Hard Drive
While your processor and the RAM it has available makes running applications a success where are these applications stored when you are not using them? In the early personal computer days these applications were kept on individual floppy disk that had to be inserted into a drive each and every time you wanted to run the application. If you wanted to save any files, you needed to insert yet another floppy to store that information.
Computers of today have hard drives inside that assume the roll of the floppy disk. Most of your applications, including your OS, are installed on this drive and when needed are accessed off the drive and pulled into your main memory. When you want to save a file you can save it to your hard drive or some other external medium for mobility (such as a floppy disk).
Hard drives have proved a great advancement as they can hold much more information that a traditional floppy. While a floppy disk can only hold 1.44 MB of data, most personal computers have hard drives that can hold anywhere from 20 to 200 GB (gigabytes). Without the hard drives of today we would be swapping floppies into eternity.
While the pictures may not accurately depict the size of the hard drives in comparison with one another, the desktop hard drive has a larger form factor than that of the laptop hard drive.
This is a Serial-ATA (SATA) drive. Using a slightly different interface than the normal IDE, it can have increased performance, and much more data throughput.
A laptop hard drive is much thinner than its desktop brethen, for the obvious reasons. This is one reason why laptop hard drives typically hold less information than desktop counterparts, only so many platters can be fit into the smaller space.
Hard Drive Performance
There are several reason why hard drives are so efficient and the primary choice for data storage. First, let's talk about what makes the hard drive work. In addition to the electronic circuits that one would expect there are two other very important pieces of equipment. One is the magnetic recording medium that is layered onto a high-precision glass or aluminum disk. This hard drive platter is polished to mirror like smoothness. In addition to the medium, we need a method to read and write from the platter. This piece of the hard drive is known as a head. For each platter there are two read/write heads - for each side of the platter. Most hard drives have multiple platters and multiple heads. This physical makeup is what makes a hard drive perform as well as it does. An internal motor spins the platters allowing the heads, which never touch the platters, to read magnetically the data stored.
To measure this performance of a drive we use to main characteristics (in addition to storage capacity), data rate and seek time. Data Rate is the bytes per second that the drive can deliver to the CPU. Seek Time is the time it takes for the hard drive to start sending the first byte of data following the initial request from the CPU.
Storing the Data
Data is stored on the surface of the platter using organizational elements known as sectors and tracks. Tracks are concentric circles that surround the entire platter. Sectors are pie shaped wedges that when grouped together make up a track. The following diagram helps to illustrate this point. A track is in red and a sector is in blue.
A sector contains a fixed number of bytes, usually 256 or 512. Either by the disc itself or by an operating system, sectors are grouped together into what are known as clusters.
Before a drive can be utilized it must be properly formatted. A low-level format is performed that establishes on the platter the tracks and sectors. By writing the starting and ending points of each sector onto the platter, this process has now prepared the disc to hold blocks of bytes. After a low-level format is performed a high-level format is completed. High level formatting prepares the disc to hold files by writing to the disc important structures for the operating system. We will discuss these structures later.
Hard Drive Interfaces
IDE (ATA)
IDE (Integrated Drive Electronics) has been around in almost all PCs for about 20 years. It's also referred to as ATA (AT Attachment) and more recently as PATA (Parallel ATA) as the need has arisen to distinguish it from SATA (more on that later). IDE controllers have two channels, which can each have two devices, a master and a slave. The master device would manage the channel and the slave would defer to it. Each channel is a shared bus so only one drive can talk at a time, and the drives can only talk to the controller, not each other. Using IDE limited computers to only 4 drives without an expansion card. Some motherboard manufacturers eventually started building in additional IDE controllers to allow for more drives. Also, IDE is only for drives internal to the computer, it is not designed for external connections. It is now being rapidly succeeded by SATA.
For more information on IDE, see http://www.techweb.com/encyclopedia/defineterm.jhtml?term=ide
SCSI
In 1986, another interface standard was developed called SCSI (Small Computer System Interface). SCSI uses a more intelligent and complicated controller to manage it's bus. Up to 15 devices can be connected to a SCSI bus and some have dual controllers. While some motherboards have SCSI controllers built in, most of the time using SCSI will require an add in card. External devices can be designed to use SCSI. Tape drives, scanners, and the first CD-ROMs drives were designed for SCSI. Like IDE, only one device can talk on a SCSI bus at a time, but devices can talk directly to each other instead of just to the controller. Many features that gave SCSI an advantage over IDE have been built into IDE over the years such as support for CD-ROMs and RAID configurations. Also, the speed gap between the two technologies narrowed enough that SCSI was pushed mostly into servers where some of it's more advanced features are more commonly used.
For more information on SCSI, see http://www.techweb.com/encyclopedia/defineterm.jhtml?term=scsi
SATA
SATA (Serial ATA) is the latest evolution of IDE. There are adapters between the two but there isn't backwards compatibility. SATA dropped IDE's parallel and two drives per cable design in favor of a direct serial connection between the motherboard (or controller) and each drive. This resulted in smaller/narrower connectors and cables in addition to increased speed. The latest version, SATA II, finally closed the speed gap with SCSI and kept the low price of IDE.
For more information on SATA, see http://www.techweb.com/encyclopedia/defineterm.jhtml?term=SerialATA
RAID
RAID (Redundant Array of Independent Disks) refers to a scheme of using multiple hard drives to create one or more logical units often to gain a benefit like fault tolerance or increased capacity. RAID historically was used only in high-end expensive server hardware, but is now affordable enough to be used in personal desktops. Vendors like Dell now even sell desktop computers with RAID built in to take advantage of fault tolerance.
RAID is not dependent to any type of hardware. There is hardware and software raid and may be used with IDE, SCSI, or SATA hard drives. It is even possible to RAID things other than hard drives, like USB drives or even floppy disk drives.
For more information including the different types of RAID:
http://en.wikipedia.org/wiki/Redundant_array_of_independent_disks
