A hard disk drive (HDD), hard disk, hard drive, or fixed disk, is an electromechanical data "Operating Systems: Three Easy Pieces, Chapter: Hard Disk Drives" (PDF). Arpaci-Dusseau Books. ^ Patterson, David; Hennessy, John (). PDF | Disk encryption is a technology which protects information by A hard disk drive (HDD), hard disk, hard drive or fixed disk is a data storage device used. PDF | A discussion about the increasing storage capacity of hard disk drives ( HDD) is presented. Today, a inch has a capacity of GB capable of storing .
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Magneto Resistive Heads. ✓ Also known as Anisotropic MR. (AMR) heads. ✓ It is the key invention that led to the creation of hard disks over 1 GB in size. we dive into more detail about one device in particular: the hard disk drive. dancindonna.info A. Chapter Hard Disk Drives. Introduction. From the early days of the computer to the present, computer storage has been classified into a primary working.
See also: Magnetic storage A modern HDD records data by magnetizing a thin film of ferromagnetic material [e] on both sides of a disk. Sequential changes in the direction of magnetization represent binary data bits. The data is read from the disk by detecting the transitions in magnetization. User data is encoded using an encoding scheme, such as run-length limited encoding, [f] which determines how the data is represented by the magnetic transitions. A typical HDD design consists of a spindle that holds flat circular disks, called platters , which hold the recorded data.
Photo: Magnets—the technology behind hard drives really is this simple! Magnetism has another very important use.
Suppose you need to leave a message for a friend and all you have is a magnet and an unmagnetized iron nail. Suppose the message is a very simple one: either you will see your friend later that day or not.
You could arrange with your friend that you will drop a nail through their letterbox. If the nail is magnetized, it means you will see them later; if the nail is unmagnetized, you won't. Your friend gets in from school and finds a nail on the doormat. They take it to the kitchen table and try to pick up a paperclip.
If the clip attaches to the magnet, it must be magnetized—and it must mean you plan to see them later. It's a pretty weird way to leave a message for someone, but it illustrates something very important: magnetism can be used to store information. If your computer has a 20 gigabyte GB hard drive, or you have a 20 GB iPod or MP3 player, it's a bit like a box containing thousand million microscopically small iron nails, each of which can store one tiny piece of information called a bit.
A bit is a binary digit—either a number zero or a number one. In computers, numbers are stored not as decimal base but as patterns of binary digits instead. For example, the decimal number is stored as the binary number Letters and other characters can also be stored as binary numbers. Thus, computers store a capital letter A as the decimal number 65 or the binary number Suppose you want to store the number in your computer in that big box of iron nails.
You need to find a row of seven unused nails. You magnetize the first one to store a 1 , leave the next five demagnetized to store five zeros , and magnetize the last one to store a 1. How a hard drive works In your computer's hard drive, there aren't really any iron nails. There's just a large shiny, circular "plate" of magnetic material called a platter, divided into billions of tiny areas.
Each one of those areas can be independently magnetized to store a 1 or demagnetized to store a 0. Magnetism is used in computer storage because it goes on storing information even when the power is switched off.
If you magnetize a nail, it stays magnetized until you demagnetize it. In much the same way, the computerized information or data stored in your PC hard drive or iPod stays there even when you switch the power off. What are the parts in a hard drive? A hard drive has only a few basic parts. There are one or more shiny silver platters where information is stored magnetically, there's an arm mechanism that moves a tiny magnet called a read-write head back and forth over the platters to record or store information, and there's an electronic circuit to control everything and act as a link between the hard drive and the rest of your computer.
After a hard-drive crash last year, I was left with an old drive that no longer worked.
I took a peek inside, and here's what I found Actuator that moves the read-write arm. In older hard drives, the actuators were stepper motors.
In most modern hard drives, voice coils are used instead. As their name suggests, these are simple electromagnets, working rather like the moving coils that make sounds in loudspeakers. They position the read-write arm more quickly, precisely, and reliably than stepper motors and are less sensitive to problems such as temperature variations.
Read-write arm swings read-write head back and forth across platter. Central spindle allows platter to rotate at high speed.
Magnetic platter stores information in binary form. In , Toshiba released the first 0. In , Seagate acquired Maxtor, further consolidating the hard drive market.
Drive capacities continued to skyrocket from there. Helium offers less drag and turbulence than air because it is less dense and lighter than air. That means drives filled with helium run cooler and faster and can have higher storage densities. In addition, a helium-filled hard disk enables manufacturers to put seven platters in the same space required for five platters in conventional hard drives.
Also in , Seagate introduced hard disks using shingled magnetic recording SMR technology to further overcome the physical limitations of conventional drives.
SMR layers magnetic tracks on each disk instead of placing them parallel to each other as in conventional hard disks, thereby increasing storage density. The tracks overlap like shingles on a roof, hence the name of the technology. Parts of a hard drive A hard drive consists of several major components inside its casing.
Hard disks include one or more aluminum, glass or ceramic platters made of substrate material with a thin magnetic surface, or media layer, to store data.
Platters store and organize data in specific structures -- tracks, sectors and clusters -- on this media layer, which is only a few millionths of an inch thick. A superthin protective and lubricating protective layer above the magnetic media guards against accidental damage and contamination by foreign material, like dust.
Hard disk components The spindle rotates the platters as needed and holds them in position. The rpm of the spindle determine how fast data is written and read. Joanne Larson, an engineer at Seagate Technology, demonstrates how a hard disk works. An intelligent circuit, or logic board, tells the actuator motor what to do. Form factor The entire disk must be mounted in an enclosure to protect the internal environment of the hard disk from outside contaminants and air.
The drive's internals, also known as the head assembly, are mounted securely to the casing and then usually covered with aluminum. The form factor of an HDD is the size and shape of this enclosure.
The HDD form factor governs its compatibility with the drive bays of desktop and portable computers, servers, storage enclosures, storage arrays or any consumer product that uses a hard disk, such as a digital video recorder DVR.
Industry standards dictate the geometry of HDD form factors , which includes the length, width and height of the HDD, in addition to the orientation and position of the host interface connector. Common enterprise-class HDD form factors are small form factor 2.
While enterprise-class HDD enclosures typically have standard lengths and widths, height can vary -- up to 15 mm and The 3. Hard disk destruction services Just because data is deleted and is no longer accessible to the application or operating system OS that created it, that doesn't mean the information isn't available on a hard disk.
The portions of the coil along the two sides of the arrowhead which point to the actuator bearing center then interact with the magnetic field of the fixed magnet. Current flowing radially outward along one side of the arrowhead and radially inward on the other produces the tangential force.
If the magnetic field were uniform, each side would generate opposing forces that would cancel each other out. Therefore, the surface of the magnet is half north pole and half south pole, with the radial dividing line in the middle, causing the two sides of the coil to see opposite magnetic fields and produce forces that add instead of canceling.
Currents along the top and bottom of the coil produce radial forces that do not rotate the head. The HDD's electronics control the movement of the actuator and the rotation of the disk, and perform reads and writes on demand from the disk controller.
Feedback of the drive electronics is accomplished by means of special segments of the disk dedicated to servo feedback.
These are either complete concentric circles in the case of dedicated servo technology , or segments interspersed with real data in the case of embedded servo technology.
The servo feedback optimizes the signal to noise ratio of the GMR sensors by adjusting the voice-coil of the actuated arm. The spinning of the disk also uses a servo motor. Modern disk firmware is capable of scheduling reads and writes efficiently on the platter surfaces and remapping sectors of the media which have failed. Error rates and handling[ edit ] Modern drives make extensive use of error correction codes ECCs , particularly Reed—Solomon error correction.
These techniques store extra bits, determined by mathematical formulas, for each block of data; the extra bits allow many errors to be corrected invisibly. The extra bits themselves take up space on the HDD, but allow higher recording densities to be employed without causing uncorrectable errors, resulting in much larger storage capacity.
The S. The "No-ID Format", developed by IBM in the mids, contains information about which sectors are bad and where remapped sectors have been located. Examples of specified uncorrected bit read error rates include: specifications for enterprise SAS disk drives state the error rate to be one uncorrected bit read error in every bits read,   specifications for consumer SATA hard drives state the error rate to be one uncorrected bit read error in every bits.
Several new magnetic storage technologies are being developed to overcome or at least abate this trilemma and thereby maintain the competitiveness of HDDs with respect to products such as flash memory —based solid-state drives SSDs.
In , Seagate introduced shingled magnetic recording SMR ,  intended as something of a "stopgap" technology between PMR and Seagate's intended successor heat-assisted magnetic recording HAMR , SMR utilises overlapping tracks for increased data density, at the cost of design complexity and lower data access speeds particularly write speeds and random access 4k speeds.