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Shahrad Rezaei Tehrani |
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Hard Disk Last Updated - 1Dec01 When the power to a PC is switched off, the contents of memory are lost. It is the PC's hard disk that serves as a non-volatile, bulk storage medium and as the repository for a user's documents, files and applications. It's astonishing to recall that back in 1954, when IBM first invented the hard disk, capacity was a mere 5MB stored across fifty 24in platters. 25 years later Seagate Technology introduced the first hard disk drive for personal computers, boasting a capacity of up to 40MB and data transfer rate of 625 KBps using the MFM encoding method. A later version of the company's ST506 interface increased both capacity and speed and switched to the RLL encoding method. It's equally hard to believe that as recently as the late 1980s 100MB of hard disk space was considered generous. Today, this would be totally inadequate, hardly enough to install the operating system alone, let alone a huge application such as Microsoft Office. The PC's upgradeability has led software companies to believe that it doesn't matter how large their applications are. As a result, the average size of the hard disk rose from 100MB to 1.2GB in just a few years and by the start of the new millennium a typical desktop hard drive stored 18GB across three 3.5in platters. Thankfully, as capacity has gone up prices have come down, improved areal density levels being the dominant reason for the reduction in price per megabyte. It's not just the size of hard disks that has increased. The performance of fixed disk media has also evolved considerably. When the Intel Triton chipset arrived, EIDE PIO mode 4 was born and hard disk performance soared to new heights, allowing users to experience high-performance and high-capacity data storage without having to pay a premium for a SCSI-based system. Construction Platters vary in size and hard disk drives come in two form factors, 5.25in or 3.5in. The trend is towards glass technology since this has the better heat resistance properties and allows platters to be made thinner than aluminium ones. The inside of a hard disk drive must be kept as dust-free as the factory where it was built. To eliminate internal contamination, air pressure is equalised via special filters and the platters are hermetically sealed in a case with the interior kept in a partial vacuum. This sealed chamber is often referred to as the head disk assembly (HDA). Typically two or three or more platters are stacked on top of each other with a common spindle that turns the whole assembly at several thousand revolutions per minute. There's a gap between the platters, making room for magnetic read/write head, mounted on the end of an actuator arm. This is so close to the platters that it's only the rush of air pulled round by the rotation of the platters that keeps the head away from the surface of the disk - it flies a fraction of a millimetre above the disk. On early hard disk drives this distance was around 0.2mm. In modern-day drives this has been reduced to 0.07mm or less. A small particle of dirt could cause a head to "crash", touching the disk and scraping off the magnetic coating. On IDE and SCSI drives the disk controller is part of the drive itself.
The heads are designed to touch the platters when the disk stops spinning - that is, when the drive is powered off. During the spin-down period, the airflow diminishes until it stops completely, when the head lands gently on the platter surface - to a dedicated spot called the landing zone (LZ). The LZ is dedicated to providing a parking spot for the read/write heads, and never contains data.
Allocating and tracking individual data sectors on a large drive would require a huge amount of overhead, causing file handling efficiency to plummet. To improve performance, data sectors are allocated in groups called clusters. Operation When it comes to accessing data already stored, the disk spins round very fast so that any part of its circumference can be quickly identified. The drive translates a read request from the computer into reality. There was a time when the cylinder/head/sector location that the computer worked out really was the data's location, but today's drives are more complicated than the BIOS can handle, and they translate BIOS requests by using their own mapping. Allocating and tracking individual data sectors on a large drive would require a huge amount of overhead, causing file handling efficiency to plummet. Therefore, to improve performance, data sectors are allocated in groups called clusters. The number of sectors in a cluster depends on the cluster size, which in turn depends on the partition size. The table below indicates the possibilities supported by the Windows 98 FAT32 file system:
When the computer wants to read data, the operating system works out where the data is on the disk. To do this it first reads the FAT (file allocation table) at the beginning of the partition. This tells the operating system in which sector on which track to find the data. With this information, the head can then read the requested data. The disk controller controls the drive's servo-motors and translates the fluctuating voltages from the head into digital data for the CPU. More often than not, the next set of data to be read is sequentially located on the disk. For this reason, hard drives contain between 256KB and 8MB of cache buffer in which to store all the information in a sector or cylinder in case it's needed. This is very effective in speeding up both throughput and access times. A hard drive also requires servo information, which provides a continuous update on the location of the heads. This can be stored on a separate platter, or it can be intermingled with the actual data on all the platters. A separate servo platter is more expensive, but it speeds up access times, since the data heads won't need to waste any time sending servo information. However, the servo and data platters can get out of alignment due to changes in temperature. To prevent this, the drive constantly rechecks itself in a process called thermal recalibration. During multimedia playback this can cause sudden pauses in data transfer, resulting in stuttered audio and dropped video frames. Where the servo information is stored on the data platters, thermal recalibration isn't required. For this reason the majority of drives embed the servo information with the data. By the new millennium the commonest means of connecting the drive to the PC were via the EIDE or SCSI interfaces. Performance Chief among them is the rotational speed of the platters. Disk RPM is a critical component of hard drive performance because it directly impacts the latency and the disk transfer rate. The faster the disk spins, the more data passes under the magnetic heads that read the data; the slower the RPM, the higher the mechanical latencies. Hard drives only spin at one constant speed, and for some time most fast EIDE hard disks span at 5,400rpm, while a fast SCSI drive was capable of 7,200rpm. In 1997 Seagate pushed spin speed to a staggering 10,033rpm with the launch of its UltraSCSI Cheetah drive and, in mid 1998, was also the first manufacturer to release an EIDE hard disk with a spin rate of 7,200rpm. In 1999 Hitachi broke the 10,000rpm barrier with the introduction of its Pegasus II SCSI drive. This spins at an amazing 12,000rpm - which translates into an average latency of 2.49ms. Hitachi has used an ingenious design to reduce the excessive heat produced by such a high spin rate. In a standard 3.5in hard disk, the physical disk platters have a 3in diameter. However, in the Pegasus II, the platter size has been reduced to 2.5in. The smaller platters cause less air friction and therefore reduce the amount of heat generated by the drive. In addition, the actual drive chassis is one big heat fin, which also helps dissipate the heat. The downside is that since the platters are smaller and have less data capacity, there are more of them and consequently the height of the drive is increased. Mechanical latencies, measured in milliseconds, include both seek time and rotational latency. "Seek Time" is measured defines the amount of time it takes a hard drive's read/write head to find the physical location of a piece of data on the disk. "Latency" is the average time for the sector being accessed to rotate into position under a head, after a completed seek. It is easily calculated from the spindle speed, being the time for half a rotation. A drive's "average access time" is the interval between the time a request for data is made by the system and the time the data is available from the drive. Access time includes the actual seek time, rotational latency, and command processing overhead time. The "disk transfer rate" (sometimes called media rate) is the speed at which data is transferred to and from the disk media (actual disk platter) and is a function of the recording frequency. It is generally described in megabytes per second (MBps). Modern hard disks have an increasing range of disk transfer rates from the inner diameter to the outer diameter of the disk. This is called a "zoned" recording technique. The key media recording parameters relating to density per platter are Tracks Per Inch (TPI) and Bits Per Inch (BPI). A track is a circular ring around the disk. TPI is the number of these tracks that can fit in a given area (inch). BPI defines how many bits can be written onto one inch of a track on a disk surface. The "host transfer rate" is the speed at which the host computer can transfer data across the IDE/EIDE or SCSI interface to the CPU. It is more generally referred to as the data transfer rate, or DTR, and can be the source of some confusion. Some vendors list the internal transfer rate, the rate at which the disk moves data from the head to its internal buffers. Others cite the burst data transfer rate, the maximum transfer rate the disk can attain under ideal circumstances and for a short duration. More important for the real world is the external data transfer rate, or how fast the hard disk actually transfers data to a PC's main memory. By late 2001 the fastest high-performance drives were capable of an average latency of less than 3ms, an average seek time of between 4 and 7ms and maximum data transfer rates in the region of 50 and 60MBps for EIDE and SCSI-based drives respectively. Note the degree to which these maximum DTRs are below the bandwidths of the current versions of the drive's interfaces - Ultra ATA/100 and UltraSCSI 160 - which are rated at 100MBps and 160MBps respectively. AV capability In the past this meant the need for specially designed, or at the very least suitably optimised, hard disk drives. However, with the progressive increase in the bandwidth of both the EIDE and SCSI interfaces over the years, the need for special AV rated drives has become less and less. Indeed, Micropolis - a company that specialise in AV drives - went out of business as long ago as 1997. The principal characteristic of an "AV drive" centred on the way that it handled thermal recalibration. As a hard drive operates, the temperature inside the drive rises causing the disk platters to expand (as most materials do when they heat up). In order to compensate for this phenomenon, hard drives would periodically recalibrate themselves to ensure the read and write heads remain perfectly aligned over the data tracks. Thermal recalibration (also known as "T-cal") is a method of re-aligning the read/write heads, and whilst it is happening, no data can be read from or written to the drive. In the past, non-AV drives entered a calibration cycle on a regular schedule regardless of what the computer and the drive happened to be doing. Drives rated as "AV" have employed a number of different techniques to address the problem. Many handled T-cal by rescheduling or postponing it until such time that the drive is not actively capturing data. Some additionally used particularly large cache buffers or caching schemes that were optimised specifically and exclusively for AV applications, incurring a significant performance loss in non-AV applications. By the start of the new millennium the universal adoption of embedded servo technology by hard disk manufacturers meant that thermal recalibration was no longer an issue. This effectively weaves head-positioning information amongst the data on discs, enabling drive heads to continuously monitor and adjust their position relative to the embedded reference points. The disruptive need for a drive to briefly pause data transfer to correctly position its heads during thermal recalibration routines is thereby completely eliminated. Capacity The read-write head technology that has sustained the hard disk drive industry through much of this period is based on the inductive voltage produced when a permanent magnet (the disk) moves past a wire-wrapped magnetic core (the head). Early recording heads were fabricated by wrapping wire around a laminated iron core analogous to the horseshoe-shaped electromagnets found in elementary school physics classes. Market acceptance of hard drives, coupled with increasing areal density requirements, fuelled a steady progression of inductive recording head advances. This progression culminated in advanced thin-film inductive (TFI) read-write heads capable of being fabricated in the necessary high volumes using semiconductor-style processors. Although it was conceived in the 1960s, it was not until the late 1970s that TFI technology was actually deployed in commercially available product. The TFI read/write head - which essentially consists of wired, wrapped magnetic cores which produce a voltage when moved past a magnetic hard disk platter - went on to become the industry standard until the mid-1990s. By this time it became impractical to increase areal density in the conventional way - by increasing the sensitivity of the head to magnetic flux changes by adding turns to the TFI head's coil - because this increased the head's inductance to levels that limited its ability to write data. The solution lay in the phenomenon discovered by Lord Kelvin in 1857 - that the resistance of ferromagnetic alloy changes as a function of an applied magnetic field - known as the anisotropic magnetoresistance (AMR) effect. MR technology In an MR head, the write element is a conventional TFI head, while the read element is composed of a thin stripe of magnetic material. The stripe's resistance changes in the presence of a magnetic field, producing a strong signal with low noise amplification and permitting significant increases in areal densities. As the disk passes by the read element, the disk drive circuitry senses and decodes changes in electrical resistance caused by the reversing magnetic polarities. The MR read element's greater sensitivity provides a higher signal output per unit of recording track width on the disk surface. Not only does magnetoresistive technology permit more data to be placed on disks, but it also uses fewer components than other head technologies to achieve a given capacity point. The MR read element is smaller than the TFI write element. In fact, the MR read element can be made smaller than the data track so that if the head were slightly off-track or misaligned, it would still remain over the track and able to read the written data on the track. Its small element size also precludes the MR read element from picking up interference from outside the data track, which accounts for the MR head's desirable high signal-to-noise ratio. Manufacturing MR heads can present difficulties. MR thin film elements are extremely sensitive to electrostatic discharge, which means special care and precautions must be taken when handling these heads. They are also sensitive to contamination and, because of the materials used in its design, subject to corrosion. MR heads also introduced a new challenge not present with TFI heads: thermal asperities, the instantaneous temperature rise that causes the data signal to spike and momentarily disrupt the recovery of data from the drive. Thermal asperities are transient electrical events, usually associated with a particle, and normally do not result in mechanical damage to the head. Although they can lead to misreading data in a large portion of a sector, new design features can detect these events. A thermal asperity detector determines when the read input signal exceeds a predetermined threshold, discounts that data value and signals the controller to re-read the sector. The various improvements offered by MR technology amount to an ability to read from areal densities about four times denser than TFI heads at higher flying heights. In practice this means that the technology is capable of supporting areal densities of at least 3 Gbits/in2. The technology's sensitivity limitations stem from the fact that the degree of change in resistance in an MR head's magnetic film is itself limited. It wasn't long before a logical progression from MR technology was under development, in the shape of Giant Magneto-Resistive (GMR) technology. GMR technology
Higher resistance occurs when the magnetic orientations of the sensor and pinned films oppose each other, because the movement of electrons of either spin direction is hampered by one or the other of these magnetic films. GMR sensors can operate at significantly higher areal densities than MR sensors, because their percent change in resistance is greater, making them more sensitive to magnetic fields from the disk. Current GMR hard disks have storage densities of 4.1Gbit/in2, although experimental GMR heads are already working at densities of 10Gbit/in2. These heads have a sensor thickness of 0.04 microns, and IBM claims that halving the sensor thickness to 0.02 microns - with new sensor designs - will allow possible densities of 40Gbit/in2. The advantage of higher recording densities is that disks can be reduced in physical size and power consumption, which in turn increases data transfer rates. With smaller disks for a given capacity, combined with lighter read/write heads, the spindle speed can be increased further and the mechanical delays caused by necessary head movement can be minimised.
"Pixie
dust"
Known technically as "antiferromagnetically-coupled (AFC) media", the new multilayer coating is expected to permit hard disk drives to store 100 billion bits (gigabits) of data per square inch of disk area by 2003 and represents the first fundamental change in disk drive design made to avoid the high-density data decay due to the superparamagnetic effect. Conventional disk media stores data on a single magnetic layer. AFC media's two magnetic layers are separated by an ultra-thin layer of ruthenium. This forces the adjacent layers to orient themselves magnetically in opposite directions. The opposing magnetic orientations make the entire multilayer structure appear much thinner than it actually is. Thus small, high-density bits can be written easily on AFC media, but they will retain their magnetisation due to the media's overall thickness. As a consequence, the technology is expected to allow densities of 100 gigabits per square inch and beyond
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This page was last updated on 03-Mar-2002.
YAHOO ID: Shrezai@yahoo.com |
Hard
disks are rigid platters, composed of a substrate and a magnetic medium. The
substrate - the platter's base material - must be non-magnetic and capable of
being machined to a smooth finish. It is made either of aluminium alloy or a
mixture of glass and ceramic. To allow data storage, both sides of each platter
are coated with a magnetic medium - formerly magnetic oxide, but now, almost
exclusively, a layer of metal called a thin-film medium. This stores data in
magnetic patterns, with each platter capable of storing a billion or so bits per
square inch (bpsi) of platter surface.
There's a 
When a
disk undergoes a low-level format, it is divided it into 
In MR material, e.g. nickel-iron alloys, conduction electrons move less freely
(more frequent collisions with atoms) when their direction of movement is
parallel to the magnetic orientation in the material. This is the "MR
effect", discovered in 1988. When electrons move less freely in a material,
the material's resistance is higher. GMR sensors exploit the quantum nature of
electrons, which have two spin directions-spin up and spin down. Conduction
electrons with spin direction parallel to a film's magnetic orientation move
freely, producing low electrical resistance. Conversely, the movement of
electrons of opposite spin direction is hampered by more frequent collisions
with atoms in the film, producing higher resistance. IBM has developed
structures, identified as spin valves, in which one magnetic film is pinned.
This means its magnetic orientation is fixed. The second magnetic film, or
sensor film, has a free, variable magnetic orientation. These films are very
thin and very close together, allowing electrons of either spin direction to
move back and forth between these films. Changes in the magnetic field
originating from the disk cause a rotation of the sensor film's magnetic
orientation, which in turn, increases or decreases resistance of the entire
structure. Low resistance occurs when the sensor and pinned films are
magnetically oriented in the same direction, since electrons with parallel spin
direction move freely in both films.
IBM
has been manufacturing merged read/write heads which implement GMR technology
since 1992. These comprise a thin film inductive write element and a read
element. The read element consists of an MR or GMR sensor between two magnetic
shields. The magnetic shields greatly reduce unwanted magnetic fields coming
from the disk; the MR or GMR sensor essentially "sees" only the
magnetic field from the recorded data bit to be read. In a merged head the
second magnetic shield also functions as one pole of the inductive write head.
The advantage of separate read and write elements is both elements can be
individually optimised. A merged head has additional advantages. This head is
less expensive to produce, because it requires fewer process steps; and, it
performs better in a drive, because the distance between the read and write
elements is less.
However, in
the summer of 2001, IBM announced a breakthrough in storage technology that
could prolong the life of the conventional hard disk drive for the foreseeable
future. The key to the breakthrough is a three-atom-thick layer of the element
ruthenium, a precious metal similar to platinum, sandwiched between two magnetic
layers. That only a few atoms could have such a dramatic impact has resulted in
scientists referring to the ruthenium layer informally as "pixie
dust".
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