Chapter 5. Managing Storage

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If there is one thing that takes up the majority of a system administrator's day, it would have to be storage management. It seems that disks are always running out of free space, becoming overloaded with too much I/O activity, or failing unexpectedly. Therefore, it is vital to have a solid working knowledge of disk storage in order to be a successful system administrator.

5.1. An Overview of Storage Hardware

Before managing storage, it is first necessary to understand the hardware on which data is stored. Unless you have at least some knowledge about mass storage device operation, you may find yourself in a situation where you have a storage-related problem, but you lack the background knowledge necessary to interpret what you are seeing. By gaining some insight into how the underlying hardware operates, you should be able to more easily determine whether your computer's storage subsystem is operating properly.
The vast majority of all mass-storage devices use some sort of rotating media and supports the random access of data on that media. This means that the following components are present in some form within nearly every mass storage device:
  • Disk platters
  • Data reading/writing device
  • Access arms
The following sections explore each of these components in more detail.

5.1.1. Disk Platters

The rotating media used by nearly all mass storage devices are in the form of one or more flat, circularly-shaped platters. The platter may be composed of any number of different materials, such aluminum, glass, and polycarbonate.
The surface of each platter is treated in such a way as to enable data storage. The exact nature of the treatment depends on the data storage technology to be used. The most common data storage technology is based on the property of magnetism; in these cases the platters are covered with a compound that exhibits good magnetic characteristics.
Another common data storage technology is based on optical principles; in these cases, the platters are covered with materials whose optical properties can be modified, thereby allowing data to be stored optically[14].
No matter what data storage technology is in use, the disk platters are spun, causing their entire surface to sweep past another component -- the data reading/writing device.

5.1.2. Data reading/writing device

The data reading/writing device is the component that takes the bits and bytes on which a computer system operates and turns them into the magnetic or optical variations necessary to interact with the materials coating the surface of the disk platters.
Sometimes the conditions under which these devices must operate are challenging. For instance, in magnetically-based mass storage the read/write devices (known as heads) must be very close to the surface of the platter. However, if the head and the surface of the disk platter were to touch, the resulting friction would do severe damage to both the head and the platter. Therefore, the surfaces of both the head and the platter are carefully polished, and the head uses air pressure developed by the spinning platters to float over the platter's surface, "flying" at an altitude less than the thickness of a human hair. This is why magnetic disk drives are sensitive to shock, sudden temperature changes, and any airborne contamination.
The challenges faced by optical heads are somewhat different than for magnetic heads -- here, the head assembly must remain at a relatively constant distance from the surface of the platter. Otherwise, the lenses used to focus on the platter does not produce a sufficiently sharp image.
In either case, the heads use a very small amount of the platter's surface area for data storage. As the platter spins below the heads, this surface area takes the form of a very thin circular line.
If this was how mass storage devices worked, it would mean that over 99% of the platter's surface area would be wasted. Additional heads could be mounted over the platter, but to fully utilize the platter's surface area more than a thousand heads would be necessary. What is required is some method of moving the head over the surface of the platter.

5.1.3. Access Arms

By using a head attached to an arm that is capable of sweeping over the platter's entire surface, it is possible to fully utilize the platter for data storage. However, the access arm must be capable of two things:
  • Moving very quickly
  • Moving very precisely
The access arm must move as quickly as possible, because the time spent moving the head from one position to another is wasted time. That is because no data can be read or written until the access arm stops moving[15].
The access arm must be able to move with great precision because, as stated earlier, the surface area used by the heads is very small. Therefore, to efficiently use the platter's storage capacity, it is necessary to move the heads only enough to ensure that any data written in the new position does not overwrite data written at a previous position. This has the affect of conceptually dividing the platter's surface into a thousand or more concentric "rings" or tracks. Movement of the access arm from one track to another is often referred to as seeking, and the time it takes the access arms to move from one track to another is known as the seek time.
Where there are multiple platters (or one platter with both surfaces used for data storage), the arms for each surface are stacked, allowing the same track on each surface to be accessed simultaneously. If the tracks for each surface could be visualized with the access stationary over a given track, they would appear to be stacked one on top of another, making up a cylindrical shape; therefore, the set of tracks accessible at a certain position of the access arms are known as a cylinder.

[14] Some optical devices -- notably CD-ROM drives -- use somewhat different approaches to data storage; these differences are pointed out at the appropriate points within the chapter.
[15] In some optical devices (such as CD-ROM drives), the access arm is continually moving, causing the head assembly to describe a spiral path over the surface of the platter. This is a fundamental difference in how the storage medium is used and reflects the CD-ROM's origins as a medium for music storage, where continuous data retrieval is a more common operation than searching for a specific data point.
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