Encyclopedia
A
hard disk drive is a digitally encoded non-volatile storage device which stores data on the
magnetic surfaces of
hard disk platters.
Hard disks were originally developed for use in connection with, or later inside, a single computer. Later, as a way of guarding against hard disk failure, they were arranged into configurations such as
redundant array of independent disks . Hard disks are also found in
network attached storage devices, but for large volumes of data may be most efficiently used in a storage area network . Over time, applications for hard disk drives have expanded beyond computers to include video recorders,
audio players,
digital organizers, and digital cameras. In 2005 the first cellular telephones to include hard disk drives were introduced by
Samsung and
Nokia.
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 2006, the "smallest" desktop hard disk still in production has a capacity of 40 gigabytes, while the largest-capacity internal drives are a 3/4 terabyte , with external drives at or exceeding one terabyte by using multiple internal disks. These new internal drives increased their storage capacities with
Perpendicular recording.
Technology
Hard drives record information by magnetizing a magnetic material and read the data back by detecting the magnetization of the material. A typical hard disk drive design consists of a spindle which holds one or more flat circular disks called
platters, on to which the data is recorded. The platters are made from a non-magnetic material, usually glass or aluminum, and are coated with a thin layer of magnetic material. Older drives used
iron oxide as the magnetic material, but current drives use a
cobalt-based alloy. The platter's magnetic surface is divided into many small regions, each of which is magnetized independently of the others.
The platters spin at high speeds. Information is written to a platter as it rotates past mechanisms called
read-and-write heads that fly very close 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 moves the heads on an arc across the platters as they spin, allowing each head to access almost the entire surface of the platter.
Drives have a mostly sealed enclosure that protects the drive internals from
dust,
condensation, and other sources of contamination. The hard disk's read-write heads fly on an air bearing which is a cushion of air only
nanometers above the disk surface. The disk surface and the drive's internal environment must therefore be kept immaculate to prevent damage from fingerprints, hair, dust, smoke particles, etc., given the submicroscopic gap between the heads and disk.
The magnetic surface of each platter is divided into many small sub-micrometre-sized magnetic regions, each of which is used to encode a single binary unit of information. As a whole, each magnetic region will have a magnetization. Thus it is a
magnetic dipole and will generate a highly localised
magnetic field nearby. In today's hard drives each of these magnetic regions is composed of a few hundred magnetic grains, which are the base material that gets magnetized. However, future hard drives may use different systems to create the magnetic regions, such as Patterned Magnetic Media.
The binary units of information are encoded through changes in magnetization from one magnetic region to the next. If the magnetization reverses across the boundary between two magnetic regions, this signifies one binary state, while no change in magnetization signifies the other state. At a boundary where the magnetization reverses, magnetic field lines will be dense and perpendicular to the medium. As the read head passes over a boundary with a magnetization reversal it will experience both an increase in the magnetic field strength and also a magnetic flux. The read sensor is designed to detect these effects. For example, today's read heads make use of the Magnetoresistive or
Giant magnetoresistive effects to detect field strength. In the past
inductors were used, and newer hard drives may have heads based on Tunnel magnetoresistance . For various reasons, the actual units of binary data are encoded using consecutive sequences of the two possible boundary states, rather than the states themselves. Most hard drives use a form of Run Length Limited coding to do this conversion.
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 2006, a typical
workstation hard disk might store between 80 GB and 750 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 up to and beyond 80 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, whereas the newest top models spin at 7,200 rpm.
History
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 , or small office or home . Before the early 1980s, most hard disks had 8-inch or 14-inch platters, required an equipment rack or a large amount of floor space , 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 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 . The IBM PC/XT had an internal hard disk, however, and this started a trend toward buying "bare" drives 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 , 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 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.
Hard disk characteristics
- Capacity, usually quoted in gigabytes.
- Physical size, usually quoted in inches:
- Almost all hard disks today are of either the 3.5" or 2.5" varieties, used in desktops and laptops, respectively. 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 and subnotebooks, 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 also usually used as storage for portable devices including digital cameras. 1" was a de facto form factor led 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, usually given in terms of Mean Time Between Failures :
- SATA 1.0 drives support speeds up to 10,000 rpm and MTBF levels up to 1 million hours under an eight-hour, low-duty cycle. Fibre Channel 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 access or 100 Sequential access operations per second.
- Power consumption .
- audible noise in dBA .
- G-shock rating .
- Transfer Rate:
- Inner Zone: from 44.2 MB/s to 74.5 MB/s.
- Outer Zone: from 74.0 MB/s to 111.4 MB/s.
- Random access time: from 5 ms to 15 ms.
Capacity measurements
Hard drive manufacturers typically specify drive capacity using
'SI prefixes', that is, the SI definition of the prefixes "giga" and "mega." This is largely for historical reasons, since disk drive storage capacities exceeded millions of bytes long before there were standard
'binary prefixes' . The IEC only standardized
'binary prefixes' in 1999. As it turned out, many practitioners early on in the computer and semiconductor industries adopted the term kilobyte to describe 2
10 bytes because 1024 is "close enough" to the metric prefix kilo, which is defined as 10
3 or 1000. Sometimes this non-SI conforming usage include a qualifier such as
'"1 kB = 1,024 Bytes"' but this qualifier was frequently omitted, particularly in marketing literature. This trend became habit and continued to be applied to the prefixes "mega," "giga," "tera," and even "peta."
Operating systems and their utilities, particularly visual operating systems such
Microsoft Windows, frequently report capacity using binary prefixes which results in a discrepancy between the drive manufacturer's stated capacity and the system's reported capacity. Obviously the difference 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, Microsoft's Windows 2000 reports drive capacity both in decimal to 12 or more significant digits and with binary prefixes to 3 significant digits. Thus a disk drive specified by a drive manufacturer as a
'30 GB' drive has its capacity reported by Windows 2000 both as
'30,065,098,568 bytes' and
'28.0 GB'. The drive manufacturer has used the SI definition of "giga," 10
9 and can be considered as an approximation of a gibibyte. Since utilities provided by the operating system probably define a gigabyte as 2
30, or 1073741824, bytes, the reported capacity of the drive will be closer to 28.0 GB, a difference of approximately 7%. For this very reason, many utilities that report capacity have begun to use the aforementioned IEC standard binary prefixes since their definitions are unambiguous.
Many people mistakenly attribute the discrepancy in reported and specified capacities to reserved space used for file system and partition accounting information. However, for large filesystems, this data rarely occupies more than a few MiB, and therefore cannot possibly account for the apparent "loss" of tens of GBs.
The capacity of a hard drive can actually manually be calculated. Given the number of cylinders, sectors, and heads, the hard drive's capacity is Cylinders × Heads × Sectors × 512 bytes per sector.
Integrity
The hard disk's spindle system relies on air pressure inside the drive to support the heads at their proper
flying height while the disk is in motion. A hard disk drive requires a certain range of air pressures in order to operate properly. The connection to the external environment and pressure occur through a small hole in the enclosure , usually with a carbon filter on the inside . If the air pressure is too low, there will not be enough lift for 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 pressurized cabin. Modern drives include temperature sensors and adjust their operation to the operating environment.
Very high humidity for extended periods can cause accelerated wear of the drive's heads and disks by corrosion. If the drive uses "Contact Start/Stop" technology to park its heads on the disk when not operating, increased humidity can also lead to increased stiction . This can cause physical damage to the disk and spindle motor and can also lead to
head crash. Breather holes can be seen 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 recirculation filter to remove any leftover contaminants from manufacture, any particles or chemicals that may have somehow entered the drive, and any particles or outgassing generated internally in normal operation.
Due to the extremely close spacing between the heads and the 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 heads in particular, a minor head crash from contamination will still result in the head temporarily overheating, due to friction with the disk surface, and can render the data unreadable for a short period until the head temperature stabilizes . Head crashes can be caused by electronic failure, a sudden power failure, physical shock, wear and tear, corrosion, or poorly manufactured disks and heads. In most desktop and server drives, when powering down, the heads are moved to a
landing zone, an area of the disk usually near its inner diameter , where no data is stored. This area is called the CSS zone. However, especially in old models, sudden power interruptions or a power supply failure can sometimes result in the drive shutting down with the heads in the data zone, which increases the risk of data loss. In fact, it used to be procedure to "park" the hard drive before shutting down your computer. Newer drives are designed such that either a spring and then rotational inertia in the platters is used to safely park the heads in the case of unexpected power loss.
The hard disk'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. 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 self-monitoring, analysis, and reporting technology , by which impending failures can be predicted, allowing the user to be alerted to prevent data loss.
Landing zones
Around 1995 IBM pioneered a technology where the landing zone is made by a precision laser process producing an array of smooth nanometer-scale "bumps" in the ID landing zone, thus vastly improving stiction and wear performance. This technology is still widely in use today . A few years after LZT, initially for mobile applications , and later also for the other HDD types, IBM introduced "head unloading" technology, where the heads are lifted off the platters onto plastic "ramps" near the outer disk edge, thus eliminating the risk of stiction altogether and greatly improving non-operating shock performance. All HDD manufacturers use these two technologies to this day. Both have a list of advantages and drawbacks in terms of loss of storage space, relative difficulty of mechanical tolerance control, cost of implementation, etc.
IBM created a technology for their
Thinkpad line of laptop computers called the Active Protection System. When a sudden, sharp movement is detected by the built-in motion sensor in the Thinkpad, internal hard disk heads automatically unload themselves into the parking zone to reduce the risk of any potential data loss or scratches made. Apple later also utilized this technology in their Powerbook, iBook, MacBook Pro, and MacBook line, known as the Sudden Motion Sensor.
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 or wear. In CSS drives the sliders carrying the head sensors 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 . 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.
Access and interfaces
Hard disks are generally accessed over one of a number of bus types, including
ATA ,
Serial ATA ,
SCSI, SAS,
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 encoding , and transferred data at a rate of 5 megabits per second. Later on, controllers using
2,7 RLL encoding increased the transfer rate 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 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.
Enhanced Small Disk Interface also supported multiple data rates , 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 . ESDI drives typically also had jumpers to set the number of sectors per track and sector size.
SCSI originally had just one speed, 5 MHz , 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 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 . 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 .
Serial ATA does away with master/slave setups entirely, placing each drive on its own channel instead.
FireWire/IEEE 1394 and USB 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.
Drive families used in personal computers
Notable drive families include:
- MFM drives required that the controller electronics be compatible with the drive electronics.
- RLL drives were named after the modulation technique that made them an improvement on MFM. They required large cables between the controller in the PC and the hard drive, the drive did not have a controller, only a modulator/demodulator.
- ESDI was an interface developed by Maxtor to allow faster communication between the PC and the disk than MFM or RLL.
The name comes from the way early families had the hard drive controller external to the drive. Moving the hard disk controller from the interface card to the drive helped to standardize interfaces, reducing cost and complexity.
The data cable was originally 40 conductor, but UDMA modes from the later drives requires using an 80 conductor cable
The interface changed from 40 pins to 39 pin. The missing pin acts as a key to prevent incorrect insertion of the connector, a common cause of drive and controller damage.
- SCSI was an early competitor with ESDI, originally named SASI for Shugart Associates. SCSI drives were standard on servers, workstations, and Apple Macintosh computers through the mid-90s, by which time most models had been transitioned to IDE family drives. Only in 2005 did the capacity of SCSI drives fall behind IDE drive technology, though the highest-performance drives 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, and then Fibre Channel interface, and then more specifically the Fibre Channel Arbitrated Loop , connected SCSI hard drives using fibre optics. FC-AL is the cornerstone of storage area networks, although other protocols like iSCSI and ATA over Ethernet have been developed as well.
- SATA . The SATA data cable has only one data pair for the differential transmission of data to the device, and one pair for receiving from the device. That requires that data be transmitted serially. The same differential transmission system is used in RS485, Appletalk,USB, Firewire,and differential SCSI. In 2005/2006 parlance, the 40 pin IDE/ATA is called "PATA" or parallel ATA, which means that there are 16 bits of data transferred in parallel at a time on the data cable.
- 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 for interacting with SAS
- EIDE was an unofficial update to the original IDE standard, with the key improvement being the use of DMA to transfer data between the drive and the computer, an improvement later adopted by the official ATA standards. DMA is used to transfer data without the CPU or program being responsible to transfer every word. That leaves the CPU/program/operating system to do other tasks while the data transfer occurs.
| Acronym | Meaning | Description |
|---|
| SASI | Shugart Associates System Interface | Predecessor to SCSI |
| SCSI | Small Computer System Interface | Bus oriented that handles concurrent operations. |
| ST-412 | | Seagate interface |
| ST-506 | | Seagate interface |
| ESDI | Enhanced Small Disk Interface | Faster and more integrated than ST-412/506, but still backwards compatible |
| ATA | Advanced Technology Attachment | Successor to ST-412/506/ESDI by integrating the drive controller completely onto the device. Incapable of concurrent operations. |
Manufacturers
Most of the world's hard disks are now manufactured by just a handful of large firms:
Seagate,
Maxtor ,
Western Digital,
Samsung, and
Hitachi which owns the former drive manufacturing division of
IBM.
Fujitsu continues to make mobile- and server-class drives but exited the desktop-class 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 Inc. or CMI; after an incident with faulty 20 MB AT drives in 1985., 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 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, after attempting to manufacture hard drives in
India using a second hand factory. 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.
Timeline of mergers and acquisitions
- 1988: Tandon sold its disk manufacturing division to Western Digital , which was then a well-known controller designer.
- 1989: Seagate Technology bought Control Data's high-end disk business, as part of CDC's exit from hardware manufacturing.
- 1990: Maxtor buys MiniScribe out of bankruptcy, making it the core of its low-end drive division.
- 1994: Quantum
...
bought
DEC's storage division, giving it a high-end drive range to go with its more consumer-oriented
ProDrive range, as well as the
DLT tape drive range.
- 1995, Conner Peripherals, which was founded by one of Seagate Technology's cofounders along with personnel from MiniScribe, announces a merger with Seagate, which was completed in early 1996.
- 1996: JTS merges with Atari, allowing JTS to bring its drive range into production. Atari was sold to Hasbro in 1998, while JTS itself went bankrupt in 1999.
- 2000: Quantum sells its disk division to Maxtor to concentrate on tape drives and backup equipment.
- 2003: Following the controversy over 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 .
- 2005: Seagate and Maxtor announce their intent to merge. US DoJ approval was given for Maxtor to be acquired by Seagate for US$1.9 billion, and the merger closed in mid-2006.
See also
References
External links
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-
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- Technical Whitepaper :
- - Less's Law and future implications of massive cheap hard disk storage
- Despatches from the magneto / flash wars
- - from the perspective of a dead hard drive recovery company
