If you were to take some form of martial arts, the instructor would show you how to do some basic punches and kicks before teaching you to spar or compete. The instructor would be well aware that jumping into combat without the basics could do you more damage than good. PC repair works the same way: before discussing the installation and configuration of IDE and SCSI devices, it is important to cover some basics.

Disk geometry

This article will introduce you to disk geometry. You will learn some of the terminology that is required in order to understand the internal construction of a disk.

Platters

A platter is a physical object (actually, a plate) that resides inside the hard disk and is responsible for storing the data. A platter is similar to a record on an old record player—the main difference being that a hard disk has many platters, while a record player only holds one record at a time.

The platters are very much like records on a record player in the sense that they spin around in a circle on a spindle that runs through the center of all the platters. Each platter has two sides for storing information, and each side of the platter has a unique ID. The ID for the first side of the first platter is 0, and each side increases by 1. For example, if there were two platters in the disk, the first platter would have Side 0 and Side 1, while the second platter would have Side 2 and Side 3.

Since there will be a writing mechanism—a head—for each side of the platter, many people use the terms “head” and “side” interchangeably. The head is more accurately called the read/write head, because it will move over the disk surface and read or write to the disk. Like a needle on a record player, the read/write head moves over the surface of the disk with the help of an arm, called the actuator arm (also known as the head positioning mechanism).

There is a read/write head for each platter surface on the disk. When information is written to the disk, the read/write head will move to the same track on all platters in a single movement and then write to the same track on all platters. The actuator arm has multiple read/write heads on it.

Tracks

Just as there are grooves, or tracks, on a record or music CD, there are also tracks on each platter. These tracks are evenly spaced across the platter’s surface.

Sectors

The platter is divided into pie slices, thus dividing the tracks into 512-byte sectors. Sectors are the actual storage areas for data, and each has an address that is made up of the platter side number, the track number, and the sector number on that track.

For the Core Hardware exam, know that a low-level format is performed by the manufacturer and is responsible for preparing the disk for data storage by creating the tracks and sectors.

Clusters

A group of any number of sectors can make up a cluster. When a partition is formatted, the file system will determine the cluster size based off the partition size. For example, a partition that is 2GB in size formatted as FAT will use a 32K-cluster size. That same 2GB partition formatted as FAT32 will use only a 4K-cluster size. Having a partition use a 4K-cluster size means there will be 8 sectors that make up a cluster. Keep in mind that once a file has been saved to the cluster, no other file can occupy that cluster. For example, if you had a 32K-cluster size and you saved a 3K file to the hard disk, the file would be saved to an empty cluster—but only 3K of it would be used, and the remaining 29K would be left unused. The remaining 29K is now considered unusable space; no other file can be saved to that unused 29 K.

Cylinders

Each platter in the disk contains the same number of tracks; these tracks are numbered from the outside in, starting with zero. For example, if there were ten tracks on a platter, the track closest to the edge of the platter would be Track 0, while the track closest to the center would be Track 9.

A cylinder consists of the same track on both sides of all the platters. In other words, when you reference Track 0, you reference a particular track on a particular platter, but when you reference Cylinder 0, you reference Track 0 on all platters. If you know the number of cylinders, heads, and sectors per track, you can calculate the size of a disk. For example, if you have a drive that has 4,092 cylinders, 16 heads, and 63 sectors per track, the size of the disk would be 2,111,864,832 bytes (2.1GB). The formula to calculate the size of the disk is Cylinders × Heads × Sectors × 512 bytes per sector.

Read/write process

Platters are divided into 512 byte sectors. These sectors are the area on the platter that data is written to. The platters have a magnetic coating applied to them that is extremely sensitive to magnetism. While the platters are rotating in a circle, the read/write heads are moved over the disk surface to the location where they need to write (or save) information. The read/write heads do not actually touch the surface of the disk platters; instead, they “hover” about ten micro-inches (or millionths of an inch) above—that’s not even enough space to place a hair between the read/write head and the platter’s surface. This design helps improve disk performance, because a read/write head that made contact with the platter would cause friction, slowing down the rotation speed of the disk.

While the platters spin around in circles, the read/write head moves from track to track until it reaches the desired one. Then it waits for the appropriate sector to move underneath it, at which time the read/write head is energized to apply a magnetic charge to the particles in the disk coating. This changes the particle binary state from zero to one, thus creating data.

Performance

Disk performance can be measured in terms of several important characteristics: seek time, latency, access time, and the spin speed of the disk.

Measuring Hard Disk Performance

Seek time is the time it takes to move the read/write heads to the desired track. Seek time is a calculated average, since the time it takes to move to the desired track will differ from one instance to another. For example, if the read/write heads are on Track 1, they will take a longer amount of time moving to Track 12 than to Track 3 (because the distance is greater between Track 1 and Track 12). Seek time is measured in milliseconds, or millionths of a second.

Latency is the time it takes for the appropriate sector to move under the read/write head. Latency is measured in milliseconds. Access time Access time is a term used to describe the overall speed of the disk. It is a combination of seek time and latency. The lower the access time, the better.

Spin speed is the speed at which the platters spin in a circle, measured in revolutions (rotations) per minute, or rpm. The larger the rpm value the faster the disk, which means less latency.

Master boot record

While I’m discussing disk geometry, I should make a brief comment about the Master Boot Record. The Master Boot Record (MBR) is the first sector on the first track of the first side of the first platter; it holds the operating system boot code that controls the loading of the operating system.

The MBR also holds drive characteristics—such as the partition table. During the boot process the system has to find a primary partition that is active and it will do this by looking in the boot record.

In general, if anything goes wrong with the MBR, you will be unable to boot the system. Since this boot record is always found in the same location on every disk, it becomes very easy for developers to write viruses that will modify or corrupt it. This is one reason you should always run virus detection software.