Computer Storage Devices

Storage Devices

Purposes and Characteristics of Storage Devices 

What good is a computer without a place to put everything? Storage media hold the data being accessed, as well as the files the system needs to operate and data that needs to be saved. The many different types of storage differ in terms of their capacity (how much they can store), access time how fast the computer can access the information, and the physical type of media used. Hard disk drive (HDD) systems (hard disks or hard drives for short) are used for perma- nent storage and quick access. Hard disks typically reside inside the computer, where they are semipermanently  mounted with no external access (although there are external and removable hard drives), and can hold more information than other forms of storage. The hard disk drive system contains three critical components:

Controller :

This component controls the drive. The controller chip controls how the drive operates, sends signals to the various motors in the disk, and receives signals from the sen- sors inside the drive. Most of today’s hard disk technologies incorporate the controller and drive into one enclosure. The most common and well-known of these are PATA and SATA, “Installing, Maintaining, and Troubleshooting Hardware.”

Hard disk :

This is the physical storage medium. Hard disk drive systems store information on small discs (from under one inch to five inches in diameter), also called platters, stacked together and placed in an enclosure.

Host adapter :

This is the translator, converting signals from the hard drive and controller to signals the computer can understand. Most motherboards  today incorporate the host adapter into the motherboard’s circuitry, offering headers for drive-cable connection. Legacy host adapters and certain modern adapters house the hard drive controller circuitry. A hard drive is constructed in a cleanroom to avoid the introduction of contaminants into the hermetically sealed drive casing. Once it’s sealed, most manufacturers seal one or more of the screws, thus sealing the casing with a warning sticker that removal of or damage to the seal will result in voiding the drive’s warranty. Even some of the smallest contaminants can damage the precision components if allowed inside the hard drive’s external shell. Inside the sealed case of the hard drive lie one or more platters, where the actual data are stored by the read/write heads. The heads are mounted on a mechanism that moves them in tandem across both surfaces of all platters. Computer repair miami knows that older drives used a stepper motor to position the heads at discrete points along the surface of the platters. Newer drives use voice coils for a more analog movement, resulting in reduced data loss because the circuitry can sense where the data is located through a servo scheme, even if the data shifts due to changes in physical disc geometry. Factory preparation for newer drives or low-level formatting in the field for legacy drives map the inherent flaws of the platters so that the drive controllers know not to place data in these compromised locations. Additionally, this phase in drive preparation creates the magnetic domains that represent the smallest units of storage on the discs’ platters, the sector. Magnetic-drive sectors commonly store only 512 bytes of data each. Sectors are created only after concentric rings, or tracks, are drawn magnetically around the surface of the platters. Sectors are then delineated within each of the tracks. The capacity of a hard drive is a function of the number of sectors it contains. The con- troller for the hard drive knows exactly how the sectors are laid out within the disk assem- bly. It takes direction from the BIOS when writing information to and reading information from the drive. The BIOS, however, does not always understand the actual geometry of the drive. For example, the BIOS does not support more than 63 sectors per track. Neverthe- less, many hard drives have tracks that contain many more than 63 sectors per track. As a result, a translation must occur from where the BIOS believes it is directing information to be written to where the information is actually written by the controller. When the BIOS detects the geometry of the drive, it is because the controller reports dimensions the BIOS can understand. The same sort of trickery occurs when the BIOS reports to the operating system a linear address space for the operating system to use when requesting data be written to or read from the drive through the BIOS. The basic hard disk geometry consists of three components: the number of sectors that each track contains, the number of read/write heads in the disk assembly, and the number of cylinders in the assembly. This set of values is known as CHS (for cylinders/heads/sec- tors). A cylinder is the number of tracks that can be found on any single surface of any single platter. It is called a cylinder because the collection of all same-number tracks on all writable surfaces of the hard disk assembly looks like a geometric cylinder when connected together vertically. Therefore, cylinder 1, for instance, on an assembly that contains three platters comprises six tracks (one on each side of each platter), each labeled track 1 on its respective surface. Figure 2.2 illustrates the key terms presented in this discussion. Because the number of cylinders indicates only the number of tracks on any one writable surface in the assembly, the number of writable surfaces must be factored into the equation to produce the total number of tracks in the entire assembly. This is where the number of heads comes in. There is a single head dedicated to each writable surface, two per platter. By multiplying the number of cylinders by the number of heads, you produce the total num-ber of tracks throughout the disk assembly. By multiplying the number of sectors per track, you discover the total number of sectors throughout the disk assembly. Dividing the result by 2 provides the number of KB the hard drive can store. This works because each sector holds 512 bytes, which is equivalent to ½KB. Each time you divide the result by 1024, you obtain a smaller number, but the unit of measure increases from KB to MB, from MB to GB, and so on.

File systems laid down on the tracks and their sectors routinely group a configurable number of sectors into equal or larger sets called clusters or allocation units. Our concept about operating system designers have to settle on a finite number of addressable units of storage and a fixed number of bits to address them uniquely. Because the units of storage can vary in size, however, the maximum amount of a drive’s storage capacity can vary accordingly, but not unless drive capacities in excess of 2TB are implemented. Be aware that volumes created with RAID (see Chapter 13) can certainly exceed 2TB. No two files are allowed to occupy the same sector, so the opportunity exists for a waste of space that defragmenting cannot correct. Clusters exacerbate the problem by having a similar foible: no two files are allowed by the operating system to occupy the same cluster. The larger the cluster size, then, the larger the potential waste. So, although you can increase the cluster size (generally to as large as 64KB, which corresponds to 128 sectors), you should keep in mind that unless you are storing a notable number of very large files, the waste will escalate astoundingly, perhaps negating or reversing your perceived storage-capacity increase. Nevertheless, if you have single files larger than 2TB, increased cluster sizes are for you. A 64KB cluster size results in a maximum volume size in Windows XP, for example, of 256TB.


One Comment

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