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We address various issues dealing with the use of disk arrays in transaction processing environments. We look at the problem of transaction undo recovery and propose a scheme for using the redundancy in disk arrays to support undo recovery. The scheme uses twin page storage for the parity information in the array. It speeds up transaction processing by eliminating the need for undo logging for most transactions. The use of redundant arrays of distributed disks to provide recovery from disasters as well as temporary site failures and disk crashes is also studied. We investigate the problem of assigning the sites of a distributed storage system to redundant arrays in such a way that a cost of maintaining the redundant parity information is minimized. Heuristic algorithms for solving the site partitioning problem are proposed and their performance is evaluated using simulation. We also develop a heuristic for which an upper bound on the deviation from the optimal solution can be established.

During the past decade, advances in processor and memory technology have given rise to increases in computational performance that far outstrip increases in the performance of secondary storage technology. Coupled with emerging small-disk technology, disk arrays provide the cost, volume, and capacity of current disk subsystems, by leveraging parallelism, many times their performance. Unfortunately, arrays of small disks may have much higher failure rates than the single large disks they replace. Redundant arrays of inexpensive disks (RAID) use simple redundancy schemes to provide high data reliability. The data encoding, performance, and reliability of redundant disk arrays are investigated. Organizing redundant data into a disk array is treated as a coding problem. Among alternatives examined, codes as simple as parity are shown to effectively correct single, self-identifying disk failures.

During the past decade, advances in processor and memory technology have given rise to increases in computational performance that far outstrip increases in the performance of secondary storage technology. Coupled with emerging small-disk technology, disk arrays provide the cost, volume, and capacity of current disk subsystems, by leveraging parallelism, many times their performance. Unfortunately, arrays of small disks may have much higher failure rates than the single large disks they replace. Redundant arrays of inexpensive disks (RAID) use simple redundancy schemes to provide high data reliability. The data encoding, performance, and reliability of redundant disk arrays are investigated. Organizing redundant data into a disk array is treated as a coding problem. Among alternatives examined, codes as simple as parity are shown to effectively correct single, self-identifying disk failures.

We address various issues dealing with the use of disk arrays in transaction processing environments. We look at the problem of transaction undo recovery and propose a scheme for using the redundancy in disk arrays to support undo recovery. The scheme uses twin page storage for the parity information in the array. It speeds up transaction processing by eliminating the need for undo logging for most transactions. The use of redundant arrays of distributed disks to provide recovery from disasters as well as temporary site failures and disk crashes is also studied. We investigate the problem of assigning the sites of a distributed storage system to redundant arrays in such a way that a cost of maintaining the redundant parity information is minimized. Heuristic algorithms for solving the site partitioning problem are proposed and their performance is evaluated using simulation. We also develop a heuristic for which an upper bound on the deviation from the optimal solution can be established.

The performance of traditional RAID Level 5 arrays is, for many applications, unacceptably poor while one of its constituent disks is non- functional. This paper describes and evaluates mechanisms by which this disk array failure-recovery performance can be improved. The two key issues addressed are the data layout, the mapping by which data and parity blocks are assigned to physical disk blocks in an array, and the reconstruction algorithm, which is the technique used to recover data that is lost when a component disk fails. The data layout techniques this paper investigates are instantiations of the declustered parity organization, a derivative of RAID level 5 that allows a system to trade some of its data capacity for improved failure-recovery performance. We show that our instantiations of parity declustering improve the failure-mode performance of an array significantly, and that a parity- declustered architecture is preferable to an equivalent-size multiple-group RAID Level 5 organization in environments where failure-recovery performance is important. The presented analyses also include comparisons to a RAID Level 1 (mirrored disks) approach. With respect to reconstruction algorithms, this paper describes and briefly evaluates two alternatives stripe-oriented reconstruction and disk-oriented reconstruction, and establishes that the latter is preferable as it provides faster reconstruction. The paper then revisits a set of previously-proposed reconstruction optimizations, evaluating their efficacy when used in conjunction with the disk-oriented algorithm. The paper concludes with a section on the reliability versus capacity trade-off that must be addressed when designing large arrays.

Redundant disk arrays provide a way for achieving rapid recovery from media failures with a relatively low storage cost for large scale database systems requiring high availability. In this paper we propose a method for using redundant disk arrays to support rapid recovery from system crashes and transaction aborts in addition to their role in providing media failure recovery. A twin page scheme is used to store the parity information in the array so that the time for transaction commit processing is not degraded. Using an analytical model, we show that the proposed method achieves a significant increase in the throughput of database systems using redundant disk arrays by reducing the number of recovery operations needed to maintain the consistency of the database.

There exists a wide variety of applications in which data availability must be continuous, that is, where the system is never taken off- line and any interruption in the accessibility of stored data causes significant disruption in the service provided by the application. Examples include on-line transaction processing systems such as airline reservation systems and automated teller networks in banking systems. In addition, there exist many applications for which a high degree of data availability is important, but continuous operation is not required. An example is a research and development environment, where access to a centrally-stored CAD system is often necessary to make progress on a design project. These applications and many others mandate both high performance and high availability from their storage subsystems. Redundant disk arrays are systems in which a high level of I/O performance is obtained by grouping together a large number of small disks, rather than building one large, expensive drive. The high component count of such systems leads to unacceptably high rates of data loss due to component failure, and so they typically incorporate redundancy to achieve fault tolerance.

Redundant disk arrays are single fault tolerant, incorporating a layer of error handling not found in nonredundant disk systems. Recovery from these errors is complex, due in part to the large number of erroneous states the system may reach. The established approach to error recovery in disk systems is to transition directly from an erroneous state to completion. This technique, known as forward error recovery, relies upon the context in which an error occurs to determine the steps required to reach completion, which implies forward error recovery is design specific. Forward error recovery requires the enumeration of all erroneous states the system may reach and the construction of a forward path from each erroneous state. We propose a method of error recovery which does not rely upon the enumeration of erroneous states or the context in which errors occur. When an error is encountered, we advocate mechanized recovery to an error-free state from which an operation may be retried. Using a form of backward error recovery, we are able to manage the complexity of error recovery in redundant disk arrays without sacrificing performance.

Redundant disk arrays provide a way for achieving rapid recovery from media failures with a relatively low storage cost for large scale database systems requiring high availability. In this paper we propose a method for using redundant disk arrays to support rapid recovery from system crashes and transaction aborts in addition to their role in providing media failure recovery. A twin page scheme is used to store the parity information in the array so that the time for transaction commit processing is not degraded. Using an analytical model and simulation results, we show that the proposed method achieves a significant increase in the throughput of database systems using redundant disk arrays by reducing the number of recovery operations needed to maintain the consistency of the database.

We describe and evaluate a strategy for declustering the parity encoding in a redundant disk array. This declustered parity organization balances cost against data rehability and performance during failure recovery in highly-available parity-based arrays for use in continuous-operation systems. It improves on standard parity organizations by reducing the additional load on surviving disks during the reconstruction of a failed disk's contents. This yields higher user throughput during recovery, and/or shorter recovery time. We first demonstrate a software implementation of declustered parity based on balanced incomplete and complete block designs. This implementation is then evaluated using a disk array simulator under a highly concurrent workload comprised of small user accesses. We show that declustered parity penalizes user response time while a disk is being repaired (before and during its recovery) less than comparable non-declustered (RAID 5) organizations without any penalty to user response time in the fault-free state. We then show that previously proposed modifications to a simple, single-sweep reconstruction algorithm further decrease user response times during recovery, but, contrary to previous suggestions, this may be achieved at the cost of slower recovery in many declustered parity arrays. This result arises from the simple model of disk access performance used in previous work, which did not consider throughput variations due to positioning delays.

Annie Elfrida McLane lives in a little brown house of the South Dakota prairie, within sight of the Black Hills. Her father is a widower and prospects for gold there; Annie lives at home with her grandmother and the servants, Marthy Stubbs and Pete Pumpernickle. Annie has no neighbours, no other children to play with, and no school to attend; she is sometimes lonely and despondent. She is dependent for company on her black cat Silvertip, the farm animals around her, and creatures of the surrounding fields and meadows that she sometimes makes her pets. In the summer when she turns six years old, Annie rescues a wounded prairie dog from a trap. It is a strange little animal, and the other creatures of Annie's world are afraid of it. Annie makes a little house for the prairie dog, but the animal soon escapes back to the wild. Follow their adventures. ( Michele Eaton )

2Autobiography of an Electron

By Charles R. Gibson

"While many scientific men now understand our place in the universe, we electrons are anxious that every person should know the very important part which we play in the workaday world. It was for this reason that my fellow-electrons urged me to write my own biography. I am pleased to say that my relationship with the scribe who has put down my story in the following pages has been of the most friendly description. I have allowed him to place what he calls "The Scribe's Note" at the beginning of each chapter, but it will be understood clearly that these are merely convenient embellishments, and that I am responsible for the story of my own experiences." (Introduction adapted from the text)

"The narrative of these two journeys is of special interest, because the first one, that made by my twin sister, Mrs. Lewis, and myself, in 1892, led to the discovery of an early and important codex of ancient Syriac gospels... while that made by us this year in company with [more Cambridge scholars and their wives] was undertaken for the purpose of deciphering the precious manuscript to which we have alluded. <br><br> "Abler pens than mine will write about these Syriac gospels... yet on me devolves the task of telling how the codex was found. Many inaccurate statements have been made by too hasty writers in our public prints, and it has occurred to me that the best means of removing misconceptions on the subject is to reprint my sister's journal of our trips to Sinai in 1892 and 1893." - Margaret Dunlop Gibson

From the series, The Library of Romance, this book introduces the reader to the modern concepts of electricity. The author discusses how man came to know about electricity, and how it powers the world of 1910.

It is not the author's purpose in the present volume to give any instruction in the practice of photography. There are many works dealing with the practical side of the subject. His object is to tell the romantic story of the discovery of this wonderful art, and the steps by which its range has been extended until it can achieve results which only a few years ago would have been thought impossible. A glance at the list of chapters will show what a wide field photography now covers, and what service it renders to man, both in his everyday life and in his most subtle scientific researches. - Summary by The Author

Those of us who have grown up with the results of <i>Modern Manufacture</i> around us are apt to miss the romance of the subject. Indeed, it is difficult for us to realize how very different were the conditions in which our great-grandfathers, and even our grandfathers, lived and worked. In the present volume the author has endeavored to trace the evolution of the different industries, and to describe in everyday language the methods of manufacture in our principle industries. In order to keep the book as readable as possible, he has omitted many names and dates, but sufficient detail is given to enable the general reader to form a clear conception of the different subjects. (Summary adapted from the Preface)

The author was an American diplomat, the Secretary of the American Legation in Brussels, at the beginning of World War I in 1914. This book consists of his edited selections from his daily journal, giving a firsthand look at the chaotic atmosphere before and throughout Germany's invasion of Belgium. His position as a diplomat of the then-neutral United States gave him an invaluable perspective from which to report on the deliberations of military and civilian decision-makers, the reactions of the public to the unfolding calamity, and the efforts of international relief organizations to meet the urgent needs of a flood of refugees. (Summary by Ted Lienhart)