All about Hard Drives

A hard disk drive (often shortened as "hard disk" or "hard drive"), is a non-volatile storage device which stores digitally encoded data on rapidly rotating platters with magnetic surfaces. Strictly speaking, "drive" refers to a device distinct from its medium, such as a tape drive and its tape, or a floppy disk drive and its floppy disk. Early HDDs had removable media; however, an HDD today is typically a sealed unit (except for a filtered vent hole to equalize air pressure) with fixed media.

History

HDDs (introduced in 1956 as data storage for an IBM accounting computer) were originally developed for use with general purpose computers. During the 1990s, the need for large-scale, reliable storage, independent of a particular device, led to the introduction of embedded systems such as RAID arrays, network attached storage (NAS) systems and storage area network (SAN) systems that provide efficient and reliable access to large volumes of data. In the 21st century, HDD usage expanded into consumer applications such as camcorders, cellphones (e.g. the Nokia N91), digital audio players, digital video players (e.g. the iPod Classic), digital video recorders, personal digital assistants and video game consoles.

Technology

HDDs record data by magnetizing ferromagnetic material directionally, to represent either a 0 or a 1 binary digit. They read the data back by detecting the magnetization of the material. A typical HDD design consists of a spindle which holds one or more flat circular disks called platters, onto which the data are recorded. The platters are made from a non-magnetic material, usually aluminum alloy or glass, and are coated with a thin layer of magnetic material. Older disks used iron(III) oxide as the magnetic material, but current disks use a cobalt-based alloy.

The platters are spun at very high speeds. Information is written to a platter as it rotates past devices called read-and-write heads that operate very close (tens of nanometers in new drives) over the magnetic surface. The read-and-write head is used to detect and modify the magnetization of the material immediately under it. There is one head for each magnetic platter surface on the spindle, mounted on a common arm. An actuator arm (or access arm) moves the heads on an arc (roughly radially) across the platters as they spin, allowing each head to access almost the entire surface of the platter as it spins. The arm is moved using a voice coil actuator or in some older designs a stepper motor.

Older drives read the data on the platter by sensing the rate of change of the magnetism in the head; these heads had small coils, and worked (in principle) much like magnetic-tape playback heads, although not in contact with the recording surface. As data density increased, read heads using magnetoresistance (MR) came into use; the electrical resistance of the head changed according to the strength of the magnetism from the platter. Later development made use of spintronics; in these heads, the magnetoresistive effect was much greater that in earlier types, and was dubbed "giant" magnetoresistance (GMR). This refers to the degree of effect, not the physical size, of the head the heads themselves are extremely tiny, and are too small to be seen without a microscope. GMR read heads are now commonplace.

HD heads are kept from contacting the platter surface by the air that is extremely close to the platter; that air moves at, or close to, the platter speed.  The record and playback head are mounted on a block called a slider, and the surface next to the platter is shaped to keep it just barely out of contact. It's a type of air bearing.

The magnetic surface of each platter is conceptually divided into many small sub-micrometre-sized magnetic regions, each of which is used to encode a single binary unit of information. In today's HDDs, each of these magnetic regions is composed of a few hundred magnetic grains. Each magnetic region forms a magnetic dipole which generates a highly localized magnetic field nearby. The write head magnetizes a region by generating a strong local magnetic field. Early HDDs used an electromagnet both to generate this field and to read the data by using electromagnetic induction. Later versions of inductive heads included metal in Gap (MIG) heads and thin film heads. In today's heads, the read and write elements are separate, but in close proximity, on the head portion of an actuator arm. The read element is typically magneto-resistive while the write element is typically thin-film inductive.

In modern drives, the small size of the magnetic regions creates the danger that their magnetic state might be lost because of thermal effects. To counter this, the platters are coated with two parallel magnetic layers, separated by a 3-atom-thick layer of the non-magnetic element ruthenium, and the two layers are magnetized in opposite orientation, thus reinforcing each other. Another technology used to overcome thermal effects to allow greater recording densities is perpendicular recording, first shipped in 2005, as of 2007 the technology was used in many HDDs.

Modern drives also make extensive use of Error Correcting Codes (ECCs), particularly ReedSolomon error correction. These techniques store extra bits for each block of data that are determined by mathematical formulas. The extra bits allow many errors to be fixed. While these extra bits take up space on the hard drive, they allow higher recording densities to be employed, resulting in much larger storage capacity for user data.

See File System for how operating systems access data on HDDs and other storage devices.

Architecture

A typical hard drive has two electric motors, one to spin the disks and one to position the read/write head assembly. The disk motor has an external rotor attached to the platters; the stator windings are fixed in place. The actuator has a read-write head under the tip of its very end (near center); a thin printed-circuit cable connects the read-write head to the hub of the actuator. A flexible, somewhat 'U'-shaped, ribbon cable, seen edge on below and to the left of the actuator arm in the first image and more clearly in the second, continues the connection from the head to the controller board on the opposite side.

The head support arm is very light, but also rigid; in modern drives, acceleration at the head reaches 250 gs.

The silver-colored structure at the upper left of the first image is the top plate of the permanent-magnet and moving coil motor that swings the heads to the desired position (it is shown removed in the second image). The plate supports a thin neodymium-iron-boron (NIB) high-flux magnet. Beneath this plate is the moving coil, often referred to as the voice coil by analogy to the coil in loudspeakers, which is attached to the actuator hub, and beneath that is a second NIB magnet, mounted on the bottom plate of the motor (some drives only have one magnet).

The voice coil, itself, is shaped rather like an arrowhead, and made of doubly-coated copper magnet wire. The inner layer is insulation, and the outer is thermoplastic, which bonds the coil together after it's wound on a form, making it self-supporting. The portions of the coil along the two sides of the arrowhead (which point to the actuator bearing center) interact with the magnetic field, developing a tangential force that rotates the actuator. Current flowing radially outward along one side of the arrowhead, and radially inward on the other produces the tangential force. (See magnetic field#Force on a charged particle.) 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 N pole, half S 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.

Access and Interfaces

Hard disk drives are accessed over one of a number of bus types, including parallel ATA (P-ATA, also called IDE or EIDE), Serial ATA (SATA), SCSI, Serial Attached SCSI (SAS), and Fibre Channel. Bridge circuitry is sometimes used to connect hard disk drives to buses that they cannot communicate with natively, such as IEEE 1394, USB and SCSI.

Back in the days of the ST-506 interface, the data encoding scheme was also important. The first ST-506 disks used Modified Frequency Modulation (MFM) encoding, and transferred data at a rate of 5  megabits per second. Later on, controllers using 2,7 RLL (or just "RLL") encoding increased the transfer rate by 50%, to 7.5 megabits per second; this also increased disk capacity by fifty percent.

Many ST-506 interface disk drives were only specified by the manufacturer to run at the lower MFM data rate, while other models (usually more expensive versions of the same basic disk drive) were specified to run at the higher RLL data rate. In some cases, a disk drive had sufficient margin to allow the MFM specified model to run at the faster RLL data rate; however, this was often unreliable and was not recommended. (An RLL-certified disk drive could run on a MFM controller, but with 1/3 less data capacity and speed.)

Enhanced Small Disk Interface (ESDI) also supported multiple data rates (ESDI disks always used 2,7 RLL, but at 10, 15 or 20 megabits per second), but this was usually negotiated automatically by the disk drive and controller; most of the time, however, 15 or 20 megabit ESDI disk drives weren't downward compatible (i.e. a 15 or 20 megabit disk drive wouldn't run on a 10 megabit controller). ESDI disk drives typically also had jumpers to set the number of sectors per track and (in some cases) sector size.

Modern hard drives present a consistent interface to the rest of the computer, no matter what data encoding scheme is used internally. Typically a DSP in the electronics inside the hard drive takes the raw analog voltages from the read head and uses PRML and ReedSolomon error correction to decode the sector boundaries and sector data, then sends that data out the standard interface. That DSP also watches the error rate detected by error detection and correction, and performs bad sector remapping, data collection for Self-Monitoring, Analysis, and Reporting Technology, and other internal tasks.

SCSI originally had just one signaling frequency of 5  MHz for a maximum data rate of 5 megabytes/second over 8 parallel conductors, but later this was increased dramatically. The SCSI bus speed had no bearing on the disk's internal speed because of buffering between the SCSI bus and the disk drive's internal data bus; however, many early disk drives had very small buffers, and thus had to be reformatted to a different interleave (just like ST-506 disks) when used on slow computers, such as early Commodore Amiga, IBM PC compatibles and Apple Macintoshes.

ATA disks have typically had no problems with interleave or data rate, due to their controller design, but many early models were incompatible with each other and couldn't run with two devices on the same physical cable in a master/slave setup. This was mostly remedied by the mid-1990s, when ATA's specification was standardised and the details began to be cleaned up, but still causes problems occasionally (especially with CD-ROM and DVD-ROM disks, and when mixing Ultra DMA and non-UDMA devices).

Serial ATA does away with master/slave setups entirely, placing each disk on its own channel (with its own set of I/O ports) instead.

FireWire/IEEE 1394 and USB(1.0/2.0) HDDs are external units containing generally ATA or SCSI disks with ports on the back allowing very simple and effective expansion and mobility. Most FireWire/IEEE 1394 models are able to daisy-chain in order to continue adding peripherals without requiring additional ports on the computer itself. USB however, is a point to point network and doesn't allow for daisy-chaining. USB hubs are used to increase the number of available ports and are used for devices that don't require charging since the current supplied by hubs is typically lower than what's available from the built-in USB ports.

Disk interface families used in personal computers

Notable families of disk interfaces include:

• Modern bit serial interfaces connect to a host bus adapter (today typically integrated into the "south bridge") with two cables, one for data/control and one for power.
  • Fibre Channel (FC), is a successor to parallel SCSI interface on enterprise market. It is a serial protocol. In disk drives usually the Fibre Channel Arbitrated Loop (FC-AL) connection topology is used. FC has much broader usage than mere disk interfaces, it is the cornerstone of storage area networks (SANs). Recently other protocols for this field, like iSCSI and ATA over Ethernet have been developed as well. Confusingly, drives usually use copper twisted-pair cables for Fibre Channel, not fibre optics. The latter are traditionally reserved for larger devices, such as servers or disk array controllers.
  • Serial ATA (SATA). The SATA data cable has one data pair for differential transmission of data to the device, and one pair for differential receiving from the device, just like EIA-422. That requires that data be transmitted serially. Similar differential signaling system is used in RS485, LocalTalk, USB, Firewire, and differential SCSI.
  • Serial Attached SCSI (SAS). The SAS is a new generation serial communication protocol for devices designed to allow for much higher speed data transfers and is compatible with SATA. SAS uses serial communication instead of the parallel method found in traditional SCSI devices but still uses SCSI commands.

• Word serial interfaces connect to a host bus adapter (today typically integrated into the "south bridge") with two cables, one for data/control and one for power. The earliest versions of these interfaces typically had a 16 bit parallel data transfer to/from the drive and there are 8 and 32 bit variants. Modern versions have serial data transfer. The word nature of data transfer makes the design of a host bus adapter significantly simpler than that of the precursor HDD controller.
  • Integrated Drive Electronics (IDE), later renamed to ATA, and then later to P-ATA ("parallel ATA", to distinguish it from the new Serial ATA). The original name reflected the innovative integration of HDD controller with HDD itself, which was not found in earlier disks. Moving the HDD controller from the interface card to the disk drive helped to standardize interfaces, and to reduce the cost and complexity. The 40 pin IDE/ATA connection of PATA transfers 16 bits of data at a time on the data cable. The data cable was originally 40 conductor, but later higher speed requirements for data transfer to and from the hard drive led to an "ultra DMA" mode, known as UDMA. Progressively faster versions of this standard ultimately added the requirement for an 80 conductor variant of the same cable; where half of the conductors provides grounding necessary for enhanced high-speed signal quality by reducing cross talk. The interface for 80 conductor only has 39 pins, the missing pin acting as a key to prevent incorrect insertion of the connector to an incompatible socket, a common cause of disk and controller damage.
  • EIDE was an unofficial update (by Western Digital) to the original IDE standard, with the key improvement being the use of direct memory access (DMA) to transfer data between the disk and the computer without the involvement of the CPU, an improvement later adopted by the official ATA standards. By directly transferring data between memory and disk, DMA eliminates the need for the CPU and operating system to copy byte per byte. And can therefore process other tasks while the data transfer occurs.
  • Small Computer System Interface (SCSI), originally named SASI for Shugart Associates System Interface, was an early competitor of ESDI. SCSI disks were standard on servers, workstations, Commodore Amiga and Apple Macintosh computers through the mid-90s, by which time most models had been transitioned to IDE (and later, SATA) family disks. Only in 2005 did the capacity of SCSI disks fall behind IDE disk technology, though the highest-performance disks are still available in SCSI and Fibre Channel only. The length limitations of the data cable allows for external SCSI devices. Originally SCSI data cables used single ended data transmission, but server class SCSI could use differential transmission, either low voltage differential (LVD) or high voltage differential (HVD).