Chapter – 2
Introduction of Hard Disks
Introduction
In today’s computer world hard disks have proved themselves as the most important part of a computer. Today hard disk is the main storage device that is most commonly used to store all type of data as well as one of the most interesting components of computer.
It will be very difficult for modern computer users to even consider what computer life would be without hard disk drives , as most of us today store billions of bytes of information in our computers.
In the very earliest computers there was no storage at all. Each time you wanted to run a program you would have to enter the program manually. Even more than that, it made most of what we consider today to be computing impossible, since there was no easy way to have a computer work with the same data over and over again. It was quickly realized that some sort of permanent storage was necessary if computers were to become truly useful tools.
The first storage medium used on computers was actually paper. Programs and data were recorded using holes punched into paper tape or punch cards. A special reader used a beam of light to scan the cards or tape. Where a hole was found it read a "1", and where the paper blocked the sensor, a "0" or vice-versa.
Though it was a great improvement over nothing but these cards were still very inconvenient to use. You basically had to write the entire program from scratch on paper, and get it working in your mind before you started trying to put it onto cards, because if you made a mistake you had to re-punch many of the cards. It was very hard to visualize what you were working with.
The next big advance over paper was the creation of magnetic tape. Recording information in a similar to way to how audio is recorded on a tape, these magnetic tapes were much more flexible, durable and faster than paper tape or punch cards.
Of course, tape is still used today on modern computers, but as a form of offline or secondary storage. Before hard disks, they were the primary storage for some computers. Their primary disadvantage is that they must be read linearly; it can take minutes to move from one end of the tape to the other, making random access impractical.
Well coming back to our topic. IBM introduced the very first hard disk that would be feasible for commercial development. It was not like disk drives that are used now days. They used rotating cylindrical drums, upon which the magnetic patterns of data were stored. The drums were large and hard to work with. The first true hard disks had the heads of the hard disk in contact with the surface of the disk. This was done to allow the low-sensitivity electronics of the day to be able to better read the magnetic fields on the surface of the disk but manufacturing techniques at that stage of time were not nearly as sophisticated as they are now, and it was not possible to get the disk's surface as smooth as it was necessary to allow the head to slide smoothly over the surface of the disk at high speed while it was in contact with it. Over time the heads would wear out, or wear out the magnetic coating on the surface of the disk.
As a critical discovery of new technology of IBM in which, contact with the surface of the disk was not necessary, took place it became the basis of the modern hard disks. The very first hard disk of this type was the IBM 305 RAMAC (Random Access Method of Accounting and Control) introduced in September 13, 1956. This hard disk could store five million characters that were approximately five megabytes with the data transfer rate of 8,800 bytes per second.
In 1962, IBM introduced the model 1301 Advanced Disk File. The key advance of this disk drive was the creation of heads that floated, or flew, above the surface of the disk on an air bearing with reducing the distance from the heads to the surface of the disks from 800 to 250 micro inches.
In 1973, IBM introduced the model 3340 disk drive, which is commonly considered to be the father of the modern hard disk which had two separate spindles, one permanent and the other removable, each with a capacity of 30 MB. IBM's model 3370 introduced in 1979 was the first disk with thin film heads. In the same year IBM introduced model 3310 which is the first disk drive with 8" platters, greatly reduced in size from the 14" that had been the standard for over a decade.
The first hard disk drive designed in the 5.25" form factor used in the first PCs was the Seagate ST-506. It featured four heads and a 5 MB capacity. IBM bypassed the ST-506 and chose the ST-412--a 10 MB disk in the same form factor--for the IBM PC/XT, making it the first hard disk drive widely used in the PC and PC-compatible world.
In the year 1983, Rodime introduced RO352, the first disk drive to use the 3.5" form factor, which became one of the most important industry standards. In 1985 Quantum introduced the Hardcard, a 10.5 MB hard disk mounted on an ISA expansion card for PCs that were originally built without a hard disk.
In 1986 Conner Peripherals introduced the CP340. It was the first disk drive to use a voice coil actuator. In the year 1988 Conner Peripherals introduced the CP3022, which was the first 3.5" drive to use the reduced 1" height now called "low profile" and the standard for modern 3.5" drives. In the same year PrairieTek introduced a drive using 2.5" platters. In 1990 IBM introduced the model 681 (Redwing), an 857 MB drive. It was the first to use MR heads and PRML.
IBM's "Pacifica" mainframe drive introduced in 1991 is the first to replace oxide media with thin film media on the platter surface. In the same year Integral Peripherals' 1820 is the first hard disk with 1.8" platters, later used for PC-Card disk drives. In the year 1992 Hewlett Packard introduced C3013A which is the first 1.3" drive.
There are a number of developments that took place in the history of hard disks to give the current design, shape performance and capacities to the today’s disks. These are difficult to count in detail within this book.
Components of Hard Disk
A hard disk has following main components in it:
- Disk Platters and Media
- Read/Write Heads
- Head Sliders, Arms and Actuator
- Hard Disk Spindle Motor
- Connectors and Jumpers
- Logic Board
- Cache and Cache Circuitry
Components of Hard Disk
Disk Platters and Media
Every hard disk uses one or more (generally more than one) round, flat disks called platters, coated on both sides with a special media material designed to store information in the form of magnetic patterns. Each surface of each platter on the disk can hold billions of bits of data.
Platters are composed of two main substances, a substrate material that forms the bulk of the platter and gives it structure and rigidity, and a magnetic media coating which actually holds the magnetic impulses that represent the data.
The quality of the platters and particularly, their media coating is critical. The size of the platters in the hard disk is the primary determinant of its overall physical dimensions, also generally called the drive's form factor; most drives are produced in one of the various standard hard disk form factors.
Sometimes hard disks are referred to by a size specification. If someone is having a 3.5-inch hard disk it means it usually refers to the disk's form factor, and normally, the form factor is named based on the platter size. The earlier hard disks had a nominal size of 5.25" but now days the most common hard disk platter size is 3.5".
Laptop drives are usually smaller, due to the expected small size and less weight of it. The platters on these drives are usually 2.5" in diameter or less; 2.5" is the standard form factor, but drives with 1.8" and even 1.0" platters are becoming more common in mobile equipment.
Though drives extend the platters to as much of the width of the physical drive package as possible, to maximize the amount of storage they can pack into the drive yet the trend overall is towards smaller platters. There are the main reasons why companies are going to smaller platters even for desktop units:
The rigid and stiff platters are more resistant to shock and vibration, and are better-suited for being mated with higher-speed spindles and other high-performance hardware. Reducing the hard disk platter's diameter by a factor of two approximately quadruples its rigidity.
Reduced size of the platters reduces the distance that the head actuator must move the heads side-to-side to perform random seeks. This improves seek time and makes random reads and writes faster.
The latest hard disk spindles are increasing in speed performance reasons. Smaller platters are easier to spin and require less-powerful motors as well as faster to spin up to speed from a stopped position.
The smallest hard disk platter size available today is 1" in diameter. IBM's amazing Micro drive has a single platter and is designed to fit into digital cameras, personal organizers, and other small equipment. The tiny size of the platters enables the Micro drive to run off battery power, spin down and back up again in less than a second.
From an engineering point of view more platters also means more mass and therefore slower response to commands to start or stop the drive. It can be compensated for with a stronger spindle motor, but that leads to other tradeoffs.
In fact, the trend recently has been towards drives with fewer head arms and platters, not more. Areal density continues to increase, allowing the creation of large drives without using a lot of platters. This enables manufacturers to reduce platter count to improve seek time without creating drives too small for the marketplace.
The form factor of the hard disk also has a great influence on the number of platters in a drive. There are several factors that are related to the number of platters used in the disk. Drives with many platters are more difficult to engineer due to the increased mass of the spindle unit, the need to perfectly align all the drives, and the greater difficulty in keeping noise and vibration under control.
Even then, though hard disk engineers wanted to put lots of platters in a particular model, the standard “slimline” hard disk form factor is limited to 1 inch in height, which limits the number of platters that can be put in a single unit. Of course, engineers are constantly working to reduce the amount of clearance required between platters, so they can increase the number of platters in drives of a given height.
The magnetic patterns that comprise your data are recorded in a very thin media layer on the surfaces of the hard disk's platters; the bulk of the material of the platter is called the substrate and does nothing but support the media layer. To be suitable, a substrate material must be rigid, easy to work with, lightweight, stable, magnetically inert, inexpensive and readily available. The most commonly used material for making platters has traditionally been an aluminum alloy, which meets all of these criteria.
Due to the way the platters spin with the read/write heads floating just above them, the platters must be extremely smooth and flat therefore alternatives to aluminum, such as glass, glass composites, and magnesium alloys have been proposed. It now is looking increasingly likely that glass and composites made with glass will be the next standard for the platter substrate. Compared to aluminum platters, glass platters have several advantages:
- Better Quality:
- Improved Rigidity:
- Thinner Platters:
- Thermal Stability:
One disadvantage of glass compared to aluminum is fragility, particularly when made very thin.
The substrate material of which the platters are made forms the base upon which the actual recording media is deposited. The media layer is a very thin coating of magnetic material which is where the actual data is stored. It is typically only a few millionths of an inch in thickness.
Older hard disks used oxide media. Oxide media is inexpensive to use, but also has several important shortcomings. The first is that it is a soft material, and easily damaged from contact by a read/write head. The second is that it is only useful for relatively low-density storage. It worked fine for older hard disks with relatively low data density, but as manufacturers sought to pack more and more data into the same space, oxide was not up to the task: the oxide particles became too large for the small magnetic fields of newer designs.
Today's hard disks use thin film media. Thin film media consists of a very thin layer of magnetic material applied to the surface of the platters. Special manufacturing techniques are employed to deposit the media material on the platters.
Compared to oxide media, thin film media is much more uniform and smooth. It also has greatly superior magnetic properties, allowing it to hold much more data in the same amount of space. After applying the magnetic media, the surface of each platter is usually covered with a thin, protective, layer made of carbon. On top of this is added a super-thin lubricating layer. These materials are used to protect the disk from damage caused by accidental contact from the heads or other foreign matter that might get into the drive.
Read/Write Heads
The heads are the read/write interface to the magnetic physical media on which the data is stored in a hard disk. The heads do the work of converting bits to magnetic pulses and storing them on the platters, and then reversing the process when the data needs to be read back. Heads are one of the more expensive parts of the hard disk to enable areal densities and disk spin speeds to increase.
However GMR heads is most popular in the today’s hard disk, there have several technologies been proposed at several times for read/write heads:
- Ferrite Heads
- Metal-In-Gap (MIG) Heads
- Thin Film (TF) Heads
- Anisotropic Magneto resistive (AMR/MR) Heads
- Giant Magneto resistive (GMR) Heads
- Colossal Magneto resistive (CMR) Heads
Read/write heads are an extremely critical component in determining the overall performance of the hard disk, since they play such an important role in the storage and retrieval of data. New head technologies are often the triggering point to increasing the speed and size of modern hard disks therefore read/write heads are the most sophisticated part of the hard disk, which is itself a technological marvel.
Each bit of data to be stored is recorded onto the hard disk using a special encoding method that translates zeros and ones into patterns of magnetic flux reversals. Each hard disk platter has two surfaces used to store the data generally and there is normally one head for each surface used on the drive. Since most hard disks have one to four platters, most hard disks have between two and eight heads. Some larger drives can have 20 heads or more. Only one head can read from or write to the hard disk at a given time. Special circuitry is used to control which head is active at any given time.
The head floats over the surface of the disk and do all of their work without ever physically touching the platters. The amount of space between the heads and the platters is called the floating height or flying height or head gap. The read/write head assemblies are spring-loaded using the spring steel of the head arms which causes the sliders to press against the platters when the disk is stationary.
This is done to ensure that the heads don't drift away from the platters therefore maintaining an exact floating height is essential for correct operation. When the disk spins up to operating speed, the high speed causes air to flow under the sliders and lift them off the surface of the disk. The distance from the platters to the heads is a specific design parameter that is tightly controlled by the manufacturers.
A modern hard disk has a floating height of 0.5 micro inches and even human hair has a thickness of over 2,000 micro inches that ’s why keeping dirt out of the hard disk is so important. It is actually quite amazing how close to the surface of the disks the heads fly without touching. Dust Particle, Finger Print even a smoke particle is a big problem for the head of a hard disk.
Hard disk drive read write structure
When the areal density of a drive is increased to improve capacity and performance, the magnetic fields are made smaller and weaker. To compensate, either the heads must be made more sensitive, or the floating height must be decreased.
Each time the floating height is decreased, the mechanical aspects of the disk must be adjusted to make sure that the platters are flatter, the alignment of the platter assembly and the read/write heads is perfect, and there is no dust or dirt on the surface of the platters. Vibration and shock also become more of a concern, and must be compensated for.
This is one reason why manufacturers are turning to smaller platters, as well as the use of glass platter substrates. Newer heads such as GMR are preferred because they allow a higher flying height than older, less sensitive heads, all else being equal.
Head Crash
Since the read/write heads of a hard disk are floating on a microscopic layer of air above the disk platters themselves, it is possible that the heads can make contact with the media on the hard disk under certain circumstances. Normally, the heads only contact the surface when the drive is either starting up or stopping.
A modern hard disk is turning over 100 times a second. If the heads contact the surface of the disk while it is at operational speed, the result can be loss of data, damage to the heads, damage to the surface of the disk, or all three. This is usually called a head crash, two of the most frightening words to any computer user. The most common causes of head crashes are contamination getting stuck in the thin gap between the head and the disk, and shock applied to the hard disk while it is in operation.
Head Parking
When the platters are not spinning, the heads rest on the surface of the disk. When the platters spin up, the heads rub along the surface of the platters until sufficient speed is gained for them to lift off and float on their cushion of air. When the drive is spun down, the process is repeated in reverse. In both of the cases, for a period of time the heads make contact with the surface of the disk while in motion.
While the platters and heads are designed with the knowledge in mind that this contact will occur, it still makes sense to avoid having this happen over an area of disk where there is data.
For this reason, most disks set aside a special track that is designated to be where the heads will be placed for takeoffs and landings. This area is called the landing zone, and no data is placed there. The process of moving the heads to this designated area is called head parking.
Almost all new operating systems have inbuilt facility to park the head automatically when it is necessary. Most early hard drives that used stepper motors did not automatically park the heads of the drive therefore as a safety precaution many small utilities were written that the user would run before shutting down the PC of those days. The utility would instruct the disk to move the heads to the landing zone, and then the PC could be shut off safely.
A parameter in the BIOS setup for the hard disk tells the system which track was the landing zone for the particular model of hard disk. Usually, it was the next consecutive-numbered track above the largest-numbered one actually used for data. Modern voice-coil actuated hard disk drives are all auto-parking. It is not necessary now to manually park the heads of modern hard disks.
Head Sliders, Arms and Actuator
When the hard disk platters are accessed for read and write operations using the read/write heads mounted on the top and bottom surfaces of each platter it is obviously, the read/write heads do not just float in space. They must be held in an exact position relative to the surfaces they are reading and also, they must be moved from track to track to allow access to the entire surface of the disk.
The heads are mounted onto a structure that facilitates this process which is often called the head assembly or actuator assembly or the head-actuator assembly. It is comprised of several different parts. The heads themselves are mounted on head sliders. The sliders are suspended over the surface of the disk at the ends of the head arms. The head arms are all mechanically fused into a single structure that is moved around the surface of the disk by the actuator.
Head Sliders
Each hard disk head is mounted to a special device called a head slider or just slider for short. The function of the slider is to physically support the head and hold it in the correct position relative to the platter as the head floats over its surface. Hard disk read/write heads are too small to be used without attaching them to a larger unit.
Sliders are given a special shape to allow them to ride precisely over the platter. As hard disk read/write heads have been shrinking in size, so have the sliders that carry them. The main advantage of using small sliders is that it reduces the weight that must be yanked around the surface of the platters, improving both positioning speed and accuracy. Smaller sliders also have less surface area to potentially contact the surface of the disk. Each slider is mounted onto a head arm to allow it to be moved over the surface of the platter to which it is mated.
Head Arms
The head arms are thin pieces of metal, usually triangular in shape onto which the head sliders carrying the read/write heads are mounted. There is one arm per read/write head, and all of them are lined up and mounted to the head actuator to form a single unit.
This means that when the actuator moves, all of the heads move together in a synchronized fashion. The arms themselves are made of a lightweight, thin material, to allow them to be moved rapidly from the inner to outer parts of the drive. Newer designs have replaced solid arms with structural shapes in order to reduce weight and improve performance.
Newer drives achieve faster seek times in part by using faster and smarter actuators and lighter, more rigid head arms, allowing the time to switch between tracks to be reduced. A recent trend in the hard disk industry has been the reduction in the number of platters in various drive families. Even some flagship drives in various families now only have three or even two platters, where four or five was commonplace a year or so ago.
One reason for this trend is that having a large number of head arms makes it difficult to make the drive with high enough precision to permit very fast positioning on random seeks. This is due to increased weight in the actuator assembly from the extra arms, and also problems aligning all the heads.
Head Actuator
The actuator is a very important part of the hard disk, because changing from track to track is the only operation on the hard disk that requires active movement. Changing heads is an electronic function, and changing sectors involves waiting for the right sector number to spin around and come under the head. Changing tracks means the heads must be shifted, and so making sure this movement can be done quickly and accurately is of paramount importance.
The actuator is the device used to position the head arms to different tracks on the surface of the platter to different cylinders, since all head arms are moved as a synchronous unit, so each arm moves to the same track number of its respective surface. Head actuators come in two general varieties:
- Stepper Motors
- Voice Coils
The main difference between the two designs is that the stepper motor is an absolute positioning system, while the voice coil is a relative positioning system.
All modern hard disks use voice coil actuators. The voice coil actuator is not only far more adaptable and insensitive to thermal issues. It is much faster and more reliable than a stepper motor. The positioning of actuator is dynamic and is based on feedback from examining the actual position of the tracks. This closed-loop feedback system is also sometimes called a servo motor or servo positioning system and is commonly used in thousands of different applications where precise positioning is important.
Spindle Motor
The spindle motor or the spindle shaft is responsible for turning the hard disk platters, allowing the hard drive to operate. A spindle motor must provide stable, reliable and consistent turning power for thousands of hours of often continuous use, to allow the hard disk to function properly because many drive failures are actually failures with the spindle motor, not the data storage systems.
The spindle motor of a hard disk must have the following quality to live long and to keep your data, secure for a long time:
- It must be of high quality, so it can run for thousands of hours, and tolerate thousands of start and stop cycles, without failing.
- It must be run smoothly and with a minimum of vibration, due to the tight tolerances of the platters and heads inside the drive.
- It must not generate excessive amounts of heat or noise.
- It should not draw too much power.
- It must have its speed managed so that it turns at the proper speed.
To meet these demands, all PC hard disks use servo-controlled DC spindle motors. Hard disk spindle motors are configured for direct connection. There are no belts or gears that are used to connect them to the hard disk platter spindle. The spindle onto which the platters are mounted is attached directly to the shaft of the motor.
The platters are machined with a hole of the exact size of the spindle, and are placed onto the spindle with separator rings between them to maintain the correct distance and provide room for the head arms. The amount of work that the spindle motor has to do is dependent on following factors:
The size and number of platters: Larger platters and more platters in a drive mean more mass for the motor to turn, so more powerful motors are required. The same is true of higher-speed drives.
o Power management: Today, users increasingly want hard disks that will spin up from a stopped position to operating speed quickly, which also requires faster or more powerful motors.
As in newer hard disks the spindle speed is supposed to be an important issue it has also become an important point in the hard disks to control the amount of noise, heat and vibration generated by the hard disks due to high spindle speed.
Some newer drives, especially 7200 and 10,000 RPM models can make a lot of noise when they are running. If possible, it is a good idea to check out a hard disk in operation before you buy it, to assess its noise level and see if it bothers you; this varies greatly from individual to individual. The noise produced also varies to some extent depending on the individual drive even in the same family. Heat created by the spindle motor can eventually cause damage to the hard disk, which is why newer drives newer hard disks are giving more attention to their cooling.
Connectors and Jumpers
There are several different connectors and jumpers in a hard disk which are used to configure the hard disk and connect it to the rest of the system. The number and types of connectors on the hard disk depend on the data interface it uses to connect to the system, the manufacturer of the drive, and any special features that the drive may possess.
Instructions for setting common jumpers are usually printed right on the drive. Hard disk drives use a standard, 4-pin male connector plug that takes one of the power connectors coming from the power supply. This leads 4-wire plastic connector provides +5 and +12 voltage to the hard disk.
There are two type of interfaces form which usually modern hard disk drives use one of them:
- IDE/ATA: It has a 40-pin rectangular connector.
- SCSI: A 50-pin, 68-pin, or 80-pin D-shaped connector. All these three pin number represent a different type of SCSI disk such as:
- A 50-pin connector means the device is narrow SCSI.
- 68 pins means wide SCSI.
- 80 pins mean wide SCSI using single connector attachment (SCA).
Connectors and Jumpers
The connectors on hard disk drives are generally in the form of a 2xN rectangular grid of pins (where N is 20, 25, 34 or 40 depending on the interface). Most of the current SCSI interface connectors are keyed to prevent incorrect insertion because they are D-shaped, this is not always the case for other interfaces.
For this reason, it is important to make sure that the cable is oriented the correct way before plugging it in. The cable has a red stripe to indicate wire 1 and the hard disk uses markers of one form or another to indicate the matching pin 1.
IDE/ATA hard disks are fairly standard in terms of jumpers. There are usually only a few jumper settings and they do not vary greatly from drive to drive. Here are the jumper’s settings you will normally find in a hard disk:
Drive Select: There may be two drives, master and slave on the same IDE channel. A jumper is normally used to tell each drive if it should function as a master or slave on the IDE channel.
For a single drive on a channel, most manufacturers instruct that the drive be jumpered as master, while some manufacturers notably Western Digital have a separate setting for a single drive as opposed to a master on a channel with a slave. The terms master and slave are misleading since the drives really have no operational relationship.
Slave Present: Some drives have an additional jumper that is used to tell a drive configured as master that there is also a slave drive on the ATA channel. This is only required for some older drives that don't support standard master/slave IDE channel signaling.
Cable Select: Some configurations use a special cable to determine which drive is master and which is slave, and when this system is used a cable select jumper is normally enabled.
Size Restriction Jumper: Some larger hard disk drives do not work properly in older computers that do not have a BIOS program or large hard disk support recognize them. To get around this, some drives have special jumpers that when set, will cause them to appear as a smaller size than they really are to the BIOS for compatibility.
For example, some 2.5 GB hard disks have a jumper that will cause them to appear as a 2.1 GB hard disk to a system that won't support anything over 2.1 GB. These are also sometimes called capacity limitation jumpers and vary from manufacturer to manufacturer.
Example of jumper setting of a Seagate Technology hard Disk model
SCSI hard disks have more sophisticated controllers than that of IDE/ATA hard disks therefore SCSI typically have many more jumpers that can be set to control their operation. They also tend to vary much more from manufacturer to manufacturer and from model to model in the number and types of jumpers they have.
Typically the following are the most common and important SCSI drives jumpers:
SCSI Device ID: Every device on a SCSI bus must be uniquely identified for addressing purposes. Narrows SCSI drives will have a set of three jumpers that can be used to assign the disk an ID number from 0 to 7. Wide SCSI drives will have four jumpers to enable ID numbers from 0 to 15. Some systems don't use jumpers to configure SCSI device IDs.
SCSI drives jumpers
Termination Activate: The devices on the ends of the SCSI bus must terminate the bus for it to function properly. If the hard disk is at the end of the bus, setting this jumper will cause it to terminate the bus for proper operation. Not all drives support termination.
Disable Auto Start: If present, this jumper will tell the drive not to automatically spin up when the power is applied, but instead wait for a start command over the SCSI bus. This is usually done to prevent excessive startup load on the power supply. Some manufacturers invert the sense of this jumper; they disable startup by default and provide an Enable Auto Start jumper.
Delay Auto Start: This jumper tells the drive to start automatically, but wait a predefined number of seconds from when power is applied. It is also used to offset motor startup load on systems with many drives.
Stagger Spin: When a system with many hard drives has this option set for each unit, the drives stagger their startup time by multiplying a user defined constant times their SCSI device ID. This ensures no two drives on the same SCSI channel will start up simultaneously.
Narrow or Wide: Some drives have a jumper to control whether they will function in narrow or wide mode.
Force SE: It allows Ultra2, Wide Ultra2, Ultra160, Ultra160+ or other LVD SCSI drives to be forced to use single-ended (SE) operation instead of LVD(low voltage differential).
Disable Parity: Turns off parity checking on the SCSI bus, for compatibility with host adapters that do not support the features.
This is not all of all. Many SCSI drives have some additional special features that are enabled through more jumpers. Some drives have replaced some of their jumpers with software commands sent over the SCSI interface.
Logic Board
The newer hard disks drives have been introduced with a lot of features and faster speed in it and development is still on progress. To control all these functions and provide the disk’s high performance features in advanced way in which they are expected to be, all modern hard disks are made with an intelligent circuit board integrated into the hard disk unit. This circuit board is called Hard Disk Logic Board. A logic board uses its following important components to provide a variety of functions and features to a hard disk:
- Control Circuitry
- Sense, Amplification and Conversion Circuits
- Interface Hardware
- Firmware
- Multiple Command Control and Reordering
Both of The two most common interfaces popular today for PC hard disks IDE (Integrated Drive Electronics) and SCSI (Small Computer Systems Interface) use integrated controllers. The more correct name for the IDE interface is AT Attachment or ATA (Advanced Technology Attachment). The modern hard disks have a very sophisticated logic board which contains more memory and faster internal processors than an entire PC of even the mid-1980s.
The logic board performs several important functions then before. Therefore the logic circuits needs to be more powerful, to handle changes like geometry translation, advanced reliability features, more complicated head technologies, faster interfaces, and higher bandwidth data streaming from the disk itself.
The internal logic board of a hard disk contains a microprocessor and internal memory, and other structures and circuits that control what happens inside the drive. Some of the most important functions of the control circuitry of the drive are as follows:
- Controlling the spindle motor, including making sure the spindle runs at the correct speed.
- Controlling the movement of actuator to various tracks.
- Managing all read and write operations.
- Implementing power management features.
- Handling geometry translation.
- Managing the internal cache and optimization features such as pre-fetch.
- Coordinating and integrating the other functions mentioned in this section, such as the flow of information over the hard disk interface, optimizing multiple requests, converting data to and from the form the read/write heads require it, etc.
- Implementing all advanced performance and reliability features.
The modern hard disks have internal microprocessors and most of them also have internal software that runs them. These routines run the control logic and make the drive work. In fact this is not really software in the conventional sense, because these instructions are embedded into read-only memory. This code is analogous to the system BIOS, low-level, hardware-based control routines, embedded in ROM. It is usually called firmware.
This is the reason why sometimes Firmware is called the middle link of hardware and software. In many drives the firmware can be updated under software control.
Cache and Cache Circuitry
The function of integrated cache (also often called a buffer) of a hard disk is to act as a buffer between a relatively fast device and a relatively slow one. For hard disks, the cache is used to hold the results of recent reads from the disk, and also to pre-fetch information that is likely to be requested in the near future, for example, the sector or sectors immediately after the one just requested.
Thus the purpose of this cache is not dissimilar to other caches used in the PC, even though it is not normally thought of as part of the regular PC cache hierarchy. You should always keep it in mind that when someone speaks generically about a disk cache, they are usually not referring to this small memory area inside the hard disk, but rather to a cache of system memory set aside to buffer accesses to the disk system.
The use of cache improves performance of any hard disk, by reducing the number of physical accesses to the disk on repeated reads and allowing data to stream from the disk uninterrupted when the bus is busy. Most modern hard disks have between 512 KB and 2 MB of internal cache memory even some high-performance SCSI drives have as much as 16 MB too.
The cache of a hard disk is important due to the sheer difference in the speeds of the hard disk and the hard disk interface. Finding a piece of data on the hard disk involves random positioning and incurs a penalty of milliseconds as the hard disk actuator is moved and the disk rotates around on the spindle. That is why hard disks have internal buffers.
The basic principle behind the operation of a simple cache is straightforward. Reading data from the hard disk is generally done in blocks of various sizes not just one 512-byte sector at a time. The cache is broken into segments or pieces each of which can contain one block of data.
When a request is made for data from the hard disk, the cache circuitry is first queried to see if the data is present in any of the segments of the cache. If it is present, it is supplied to the logic board without access to the hard disk platters being necessary. If the data is not in the cache, it is read from the hard disk, supplied to the controller and then placed into the cache in the event that it gets asked for again.
Since the cache is limited in size, there are only so many pieces of data that can be held before the segments must be recycled. Typically the oldest piece of data is replaced with the newest one. This is called circular, first-in, first-out (FIFO) or wrap-around caching.
In an effort to improve performance, most hard disk manufacturers today have implemented enhancements to their cache management circuitry, particularly on high-end SCSI drives:
Adaptive Segmentation: Conventional caches are chopped into a number of equal sized segments. Since requests can be made for data blocks of different sizes, this can lead to some of the storage of the cache in some segments being left over and hence wasted. Many newer drives dynamically resize the segments based on how much space is required for each access, to ensure greater utilization. It can also change the number of segments. This is more complex to handle than fixed-size segments, and it can result in waste itself if the space is not managed properly.
Pre-Fetch: The cache logic of a drive, based on analyzing access and usage patterns of the drive, attempts to load into part of the cache data that has not been requested yet but that it anticipates will be requested soon. Usually, this means loading additional data beyond that which was just read from the disk, since it is statistically more likely to be requested next. When done correctly, this will improve performance to some degree.
User Control: High-end drives have implemented a set of commands that allows the user detailed control of the drive cache's operation. This includes letting the user enable or disable caching, set the size of segments, turn on or off adaptive segmentation and pre-fetch etc.
Though internal buffer is obviously improving performance yet it also has the limitations. It helps very little if you are doing a lot of random accesses to data in different parts of the disk, because if the disk has not loaded a piece of data recently in the past, it will not be in the cache.
The buffer is also of little help if you are reading a large amount of data from the disk because normally it will be very small if you are copying a 50 MB file. For example, on a typical disk with a 512 Bytes buffer a very small part of the file could be in the buffer and the rest must be read from the disk itself.
Due to these limitations, the cache does not have as much of an impact on overall system performance as you might think. How much it helps depends on its size to some extent, but at least as much on the intelligence of its circuitry; just like the hard disk's logic overall. And just like the logic overall, it's hard to determine in many cases exactly what the cache logic on a given drive is like. However the size of the cache of the disk is important to its overall impact in improving the performance of the system.
Caching reads from the hard disk and caching writes to the hard disk are similar in some ways, but very different in others. They are the same in their overall objective that is to decouple the fast computer from the slow mechanics of the hard disk. The key difference is that a write involves a change to the hard disk while a read does not.
With no write caching, every write to the hard disk involves a performance hit while the system waits for the hard disk to access the correct location on the hard disk and write the data. This takes at least 10 milliseconds on most drives, which is a long time in the computer world and really slows down performance as the system waits for the hard disk. This mode of operation is called write-through caching.
When write caching is enabled and the system sends a write to the hard disk, the logic circuit records the write in its much faster cache and then immediately sends back an acknowledgement to the operating system for completion of process. The rest of the system can then proceed on its way without having to sit around waiting for the actuator to position and the disk to spin, and so on. This is called write-back caching, because the data is stored in the cache and only written back to the platters later on. Write-back functionality of course improves performance.
Since cache memory is volatile, if the power goes out, its contents are lost. If there were any pending writes in the cache that were not written to the disk yet, they are gone forever and the rest of the system has no way to know this because when it is told by the hard disk as the completion. Therefore not only is some data lost but also the system does not even know which data, or even that it happened. The end result can be file consistency problems, operating system corruption, and so on. Due to this risk, in some situations write caching is not used at all.
This is especially true for applications where high data integrity is critical. Due to the improvement in performance that write caching offers, however, it is increasingly being used despite the risk, and the risk is being mitigated through the use of additional technology.
The most common technique is simply ensuring that the power does not go off. For added peace of mind, better drives that employ write caching have a write flush feature that tells the drive to immediately write to disk any pending writes in its cache. This is a command that would commonly be sent before the UPS batteries ran out if a power interruption was detected by the system or just before the system was to be shut down for any other reason.
Low-level hard disk geometry
When we say low level hard disk geometry, we have not very much concerned to know the physical circuitry of the disk. Here we are going to discuss the terms with which we are going to deal now to understand the disk troubleshooting and data recovery programming above after.
The low level hard disk geometry is usually concerned with the following terms:
- Track
- Cylinder
- Sector
- Head or Side
Low-level hard disk geometry
The platters of a hard disk have two sides for recording the data. Every surface of the platter has invisible concentric circles on it, which are written on the surface as magnetic information during the formatting of the hard disk. These circles are called tracks. All information stored on a hard disk is recorded in tracks. The tracks are numbered, starting from 0, starting at the outside of the platter and increasing as you go in.
About the maximum number of tracks and cylinders, we shall discuss in detail in the next chapters. However for now we can get the knowledge of physical low level geometry of maximum numbers of Cylinders, Tracks, Heads (sides) and sectors.
| Name |
Start From |
End Limit |
Total Number |
| Cylinders |
0 |
1023 |
1024 |
| Heads |
0 |
255 |
256 |
| Sectors |
1 |
63 |
63 |
In the surface of the platter of a hard disk, the data is accessed by moving the heads from the inner to the outer part of the disk. This organization of data allows for easy access to any part of the disk, which is why disks are called random access storage devices.
Low-level hard disk geometry track
Each track can hold thousands of bytes of data and generally this storage is more than 5000 bytes. Therefore if we make a track the smallest unit of storage on the disk it will be the wastage of disk space, because by doing this the small files having the size less then 5000 bytes will waste the amount of space and generally it is quite possible to having a number of files in the disk which are much smaller than this size.
In this way making a track the smallest unit of storage will cause the small files to waste a large amount of space. Therefore, each track is broken into smaller units called sectors. The size of each sector is 512 bytes i.e. a sector can hold 512 bytes of information.
Thus the basic unit of data storage on a hard disk is the sector. The name sector refers to a pie-shaped angular section of a circle, bounded on two sides by radii and the third by the perimeter of the circle. You can see a logical figure representing sectors on a track given next.
Thus on a hard disk containing concentric circular tracks that shape would define a sector of each track of the platter surface that it intercepted. This is what is called a sector in the hard disk world is a small segment along the length of a track.
As according to the standard, each sector of a hard disk can store 512 bytes of user data. However, actually sector holds much more than 512 bytes of information. Additional bytes are needed for control structures and other information necessary to manage the drive, locate data and perform other support functions.
The exact details of how a sector is structured depend on the drive model and manufacturer. However, the contents of a sector usually include the following general elements:
ID Information: Conventionally, space is left in each sector to identify the sector's number and location. This is used for locating the sector on the disk and also includes status information about the sector in this area. For example, a bit is commonly used to indicate if the sector has been marked defective and remapped.
Synchronization Fields: These are used internally by the drive controller to guide the read process.
Data: The actual data in the sector.
Error Correcting Codes (ECC): Error correcting codes are used to ensure data integrity.
Gaps: Gaps are basically one or more spacers added as necessary to separate other areas of the sector, or provide time for the controller to process what it has read before reading more bits.
In addition to the sectors, each containing the items described, space on each track is also used for servo information. The amount of space taken up by each sector for overhead items is important, because the more bits used for this management, the fewer overall that can be used for data.
This is the reason that the hard disk manufacturers strive to reduce the amount of non-user data information that must be stored on the disk. The percentage of bits on each disk that are used for data, as opposed to other things as described before is known as format efficiency. Therefore the higher format efficiency is an expected feature of a drive.
In the latest approach to get the higher format efficiency now days, the ID fields are removed from the sector format and instead of labeling each sector within the sector header a format map is stored in memory and referenced when a sector must be located.
This map also contains information about the sectors which have been marked bad and relocated where the sectors are relative to the location of servo information and so on. This approach not only improves format efficiency allowing up to 10% more data to be stored on the surface of each platter but also improves performance. Since this critical positioning information is present in high-speed memory, it can be accessed much more quickly.
Hard disk hd hdd drives physical circuitry structure
Each platter of the hard disk uses two heads (except some special cases) to record and read data, one for the top of the platter and one for the bottom. The heads that access the platters are locked together on an assembly of head arms therefore all the heads move in and out together, so each head is always physically located at the same track number.
This is the reason that it is not possible to have one head at track 0 and another at track 1,000. Because of this arrangement, often the track location of the heads is not referred to as a track number but rather as a cylinder number.
A cylinder is basically the set of all tracks that all the heads are currently located at. If a disk has four platters, in general case it would have eight heads. Now suppose it has cylinders number 720.
Hard disk drives cylinder physical circuitry geometry structure
It would be made up of the eight set of tracks, one per platter surface with tracks number 720. The name comes from the fact that these tracks form a skeletal cylinder because they are equal-sized circles stacked one on top of the other in space, as shown in the figure given before.
The addressing of the factors of the disk is traditionally done by referring to cylinders, heads and sectors (CHS).
Formatting
Every storage media must be formatted before it can be used. The utilities used for formatting behave differently when acting on hard disks than when used for floppy disks. Formatting a hard disk involves the following steps:
Partitioning:
This process divides the disk into logical parts that assign different hard disk volumes or drive letters.
Hard drive partitioning is one of the most effective methods available for organizing hard drives. Partitions provide a more general level of organization than directories and files. They also offer greater security by separating data from operating systems and applications.
Partitions allow you to separate data files, which must be backed up regularly from program and operating system files. Partitioning becomes a necessity for the hard drive if you are willing to load, more than one operating systems in the disk otherwise in most of the cases it is possible that you may lose your data.
The first sector of any hard drive contains a partition table. This partition table only has room to describe four partitions. These are called primary partitions. One of these primary partitions can point to a chain of additional partitions. Each partition in this chain is called a logical partition. We shall discuss the partition basics with logical approach in details, in the next chapters.
High-Level Formatting:
It defines the logical structures on the partition and places at the start of the disk any necessary operating system files. This step is also an operating system-level command.
The FORMAT command of DOS that is FORMAT.COM, behaves differently when it is used on a hard disk than when it is used on a floppy disk. Floppy disks have simple, standard geometry and cannot be partitioned, so the FORMAT command is programmed to automatically both low-level and high-level format a floppy disk, if necessary but in case of hard disks, FORMAT will only do a high-level format.
When we have completed low–level formatting, we have a disk with tracks and sectors but nothing written on them. High-level formatting is the process of writing the file system structures on the disk that let the disk be used for storing programs and data.
If you are using DOS, the FORMAT command (that is FORMAT.COM), performs this work by writing such structures as the DOS boot record file allocation tables and root directories to the disk. High-level formatting is done after the hard disk has been partitioned.
Formatted and Unformatted Storage Capacity
The total storage of a hard disk depends on, if you are looking at the formatted or unformatted capacity. Some portion of the space on a hard disk is taken up by the formatting information that marks the start and end of sectors, ECC (Error Correction Codes) and other overhead information. For this reason, the difference can be quite significant.
Older drives that were typically low-level formatted by the user often had their size listed in terms of unformatted capacity.
For example : take the Seagate ST-412, the first drive used on the original IBM PC/XT in the early 1980s. The "12" in this model number refers to the drive's unformatted capacity of 12.76 MB. Formatted, it is actually a 10.65 MB drive.
Unformatted capacity of a hard disk is generally 19% (19 percent) higher than its formatted capacity. Since nobody can use a drive that is unformatted, the only thing that matters is the formatted capacity and therefore modern drives are always low-level formatted by the manufacturers.
The capacity of a hard disk can be expressed in the following four ways:
- Formatted capacity in millions of bytes
- Formatted capacity in megabytes
- Unformatted capacity in millions of bytes
- Unformatted capacity in megabytes
Now if I have a hard disk with C–H–S = 1024*63*63 (It means that the disk has number of cylinder = 1024, number of heads or sides = 63 number of sectors per track = 63) and every sector having 512 bytes. The formula that will calculate the size of the disk is as follows:
Total Size of the Disk (Bytes) = (Cylinders) X (Heads) X (Sectors) X
(Bytes per Sector) |
By this formula when we calculate the size of the given hard disk in bytes, it will be
= 1024 X 63 X 63 X 512
= 2080899072 Bytes
Now if I calculate the Size of my disk in millions of bytes, it will be approximately
= 2080.899072
~ 2081 millions of byte
Traditionally size in millions of byte is represented by M. Therefore the size of my disk in millions of bytes is approximately 2081 M.
But when I tell the capacity of my hard disk in Megabytes, It will be approximately 1985 and will be written as 1985 Meg.
In this way the general formula to calculate the capacity of disk in Millions of byte will be as follows:
And general formula for calculating the capacity of the disk in Megabyte will be given as follows:
