Chapter One Preview: Introduction to Disk Architecture

Computers were initially designed to store and process data. Little has changed in this regard since the invention of the modern computer in the mid-20th century. However, the scale has increased tremendously. Computers process an immense amount of data, and that data must be stored somewhere. On modern computers, that storage is usually a hard disk.

Storing data on a disk has become less expensive and more convenient in modern times. Hard disk prices are, at the time of this writing, incredibly inexpensive. 750GB of hard disk storage, which just 5 years ago required a large disk array and cost tens of thousands of dollars to plan and implement, costs less than $400 for a single off-the-shelf drive unit.

But with that increase in storage capacity and decrease in price comes a problem of management. Unfortunately, most modern administrators are complacent about their hard disks. Little care is taken to ensure that the disks continue to perform at their best based on numerous recent Gartner Group surveys. But with very little work, these disks can be maintained in optimal condition and provide exceptional performance for years.

This guide will explore the storage of data on hard disks. This chapter will examine how modern computers and operating systems (OSs) implement disk storage and access. Later chapters will explore one pervasive problem with such storage—fragmentation. This guide will also examine how fragmentation affects a computer system and what approaches are effective in reducing the effects.

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Chapter Two Preview: Issues with Disk Fragmentation

Chapter 1 explored how disks work. They were designed as efficient long-term data storage devices, and they’ve lived up to that design criteria well. The first disks were large, clunky, fragile, and had very limited storage capacity. Over time, disks have significantly evolved. A disk today might fit on a postage stamp, draw virtually no power, have a lifetime measured in decades, and have the capacity to store the entire Library of Congress. Performance has also come a long way with today’s disk throughput being orders of magnitude more than even a decade ago.

Cost has always been a concern about disks. In Chapter 1, we learned that disks used to be extremely expensive and hence very rare. Today they’re virtually commodity items. You can buy a reliable, high-capacity disk drive at any office supply store for less than the cost of a nice chair.

Overall the disk storage market has boomed and products are keeping up with demand. As an example of the drastic evolution in the market, at the time of this writing, a fully redundant disk array that provides one terabyte of storage can be implemented for less than $1000 using off-the-shelf hardware and does not require specialized knowledge or extensive consulting. Such disk arrays were scarcely available to consumers and small businesses even 5 years ago and, when available, required extensive consulting with storage experts, specialized hardware implementations, and cost tens of thousands of dollars or more. In short, disk-based storage is getting cheaper, easier, and more commonplace.

Disk operation is not all paradise, though. There are many issues to consider when operating disks. None of them should prevent you from using disk storage. However, they should be taken into account when implementing and operating any disk-reliant system. These issues can include:

• Disk lifetime—How long will each disk drive work before it fails?
• Throughput—How quickly is data getting from the storage system to the computer?
• Redundancy—Is the system truly redundant and fault tolerant?
• Fragmentation—Is the disk system operating at optimum efficiency?

This chapter explores the most common issue in disk operation—fragmentation. It happens to all disks on all operating systems (OSs). It can affect the health of the system. And it’s easily repairable.

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Chapter Three Preview: Solving Disk Fragmentation Issues

Chapter 1 explored how disks work. They were designed as efficient long-term data storage devices, and they’ve lived up to that design criteria well. The first disks were large, clunky, fragile, and had very limited storage capacity. Over time, disks have significantly evolved. A disk today might fit on a postage stamp, draw virtually no power, have a lifetime measured in decades, and have the capacity to store the entire Library of Congress. Performance has also come a long way with today’s disk throughput being orders of magnitude more than even a decade ago.

Cost has always been a concern about disks. In Chapter 1, we learned that disks used to be extremely expensive and hence very rare. Today they’re virtually commodity items. You can buy a reliable, high-capacity disk drive at any office supply store for less than the cost of a nice chair. Overall the disk storage market has boomed and products are keeping up with demand. As an example of the drastic evolution in the market, at the time of this writing, a fully redundant disk array that provides one terabyte of storage can be implemented for less than $1000 using off-the shelf hardware and does not require specialized knowledge or extensive consulting. Such disk arrays were scarcely available to consumers and small businesses even 5 years ago and, when available, required extensive consulting with storage experts, specialized hardware implementations, and cost tens of thousands of dollars or more. In short, disk-based storage is getting cheaper, easier, and more commonplace.

Disk operation is not all paradise, though. There are many issues to consider when operating disks. None of them should prevent you from using disk storage. However, they should be taken into account when implementing and operating any disk-reliant system. These issues can include:

• Disk lifetime—How long will each disk drive work before it fails?
• Throughput—How quickly is data getting from the storage system to the computer?
• Redundancy—Is the system truly redundant and fault tolerant?
• Fragmentation—Is the disk system operating at optimum efficiency?

Then in Chapter 2 we explored one problem, disk fragmentation, in great detail. In a nutshell, bits of a file get scattered all over a disk. This makes reading the file more difficult for the hardware to accomplish, decreasing the efficiency of disks and slowing down disk throughput. When critical files or often used files become fragmented the impact can be profound.

Fragmentation is a problem that can actually result from a number of different causes. Unfortunately these causes include normal daily operation of a disk. Over time, disks will become fragmented. There are preventative measures that we can take, and many that are designed right into our modern operating and file systems. But these measures only delay the inevitable.

The problems caused by fragmentation can generally be broken up into three categories: performance, backup and restore, and stability.

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