HARD DRIVES

Alternatively referred to as a and abbreviated as HD or HDD, thehard drive is the computer's main storage media device that permanently stores all data on the computer. The hard drive was first introduced on September 13, 1956 and consists of one or more hard drive platters inside of air sealed casing. Most computer hard drives are in an internal drive bay at the front of the computer and connect to themotherboard using either ATA, SCSI, or a SATA cable and power cable. Below, is a picture of what the inside of a hard drive looks like for a desktop and laptop hard drive.

As can be seen in the above picture, the desktop hard drive has six components: the head actuator, read/write actuator arm, read/write head, spindle, and platter. On the back of a hard drive is a circuit board called the disk controller. Tip: New users often confuse memory (RAM) with disk drive space. See our memory definition for a comparison between memory and storage. How is data read and stored on a hard drive?

Data sent to and from the hard drive is interpreted by the disk controller, which tells the hard drive what to do and how to move the components within the drive. When the operating system needs to read or write information, it examines the hard drives File Allocation Table (FAT) to determine file location and available areas. Once this has been determined, the disk controller instructs the actuator to move the read/write arm and align the read/write head. Because files are often scattered throughout the platter, the head needs to move to different locations to access all information. All information stored on a traditional hard drive, like the above example, is done magnetically. After completing the above steps, if the computer needs to read information from the hard drive it would read the magnetic polarities on the platter. One side of the magnetic polarity is 0 and the other is 1, reading this as binary data the computer can understand what the data is on the platter. For the computer to write information to the platter, the read/write head aligns the magnetic polarities, writing 0's and 1's that can be read later. External and Internal hard drives

Although most hard drives are internal hard drives, many users also use external hard drives to backup data on their computer and expand the total amount of space available to them. External drives are often stored in an enclosure that helps protect the drive and allow it to interface with the computer, usually over USB or eSATA. A great example of a backup external device that supports multiple hard drives is theDrobo.

External hard drives come in many shapes and sizes. Some are large, about the size of a book while others are about the size of a cell phone. External hard drives can be very useful for backing up important data and taking with you on the go since they usually offer more than ajump drive and are still portable. The picture is an example of a laptop hard disk drive enclosurefrom Adaptec. With this enclosure, the user installs any size of laptop hard drive they desire into the enclosure and connect it to a USB port on the computer. HDD being replaced by SSD

Solid State Drives (SSDs) are starting to replace hard disk drives (HDDs) in many computers because of the clear advantages these drives have over HDD. While SSD is becoming more and more popular, HDD will continue to be in desktop computers with SSD because of the available capacity HDD offers over SSD. Advantages of SSD over HDD

The hard drive (HDD) has been the predominant storage device for computers, both desktops and laptops, for a long time. The main draw is the storage size and low cost. Computer manufacturers can include large hard drives at a small cost, so they've continued to use HDDs in their computers. The solid state drive (SSD) is available and can replace a HDD relatively easily. As you'll find by reading the below pros and cons, the SSD is a clear winner, but because of the price it still doesn't make sense to use SSDs for all uses. For most computer users, we suggest using SSD as a primary drive for your operating system and most important programs and then having either one or more HDD inside the same computer or an external HDD to store files like pictures and music, which doesn't need the fast access times of SSD. Topic SSD HDD

Access A SSD has access speeds of 35 to 100 micro-seconds, which is A typical HDD takes about 5,000 to 10,000 micro- time nearly 100 times faster. This faster access speed means programs seconds to access data. can run more quickly, which is very significant, especially for programs that access large amounts of data often like your operating system.

Price The price of a solid state drive is much more than a HDD which is HDD is much cheaper than SSD, especially for drives why most computers with a SSD only have a few hundred over 500GB. of storage. Desktop computers with a SSD may also have one or more HDDs for additional storage.

Reliability The SSD drive has no moving parts. It uses to store The HDD has moving parts and magnetic platters, data, which provides better performance and reliability over a meaning the more use they get, the faster they wear HDD. down and fail.

Capacity Although there are large SSDs realistically for most people's Several terabyte hard disk drives are available for very budgets anything over 512GB SSD is beyond their price range. reasonable prices.

Power The SSD uses less power than a standard HDD, which means a With all the parts and requirements to spin the platters lower energy bill over time and for laptops an increase of battery the HDD uses more power than a SSD. life.

Noise With no moving parts SSD generates no noise. With the spinning platters and moving read/write heads a HDD can sometimes be one of the loudest components in your computer.

Size SSD is available in 2.5", 1.8", and 1.0", increasing the available HDDs are usually 3.5" and 2.5" in size, for desktop and space available in a computer, especially a desktop or server. laptops respectively with no options for anything smaller.

Heat Because there are no moving parts and due to the nature of flash With moving parts comes added heat, which is why the memory, the SSD generates less heat, helping to increase its HDD generates more heat. Heat can slowly damage lifespan and reliability. electronics over time, so the higher the heat, the greater the potential of damage being done.

Magnetism SSD is not affected by magnetism. Because a hard drive relies off magnetism to write information to the platter information could be erased from a HDD using strong magnets.

(http://www.computerhope.com/jargon/h/harddriv.htm)

File system use[edit] Main article: Disk formatting

The presentation of an HDD to its host is determined by its controller. This may differ substantially from the drive's native interface particularly in mainframes or servers.

Modern HDDs, such as SAS[53] and SATA[54] drives, appear at their interfaces as a contiguous set of logical blocks; typically 512 long but the industry is in the process of changing to 4,096- logical blocks; see Advanced Format.[56]

The process of initializing these logical blocks on the physical disk platters is called low level formatting which is usually performed at the factory and is not normally changed in the field.[e] High level formatting then writes the file system structures into selected logical blocks to make the remaining logical blocks available to the host OS and its applications.[57] The operating system file system uses some of the disk space to organize files on the disk, recording their file names and the sequence of disk areas that represent the file. Examples of data structures stored on disk to retrieve files include the File Allocation Table (FAT) in the DOS file system and inodes in many UNIX file systems, as well as other operating system data structures. As a consequence not all the space on an HDD is available for user files. This file system overhead is usually less than 1% on drives larger than 100 MB. Units[edit] See also:

Unit prefixes[58][59]

Advertised capacity by Expected capacity by consumers in Reported capacity manufacturer class action (using binary multiples)

(usingdecimal multiples) Windows (using Mac OS X 10.6+ binary multiples) (using decimal

With prefix Bytes Bytes Diff. multiples)

100 MB 100,000,000 104,857,600 4.86% 95.4 MB 100 MB

100 GB 100,000,000,000 107,374,182,400 7.37% 93.1 GB, 95,367 MB 100 GB

1 TB 1,000,000,000,000 1,099,511,627,776 9.95% 931 GB, 953,674 MB 1,000 GB, 1,000,000 MB

The total capacity of HDDs is given by manufacturers in (1 MB = 1,000,000 bytes), gigabytes (1 GB = 1,000,000,000 bytes) or terabytes (1 TB = 1,000,000,000,000 bytes).[58][60][61][62][63][64]This numbering convention, where prefixes like mega- and giga- denote powers of 1000, is also used for data transmission rates and DVD capacities. However, the convention is different from that used by manufacturers of memory (RAM, ROM) and CDs, where prefixes like kilo- and mega- mean powers of 1024.

The practice of using prefixes assigned to powers of 1,000 within the HDD and computer industries dates back to the early days of computing.[65] By the 1970s million, mega and M were consistently being used in the powers of 1,000 sense to describe HDD capacity.[66][67][68]

Computers do not internally represent HDD or memory capacity in powers of 1,024; reporting it in this manner is just a convention.[69] uses the powers of 1,024 convention when reporting HDD capacity, thus an HDD offered by its manufacturer as a 1 TB drive is reported by these OSes as a 931 GB HDD. Mac OS X 10.6 ("Snow Leopard"), uses powers of 1,000 when reporting HDD capacity.

In the case of "mega-", there is a nearly 5% difference between the powers of 1,000 definition and the powers of 1,024 definition. Furthermore, the difference is compounded by 2.4% with each incrementally larger prefix (, terabyte, etc.). The discrepancy between the two conventions for measuring capacity was the subject of several class action suits against HDD manufacturers. The plaintiffs argued that the use of decimal measurements effectively misled consumers[70][71] while the defendants denied any wrongdoing or liability, asserting that their marketing and advertising complied in all respects with the law and that no class member sustained any damages or injuries.[72]

In December 1998, standards organizations addressed these dual definitions of the conventional prefixes by standardizing on unique binary prefixes and prefix symbols to denote multiples of 1,024, such as "mebibyte (MiB)", which exclusively denotes 220 or 1,048,576 bytes.[73] This standard has seen little adoption by the computer industry, and the conventionally prefixed forms of "byte" continue to denote slightly different values depending on context.[74][75] They are occasionally used, however.

CLOCK SPEED

Also called , the speed at which a microprocessorexecutes instructions. Every computer contains an internal clock that regulates the rate at which instructions are executed and synchronizes all the various computer components. The CPUrequires a fixed number of clock ticks (or clock cycles) to execute each instruction. The faster the clock, the more instructions the CPU can execute per second. Clock speeds are expressed in megahertz (MHz) or gigahertz ((GHz). The internal architecture of a CPU has as much to do with a CPU's performance as the clock speed, so two CPUs with the same clock speed will not necessarily perform equally. Whereas an Intel 80286 microprocessor requires 20 cycles to multiply two numbers, an Intel 80486 or later processor can perform the same calculation in a single clock tick. (Note that clock tick here refers to the system's clock, which runs at 66 MHz for all PCs.) These newer processors, therefore, would be 20 times faster than the older processors even if their clock speeds were the same. In addition, some microprocessors are superscalar, which means that they can execute more than one instruction per clock cycle. Like CPUs, expansion buses also have clock speeds. Ideally, the CPU clock speed and the clock speed should be the same so that neither component slows down the other. In practice, the bus clock speed is often slower than the CPU clock speed, which creates a bottleneck. This is why new local buses, such as AGP, have been developed.

Speed is the actual speed for interfaces. Bandwidth is taken into account in QoS and by routing protocols such as OSPF / EIGRP for metric calculation. It does not influence how fast an interface sends or receives data. As an example, let's consider FastEthernet which only supports two transmission speeds (10 Mbps and 100 Mbps). Setting the bandwidth to a certain value such as configuring bandwidth 50000 on the interface won't change the speed. Clock rate is the physical speed which is being set on the DCE end of a serial link. If you have noticed, configuring "speed" is not available in serial interface config mode. What should be noted is an interface's speed strongly depends on the physical layer and bandwidth has no bearing with it.