Hard disk

A hard disk drive (HDD, or also hard drive) is a non-volatile data storage device that stores data on a magnetic surface layered onto hard disk platters.

Mechanics

A hard disk uses rigid rotating platters (disks). Each platter has a planar magnetic surface on which digital data may be stored. Information is written to the disk by transmitting an electromagnetic flux through an antenna or read-write head that is very close to a magnetic material, which in turn changes its polarization due to the flux. The information can be read by a read-write head which senses electrical change as the magnetic fields pass by in close proximity as the platter rotates.

A typical hard disk drive design consists of a central axis or spindle upon which the platters spin at a constant rotational velocity. Moving along and between the platters on a common armature are read-write heads, with one head for each platter surface. The armature moves the heads radially across the platters as they spin, allowing each head access to the entirety of the platter.

The associated electronics control the movement of the read-write armature and the rotation of the disk, and perform reads and writes on demand from the disk controller. Modern drive firmware is capable of scheduling reads and writes efficiently on the disk surfaces and remapping sectors of the disk which have failed.

Also, most major hard drive and motherboard vendors now support S.M.A.R.T. technology, by which impending failures can often be predicted, allowing the user to be alerted in time to prevent data loss.

The (mostly) sealed enclosure protects the drive internals from dust, condensation, and other sources of contamination. The hard disk's read-write heads fly on an air bearing (a cushion of air) only nanometres above the disk surface. The disk surface and the drive's internal environment must therefore be kept immaculately clean to prevent damage from fingerprints, hair, dust, smoke particles, etc. given the submicroscopic gap between the heads and disk.

Contrary to popular belief a hard disk drive does not contain a vacuum. Instead, the system relies on air pressure inside the drive to support the heads at their proper flying height while the disk is in motion. Another common misconception is that a hard drive is totally sealed. A hard disk drive requires a certain range of air pressures in order to operate properly. If the air pressure is too low, the air will not exert enough force on the flying head, the head will not be at the proper height, and there is a risk of head crashes and data loss. (Specially manufactured sealed and pressurized drives are needed for reliable high-altitude operation, above about 10,000 feet. This does not apply to pressurized enclosures, like an airplane cabin.) Modern drives include temperature sensors and adjust their operation to the operating environment.

Hard disk drives are not airtight. They have a permeable filter (a breather filter) between the top cover and inside of the drive, to allow the pressure inside and outside the drive to equalize while keeping out dust and dirt. The filter also allows moisture in the air to enter the drive. Very high humidity year-round will cause accelerated wear of the drive's heads (by increasing stiction, or the tendency for the heads to stick to the disk surface, which causes physical damage to the disk and spindle motor). You can see these breather holes on all drives -- they usually have a warning sticker next to them, informing the user not to cover the holes. The air inside the operating drive is constantly moving too, being swept in motion by friction with the spinning disk platters. This air passes through an internal filter to remove any leftover contaminants from manufacture, any particles that may have somehow entered the drive, and any particles generated by head crash.

Due to the extremely close spacing of the heads and disk surface, any contamination of the read-write heads or disk platters can lead to a head crash — a failure of the disk in which the head scrapes across the platter surface, often grinding away the thin magnetic film. For Giant Magnetoresistive (GMR) heads in particular, a minor head crash from contamination (that does not remove the magnetic surface of the disk) will still result in the head temporarily overheating, due to friction with the disk surface, and renders the disk unreadable until the head temperature stabilizes. Head crashes can be caused by electronic failure, a sudden power failure, physical shock, wear and tear, or poorly manufactured disks. Normally, when powering down, a hard disk moves its heads to a safe area of the disk, where no data is ever kept (the landing zone). However, especially in old models, sudden power interruptions or a power supply failure can result in the drive shutting down with the heads in the data zone, which increases the risk of data loss. Newer drives are designed such that the rotational inertia in the platters is used to safely park the heads in the case of unexpected power loss. IBM pioneered drives with "head unloading" technology that lifts the heads off the platters onto "ramps" instead of having them rest on the platters, reducing the risk of stiction. Other manufacturers also use this technology.

Apple Computer has created a technology for their new PowerBook line of laptop computers called Sudden Motion Sensor, or SMS. When a sudden, sharp movement is detected by the built-in motion sensor in the PowerBook, internal hard disk heads automatically unload themselves into the parking zone to reduce the risk of any potential data loss or scratches made.

Spring tension from the head mounting constantly pushes the heads towards the disk. While the disk is spinning, the heads are supported by an air bearing and experience no physical contact wear. The sliders (the part of the heads that are closest to the disk and contain the pickup coil itself) are designed to reliably survive a number of landings and takeoffs from the disk surface, though wear and tear on these microscopic components eventually takes its toll. Most manufacturers design the sliders to survive 50,000 contact cycles before the chance of damage on startup rises above 50%. However, the decay rate is not linear — when a drive is younger and has fewer start/stop cycles, it has a better chance of surviving the next startup than an older, higher-mileage drive (as the head literally drags along the drive's surface until the air bearing is established). For example, the Maxtor DiamondMax series of desktop hard drives are rated to 50,000 start-stop cycles. This means that no failures attributed to the head-disk interface were seen before at least 50,000 start-stop cycles during testing.

Using rigid platters and sealing the unit allows much tighter tolerances than in a floppy disk. Consequently, hard disks can store much more data than floppy disk, and access and transmit it faster. In 2005, a typical workstation hard disk might store between 80 GB and 500 GB of data, rotate at 7,200 to 10,000 rpm, and have a sequential media transfer rate of over 50 MB/s. The fastest workstation and server hard drives spin at 15,000 rpm, and can achieve sequential media transfer speeds of up to 100 MB/s. Notebook hard drives, which are physically smaller than their desktop counterparts, tend to be slower and have less capacity. Most spin at only 4,200 rpm or 5,400 rpm, though the newest top models spin at 7,200 rpm.

Access and interfaces

A hard disk is generally accessed over one of a number of bus types, including ATA (IDE, EIDE), Serial ATA, SCSI, SAS, FireWire (aka IEEE 1394), USB, and Fibre Channel.

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 (which is still used on the common "1.44 MB" (1.4 MiB) 3.5-inch floppy), and ran at a data rate of 5 megabits per second. Later on, controllers using 2,7 RLL (or just "RLL") encoding increased this by half, to 7.5 megabits per second; it also increased drive capacity by half.

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

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

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

ATA drives 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 in a master/slave setup (two drives on the same cable). This was mostly remedied by the mid-1990s, when ATA's specfication was standardised and the details began to be cleaned up, but still causes problems occasionally (especially with CD-ROM and DVD-ROM drives, and when mixing Ultra DMA and non-UDMA devices).

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

FireWire/IEEE 1394 and USB(1.0/2.0) hard disks are external units containing generally ATA or SCSI drives 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.

Other characteristics

• Capacity (measured in gigabytes)
• Physical size (inches)
  • Almost all hard disks today are of either the 3.5", used in desktops, or 2.5", used in laptops, variety. 2.5" drives are usually slower and have less capacity but use less power and are more tolerant of movement. An increasingly common size is the 1.8" drives used in portable MP3 players, which have very low power consumption and are highly shock-resistant. Additionally, there is the 1" form factor designed to fit the dimensions of CF Type II, which is usually used as storage for portable devices such as mp3 players and digital cameras. 1" was a de facto form factor lead by IBM's Microdrive, but is now generically called 1" due to other manufacturers producing similar products. There is also a 0.85" form factor produced by Toshiba for use in mobile phones and similar applications. The size designations can be slightly confusing, for example a 3.5" disk drive has a case that is 4" wide. Furthermore, server-class hard disks also come in both 3.5" and 2.5" form factors.
• Reliability: Mean Time Between Failures (MTBF)
  • SATA 1.0 drives support speeds up to 10,000 rpm and mean time between failure (MTBF) levels up to 1 million hours under an eight-hour, low-duty cycle. Fibre Channel (FC) drives support up to 15,000 rpm and an MTBF of 1.4 million hours under a 24-hour duty cycle.
• Number of I/O operations per second
  • Modern disks can perform around 50 random or 100 sequential OPS
• Power consumption (especially important in battery-powered laptops)
• audible noise (in dBA)
• G-shock rating (surprisingly high in modern drives)
• Transfer Rate
  • Inner Zone: from 44.2 MB/sec to 74.5 MB/sec
  • Outer Zone: from 74.0 MB/sec to 111.4 MB/sec
• Random access time: from 5 ms to 15 ms

Addressing modes There are two modes of addressing the data blocks on more recent hard disks. The older mode is CHS addressing (Cylinder-Head-Sector), used on old ST-506 and ATA drives and internally by the PC BIOS. The more recent mode is the LBA (Logical Block Addressing), used by SCSI drives and newer ATA drives (ATA drives power up in CHS mode for historical reasons).

CHS describes the disk space in terms of its physical dimensions, data-wise; this is the traditional way of accessing a disk on IBM PC compatible hardware, and while it works well for floppies (for which it was originally designed) and small hard disks, it caused problems when disks started to exceed the design limits of the PC's CHS implementation. The traditional CHS limit was 1024 cylinders, 16 heads and 63 sectors; on a drive with 512-byte sectors, this comes to 504 MiB (528 megabytes). The origin of the CHS limit lies in a combination of the limitations of IBM's BIOS interface (which allowed 1024 cylinders, 256 heads and 64 sectors; sectors were counted from 1, reducing that number to 63, giving an addressing limit of 8064 MiB or 7.8 GiB), and a hardware limitation of the AT's hard disk controller (which allowed up to 65536 cylinders and 256 sectors, but only 16 heads, putting its addressing limit at 2^28 bits or 128 GiB).

When drives larger than 504 MiB began to appear in the mid-1990s, many system BIOSes had problems communicating with them, requiring LBA BIOS upgrades or special driver software to work correctly. Even after the introduction of LBA, similar limitations reappeared several times over the following years: at 2.1, 4.2, 8.4, 32, and 128 GiB. The 2.1, 4.2 and 32 GiB limits are hard limits: fitting a drive larger than the limit results in a PC that refuses to boot, unless the drive includes special jumpers to make it appear as a smaller capacity. The 8.4 and 128 GiB limits are soft limits: the PC simply ignores the extra capacity and reports a drive of the maximum size it is able to communicate with.

SCSI drives, however, have always used LBA addressing, which describes the disk as a linear, sequentially-numbered set of blocks. SCSI mode page commands can be used to get the physical specifications of the disk, but this is not used to read or write data; this is an artifact of the early days of SCSI, circa 1986, when a disk attached to a SCSI bus could just as well be an ST-506 or ESDI drive attached through a bridge (and therefore having a CHS configuration that was subject to change) as it could be a native SCSI device. Because PCs use CHS addressing internally, the BIOS code on PC SCSI host adapters does CHS-to-LBA translation, and provides a set of CHS drive parameters that tries to match the total number of LBA blocks as closely as possible.

ATA drives can either use their native CHS parameters (only on very early drives; hard drives made since the early 1990s use zone bit recording, and thus don't have a set number of sectors per track), use a "translated" CHS profile (similar to what SCSI host adapters provide), or run in ATA LBA mode, as specified by ATA-2. To maintain some degree of compatibility with older computers, LBA mode generally has to be requested explicitly by the host computer. ATA drives larger than 8 GiB are always accessed by LBA, due to the 8 GiB limit described above.

See also: hard disk drive partitioning, master boot record, file system, drive letter assignment, boot sector.

Manufacturers

Most of the world's hard disks are now manufactured by just a handful of large firms: Seagate, Maxtor (now owned by Seagate), Western Digital, Samsung, and Hitachi, the former drive manufacturing division of IBM. Fujitsu continues to make specialist notebook and SCSI drives but exited the mass market in 2001. Toshiba is a major manufacturer of 2.5-inch and 1.8-inch notebook drives.

Firms that have come and gone

Dozens of former hard drive manufacturers have gone out of business, merged, or closed their hard drive divisions; as capacities and demand for products increased, profits became hard to find, and there were shakeouts in the late 1980s and late 1990s. The first notable casualty of the business in the PC era was Computer Memories International or CMI; after the 1985 incident with the faulty 20MB AT drives, CMI's reputation never recovered, and they exited the hard drive business in 1987. Another notable failure was MiniScribe, who went bankrupt in 1990 after it was found that they had "cooked the books" and inflated sales numbers for several years. Many other smaller companies (like Kalok, Microscience, LaPine, Areal, Priam and PrairieTek) also did not survive the shakeout, and had disappeared by 1993; Micropolis was able to hold on until 1997, and JTS, a relative latecomer to the scene, lasted only a few years and was gone by 1999. Rodime was also an important manufacturer during the 1980s, but stopped making drives in the early 1990s amid the shakeout and now concentrates on technology licensing; they hold a number of patents related to 3.5-inch form factor hard drives.

There have also been a number of notable mergers in the hard disk industry:

• Tandon sold its disk manufacturing division to Western Digital (which was then a controller maker and ASIC house) in 1988; by the early 1990s Western Digital disks were among the top sellers.
• In 1995, Conner Peripherals announced a merger with Seagate (who had earlier bought Imprimis from CDC), which was completed in early 1996.
• JTS infamously merged with Atari in 1996, giving it the capital it needed to bring its drive range into production.
• In 2003, following the controversy over the mass failures of its Deskstar 75GXP range, hard disk pioneer IBM sold the majority of its disk division to Hitachi, who renamed it Hitachi Global Storage Technologies.
• Quantum bought DEC's storage division in 1994, and later (2000) sold the hard disk division to Maxtor to concentrate on tape drives. In December 2005, however, Maxtor itself was acquired by Seagate for USD1.9 billion.

In the United Kingdom, Cumana, a manufacturer of disk drives for Acorn computers, ceased manufacturing drives in 1995.

"Marketing" capacity versus true capacity

Hard drive manufacturers often use the metric definition of the prefixes "giga" and "mega", whilst nearly all operating system utilities report capacities using binary definitions for the prefixes. This is largely for historical reasons, since when storage capacities started to exceed thousands of bytes, there were no standard binary prefixes. The IEC only standardized binary prefixes in 1999, so 2¹⁰ (1024) bytes was called a kilobyte because 1024 is "close enough" to the metric prefix kilo, which is defined as 10³ or 1000. This trend became habit and continued to be applied to the prefixes "mega," "giga," and even "tera." Obviously the discrepancy becomes much more noticeable in reported capacities in the multiple gigabyte range, and users will often notice that the volume capacity reported by their OS is significantly less than that advertised by the hard drive manufacturer. For example, a drive advertised as 200 GB can be expected to store close to 200 x 10⁹, or 200 billion, bytes. This uses the proper SI definition of "giga," 10⁹ and can be considered as an approximation of a gibibyte. Since utilities provided by the operating system probably define a gigabyte as 2³⁰, or 1073741824, bytes, the reported capacity of the drive will be closer to 186.26 GB (actually, GiB), a difference of well over 7 percent. For this very reason, many utilities that report capacity have begun to use the aforementioned IEC standard binary prefixes (e.g. KiB, MiB, GiB) since their definitions are unambiguous.

Another side point is that many people mistakenly attribute the discrepancy in reported and advertised capacities to reserved space used for file system and partition accounting information. However, for large (several GiB) filesystems, this data rarely occupies more than several MiB, and therefore cannot possibly account for the apparent "loss" of tens of GBs.

Hard disk usage

From the original use of a hard drive in a single computer, techniques for guarding against hard disk failure were developed such as the redundant array of independent disks (RAID). Hard disks are also found in network attached storage (NAS) devices, but for large volumes of data are most efficiently used in a storage area network (SAN). Applications for hard disk drives expanded to include personal video recorders, digital audio players, digital organizers and digital cameras. In 2005 the first cellular telephones to include hard disk drives were introduced by Samsung and Nokia.

History

The first hard disk drive was the IBM 350 Disk File, invented by Reynold Johnson and introduced in 1955 with the IBM 305 computer. This drive had fifty 24 inch platters, with a total capacity of five million characters. A single head was used for access to all the platters, making the average access time very slow.

The IBM 1301 Disk Storage Unit, announced in 1961, introduced the usage of a separate head for each data surface.

The first disk drive to use removable media was the IBM 1311 drive, which used the IBM 1316 disk pack to store two million characters.

In 1973, IBM introduced the 3340 "Winchester" disk system the first to use a sealed head/disk assembly (HDA). Almost all modern disk drives now use this technology, and the term "Winchester" became a common description for all hard disks, though generally falling out of use during the 1990s. Project head designer/lead designer Kenneth Haughton named it after the Winchester 30-30 rifle after the developers called it the "30-30" because of its two 30 MB spindles.

For many years, hard disks were large, cumbersome devices, more suited to use in the protected environment of a data center or large office than in a harsh industrial environment (due to their delicacy), or small office or home (due to their size and power consumption). Before the early 1980s, most hard disks had 8-inch (20 cm) or 14-inch (35 cm) platters, required an equipment rack or a large amount of floor space (especially the large removable-media drives, which were often referred to as "washing machines"), and in many cases needed high-amperage or even three-phase power hookups due to the large motors they used. Because of this, hard disks were not commonly used with microcomputers until after 1980, when Seagate Technology introduced the ST-506, the first 5.25-inch hard drive, with a capacity of 5 megabytes. In fact, in its factory configuration the original IBM PC (IBM 5150) was not equipped with a hard drive.

Most microcomputer hard disk drives in the early 1980s were not sold under their manufacturer's names, but by OEMs as part of larger peripherals (such as the Corvus Disk System and the Apple ProFile). The IBM PC/XT had an internal hard disk, however, and this started a trend toward buying "bare" drives (often by mail order) and installing them directly into a system. Hard disk makers started marketing to end users as well as OEMs, and by the mid-1990s, hard disks had become available on retail store shelves.

While internal drives became the system of choice on PCs, external hard drives remained popular for much longer on the Apple Macintosh and other platforms. Every Mac made between 1986 and 1998 has a SCSI port on the back, making external expansion easy; also, "toaster" Macs did not have easily accessible hard drive bays (or, in the case of the Mac Plus, any hard drive bay at all), so on those models, external SCSI disks were the only reasonable option. External SCSI drives were also popular with older microcomputers such as the Apple II series and the Commodore 64, and were also used extensively in servers, a usage which is still popular today. The appearance in the late 1990s of high-speed external interfaces such as USB and FireWire has made external disk systems popular among regular users once again, especially for users who move large amounts of data between two or more locations, and most hard disk makers now make their disks available in external cases.

The capacity of hard drives has grown exponentially over time. With early personal computers, a drive with a 20 megabyte capacity was considered large. In the latter half of the 1990s, hard drives with capacities of 1 gigabyte and greater became available. As of early 2005, the "smallest" desktop hard disk in production has a capacity of 40 gigabytes, while the largest-capacity internal drives are a half terabyte (500 gigabytes), with external drives at or exceeding one terabyte. As far as PC history is concerned, the major drive families have been MFM, RLL, ESDI, SCSI, IDE and EIDE, and now SATA. MFM drives required that the electronics on the "controller" be compatible with the electronics on the drive — disks and controllers had to be compatible. RLL (Run Length Limited) was a way of encoding bits onto the platters that allowed for better density. Most RLL drives also needed to be "compatible" with the controllers that communicated with them. ESDI was an interface developed by Maxtor. It allowed for faster communication between the PC and the disk. SCSI (originally named SASI for Shugart (sic) Associates) or Small Computer System Interface was an early competitor with ESDI. When the price of electronics dropped (and because of a demand by consumers) the electronics that had been stored on the controller card was moved to the disk drive itself. This advance was known as "Integrated Drive Electronics" or IDE. Eventually, IDE manufacturers wanted the speed of IDE to approach the speed of SCSI drives. IDE drives were slower because they did not have as big a cache as the SCSI drives, and they could not write directly to RAM. IDE manufacturers attempted to close this speed gap by introducing Logical Block Addressing (LBA). These drives were known as EIDE. While EIDE was introduced, though, SCSI manufacturers continued to improve SCSI's performance. The increase in SCSI performance came at a price — its interfaces were more expensive. In order for EIDE's performance to increase (while keeping the cost of the associated electronics low), it was realized that the only way to do this was to move from "parallel" interfaces to "serial" interfaces, the result of which is the SATA interface. However, as of 2005, performance of SATA and PATA disks is comparable. Fibre channel (FC) interfaces are left to discussions of server drives.

Timeline of capacity and other technical improvements

• (CS) denotes an improvement in the consumer market.

1950s

• 1956 - first commercial hard disk, the IBM 350 RAMAC disk drive, 5 megabyte.

1960s

1970s

1980s

• 1980 - first 5.25-inch Winchester drive, the Shugart ST-506, 5 megabyte (CS)
• 1986 - Standardization of SCSI

1990s

• 1991 - 100 megabyte hard drive (CS)
• 1994 - ATA-1 standardized
• 1995 - 2 gigabyte hard drive (CS)
• 1997 - 10 gigabyte hard drive (CS)
• 1998 - UltraDMA/33 and ATAPI standardized

2000s

• 2002 - 137 GB addressing space barrier broken
• 2003 - Serial ATA introduced
• 2005 - 500 GB hard drive
• 2005 - Serial ATA 3G standardized
• 2005 - Introduction of faster SAS (Serial Attached SCSI)