and 4, 6 and 8 mm in co-rotating disks and for the various rotating Reynolds numbers of 5.53 × 10⁴, 8.53 × 10⁴ and 1.13 × 10⁵. To accommodate the general operating conditions of HDD, the experiments are also conducted with an obstruction of rectangular cross-section in the space, which simulates a read-write head arm. A naphthalene sublimation technique is employed to determine the detailed local heat transfer coefficients on the rotating disks using the heat and mass transfer analogy. Flow field measurements are conducted using laser Doppler anemometry (LDA) and numerical calculations are performed simultaneously to analyze the flow patterns induced by disk rotation. The results of a single rotating disk show that the heat transfer on the rotating disk is enhanced considerably according to the reduction of the hub height and the increase of the rotating Reynolds number. The head arm inserted in the cavity between the rotating disk and the cover enhances uniformity of the heat/mass transfer on the disk due to the deficit of the momentum in the average flow despite the enhancement of the tangential component of fluctuation velocity. The heat/mass transfer rates on the co-rotating disks have very low values near the hub in the inner region of the solid-body rotation and increase rapidly toward the outer region. The change of heat/mass transfer for various hub heights is negligible.The authors wish to acknowledge support for this study by the Ministry of Science and Technology through their National Research Laboratory program and by the KOSEF through the Center of Information Storage Device.     

Scheduling video streams in a large-scale video-on-demand server

This paper addresses the design problems concerning a large-scale, parallel video-on-demand server that consists of multiple clusters of nodes connected by a high performance interconnection network. In order to efficiently control the flow of video streams, we propose two scheduling algorithms for data retrieval and communication. First, we present a disk scheduling algorithm called round scheduling which fully utilizes disk bandwidth, minimizing the disk idle time while the server retrieves data blocks. Second, a communication scheduling algorithm is developed to guarantee conflict-free communication over the multistage interconnection network that is topologically equivalent to the Omega network. We also show some simulation results on the server configuration. Analysis of tradeoffs between the server utilization and the start-up latency helps to determine the proper number and size of server clusters for a set of given nodes.     

A dynamic policy of segment replication for load-balancing in video-on-demand servers

In a video-on-demand server, resource reservation is needed for continuous delivery. Hence, any given server can serve only a fixed maximum of clients. Different videos can be placed on different disks or disk array groups. Since the access rates to various movies are not uniform, load imbalance can occur among the disks in the system. In this paper, we propose a dynamic policy that replicates segments of files to balance the load across the disks. By using simulation, we show that the proposed policy is responsive to quick load surges and is superior to a policy based on the static replication of hot movies.     

Adaptive grid generation technique of sub-5?nm flying height air bearing slider with clearance discontinuities

An adaptive grid generation technique including three kinds of weight functions related to the maximum pressure gradient is proposed to simulate the pressure distribution of a sub-5?nm flying height air bearing slider with clearance discontinuities in the interface of the head and the disk in hard disk drives. Considering the clearance discontinuities of a slider with complex geometrical shape, we have defined a discontinuous factor to describe the mass flux crossing the discontinuous boundaries. The effect of different parameters in the weight functions on the node distribution of a typical slider is investigated. The pressure profile of a slider with sub-5?nm flying height is obtained based on the grid distribution calculated from the weight functions. The computational efficiency for simulating the pressure distributions is compared for different kinds of weight functions.     

Highly available and heterogeneous continuous media storage systems

A number of recent technological trends have made data intensive applications such as continuous media (audio and video) servers a reality. These servers store and retrieve large volumes of data using magnetic disks. Servers consisting of multiple nodes and large arrays of heterogeneous disk drives have become a fact of life for several reasons. First, magnetic disks might fail. Failed disks are almost always replaced with newer disk models because the current technological trend for these devices is one of annual increase in both performance and storage capacity. Second, storage requirements are ever increasing, forcing servers to be scaled up progressively. In this study, we present a framework to enable parity-based data protection for heterogeneous storage systems and to compute their mean lifetime. We describe the tradeoffs associated with three alternative techniques: independent subservers, dependent subservers, and disk merging. The disk merging approach provides a solution for systems that require highly available secondary storage in environments that also necessitate maximum flexibility.     

基于FC接口的Hyper-RAID设计与研究

超级磁盘阵列（Hyper-RAID）在其内部划分逻辑卷和逻辑单元,实现了层次化的多级复合阵列结构.它与服务器的连接采用光纤通道接口（FC）,利用FCP-SCSI协议,将阵列内部的每个逻辑单元映射为服务器端的一个物理磁盘设备,从而实现了多阵列功能.研究分析表明,这种多层次的多阵列结构和普通的单阵列相比具有更好的容错性能,而且还能对不不同重要等级的数据分类存储.     

Performance analysis of advanced I/O architectures for PC-based video servers

In the personal computing and workstation environments, more and more I/O adapters are becoming complete functional subsystems that are intelligent enough to handle I/O operations on their own without much intervention from the host processor. The IBM subsystem control block (SCB) architecture has been defined to enhance the potential of these intelligent adapters by defining services and conventions that deliver command information and data to and from the adapters. In recent years, a new storage architecture, the redundant array of independent disks (RAID), has been quickly gaining acceptance in the world of computing. In this paper, we discuss and present a performance analysis of the SCB architecture and disk array technology in typical video server environments. In particular, we would like to see whether a disk array can outperform a group of disks (of the same type, the same data capacity, and same cost) operating independently (not in parallel as in a disk array) in a video server environment where most disk I/O operations are large sequential reads.     

Response time distribution in a multiprogrammed computer with terminal traffic

The central server model has been used to study a computer with a CPU and disks and operating with a fixed limit on the level of multiprogramming. An approximation of this model is to treat the CPU and disk subsystem as a single state-dependent server. Under some assumptions, such an approximation is known to give good results for the mean response time. This paper is concerned with two issues about this approximation. First, we obtain a complete characterization of the response time distribution. Second, based on this characterization, a numerically feasible method is developed for computing the response time distribution. When applied to several examples involving a CPU and several disks, this method yielded results which compared well with those obtained by a simulation of the central server model.     

Design of suspension-based and collocated dual stage actuated suspensions

Dual stage actuators have recently been implemented in hard disk drive suspensions in order to increase the track density of hard disk drives. This paper investigates the lateral deflection (lateral stroke), stress distribution and design characteristics of a suspension-based and a collocated flexure-based dual stage actuated suspension design using finite element analysis. Design parameters for each suspension design are examined and guidelines for improved lateral deflection characteristics are proposed.     

Storage System for Supporting More Video Streams in Video Server

Video server needs a storage system with large bandwidth in order to provide concurrently more users with the real time retrieval requests for video streams. So, the storage system generally has the structure of disk array, which consists of multiple disks. When the storage system serves multiple video stream requests, it's bottlenecks come from the seeking delay caused by the random movement of disk head and from unbalanced disk access due to disk load unbalance among multiple disks.This paper presents a novel placement and retrieval policy. The new policy retrieves the requested data through sequential movement of disk heads and maintaining disk load balance so that it can diminish the bottlenecks on retrieving and can provide the concurrent real time retrieval services for more users simultaneously. In addition, the novel policy reduces the startup latency for the requests. The correctness of the novel placement and retrieval policy is analyzed with theoretical views. Performance analysis of the novel placement and retrieval policy is provided with simulations.