Detailed description of hardware structure and working principle inside the hard disk

Generally, the front of the hard disk is labeled with a product label, which mainly includes the manufacturer information and product information, such as the trademark, model, serial number, production date, capacity, parameters, and master-slave setting method. This information is the basic basis for correct use of the hard disk. The following describes their meanings.

The hard disk consists of a disk, a control circuit board, and interface components, as shown in Figure 1-1. The disk is a sealed cavity. The internal structure of a hard disk usually refers to the internal structure of the disk body. The control circuit board mainly includes hard disk bios, hard disk cache (that is, cache), master control chip, and other units, as shown in 1-2; hard Disk interfaces include power sockets, data interfaces, and master and slave jumpers, as shown in 1-3.

Figure 1-1 hard disk appearance

Figure 1-2 control circuit board

Figure 1-3 hard disk interface

The power outlet is connected to the power supply, providing power assurance for hard drive operation. The data interface is a channel for data exchange between the hard disk and the motherboard and memory. It is connected using a 40-pin, 40-wire (early), or 40-wire, 80-wire (current) ide interface cable. The newly added 40 lines are signal shielding lines used to shield crosstalk during high-speed and high-frequency data transmission. The main and slave jumper sockets in the middle are used to set the access sequence of the master and slave hard disks. The setting method is usually marked on the external disk label, some labels are also marked at the interface, the early hard disk may also be printed on the circuit board.

In addition, there is a breathable hole on the hard disk surface (see Figure 1-1), which is used to make the internal pressure of the hard disk consistent with the external atmospheric pressure. Because the disk body is sealed, the breathable hole is not directly connected to the internal, but is connected to the disk body through an efficient filter to ensure that the inside of the disk body is clean and dust-free, do not cover it during use.

1.2 internal structure of the hard disk

The internal structure of a hard disk usually refers to the internal structure of the disk. The disk is a sealed cavity with a head, disk (disk, disk), and other components, as shown in 1-4.

Figure 1-4 hard disk Internal Structure

The hard disk is a hard magnetic alloy disk, which is about 5mm thick and has a diameter of 1.8in (1In = 25.4mm), 2.5in, 3.5in, and 5.25in, among them, 2.5in and 3.5in are the most widely used. The speed of the disk depends on the disk size. Considering the inertia and Stability of the disk, the larger the disk, the lower the speed. Generally speaking, the 400 in Hard Drive Speed is 5 r/min ~ 200 r/min; the speed of the 3.5in hard disk is 4 500 r/min ~ 400 r/min, while 600 in Hard Drive Speed is 3 r/min ~ 4: 500 r/min. With the advancement of technology, the speed of the 2.5in hard disk has reached a maximum of r/min, and the speed of the 3.5in hard disk has reached a maximum of r/min.

Some hard disks only have one disk, while some hard disks have multiple disks. These disks are mounted on the rotating shaft of the spindle motor and rotate at high speed under the drive of the spindle motor. The capacity of each disk is called the capacity of a single disk, and the capacity of a hard disk is the total capacity of all disks. Because of the low capacity of a single hard disk in the early days, there were a large number of disks, and some even had more than 10 disks. Modern hard disks generally had only a few disks. All disks in a hard disk are identical, otherwise the control part is too complicated. A series of one brand of disks generally use the same disk. A series of Hard Disk products with different capacities appear when different numbers of disks are used.

The complete structure of the disk is 1-5.

Figure 1-5 complete disk structure

The hard drive adopts a high-precision, lightweight head drive/positioning system. This system allows the head to quickly move on the disk surface and precisely locate on the track specified by computer instructions in a very short time. At present, the track density is as high as TPI (number of tracks per inch) or higher; people are still studying new methods, such as extrusion (or etching) on the disk) images, grooves, and spots are used as positioning and tracking marks to increase the track density equal to that of the disc. This greatly improves the storage capacity while maintaining the high speed, high density, and high reliability of the hosts.

The motor in the hard drive is a brushless motor, and the mechanical wear is very small under the support of high-speed bearings, and can work continuously for a long time. High-speed rotation of the disk has a significant gyro effect, so it is not recommended to move the hard disk, otherwise, it will increase the bearing workload. In order to store and read information at high speed, the hard drive has low head quality and Low Inertia. Therefore, the hard drive tracing speed is significantly faster than that of the soft drive and optical drive.

The hard drive head is integrated with the head arm and servo positioning system. The servo positioning system consists of the coil behind the head arm and the Electromagnetic Control System fixed on the bottom plate. Due to positioning system restrictions, the head arm can only move between the internal and external magnetic channels of the disk. Therefore, the head is always on the disk, no matter whether it is started or shut down. The difference is that the head stays in the Start and Stop area of the disk when it is turned off, and the head is "Flying" above the disk.

 

How is data on a hard disk organized and managed? The hard disk is logically divided into tracks, cylinders, and sectors. Its structure is 1-6.

Figure 1-6 head, cylinder, and sector

Each disk has a read/write head on each side, as shown in area 1-7. The head is close to the contact surface of the main shaft, that is, the place with the smallest wire speed. It is a special area that does not store any data. It is called the start/stop zone or the landing zone ), the data zone is used outside the start and stop areas. In the outermost ring, the farthest place from the spindle is the "0" track, and the storage of Hard Disk Data starts from the outermost ring. Then, how does the head locate the position of the "0" track? From Figure 1-5, we can see that there is a "0" track detector that completes the initial positioning of the hard disk. The "0" track is so important that many hard disks are scrapped only because the "0" track is damaged. This is a pity. The fault repair technology is described in detail in the following sections.

Figure 1-7 Start and Stop zones and data zones of Hard Disk Disks

A program called parking needs to be run before each shutdown of a hard disk in the early days. Its function is to let the head back to the start and stop area. The Design of Modern hard drives has abandoned this small defect that is not complex but unpleasant. When the hard disk does not work, the head stays in the Start and Stop area. When you need to read and write data from the hard disk, the disk starts to rotate. When the rotation speed reaches the rated high speed, the head will be lifted by the airflow generated by disk rotation, then the head will move to the area where the disk is stored. The airflow generated by disk rotation is strong enough to hold the head and keep a tiny distance from the disk surface. The smaller the distance, the higher the sensitivity of the head to read and write data, of course, the higher the requirements for each part of the hard disk. The disk drive was designed to keep the head flying several microns above the disk. Later, some designs reduced the flying height of the head on the disk to about 0.1 μm ~ 0.5 μm, the current level has reached 0.005 μm ~ 0.01 μM, which is only 1‰ of the diameter of human hair. The airflow not only separates the head from the opening surface, but also keeps it close enough to the disk surface, closely following the ups and downs of the disk surface, so that the head flight is strictly controlled. The head must fly above the disk surface rather than touching the disk surface. This position can avoid scratches on the magnetic coating, and more importantly, prevent the magnetic coating from damaging the head. However, the head cannot be too far away from the disk surface. Otherwise, the disk surface cannot be magnetization enough, and it is difficult to read the magnetization of the disk (pole conversion form, is the method of actually recording data on the disk ).

The floating height of the hard drive head is low and the speed is fast. Once a small amount of dust enters the hard drive sealing cavity, or if the head and the disk body collide, data may be lost, the formation of Bad blocks may even cause damage to the head and disk. Therefore, the hard disk system must be sealed reliably. In non-professional circumstances, it is absolutely impossible to enable the hard disk sealed cavity. Otherwise, the damage to the hard disk will be accelerated after dust enters. In addition, the track-seeking servo motor of the hard drive head mostly uses the audio ring rotation or linear motion stepping motor to precisely track the track of the disc under the adjustment of the Servo Tracking, when the hard disk is working, do not have an impact collision. Be careful when moving it.

This type of hard disk is a hard disk created using Winchester technology, so it is also called a warm disk. Its structure features are as follows.

① The head, disc and motion mechanism are sealed in the disc body.

② When the head is started or stopped, it is in contact with the disk. during work, the disk is rotated at high speed, and the head is "suspended" on the disk (aerodynamic principle ), the height of "floating" is about 0.1 μm ~ 0.3 μm. The height is very small. Figure 1-8 shows the relationship between the height and the size of hair, smoke, and fingerprint, here we can intuitively see how high this height is ".

Figure 1-8 disk structure and head height

③ When the head is working, it is not directly in contact with the disc. Therefore, the head can be loaded with a small size, and the head can be very delicate. The ability to detect the track is very strong, which can greatly improve the bit density.

④ The disk surface is very smooth and can be used as a mirror.

The following describes the meanings of "disk", "track", "Cylinder", and "Sector" one by one.

1. Disk ID

Hard Disk disks are generally made of aluminum alloy materials as substrates, and high-speed hard disks may also be made of glass as substrates. The glass substrate is easier to achieve the required flatness and finish, and has a high hardness. A head drive device is a device that enables the head parts to move in a radial direction. there are usually two types of drive devices. One is the Step Motor Drive Device of rack drive and the other is the sound ring motor drive device. The former is a fixed-estimate drive positioner, while the latter uses servo feedback to return to the correct position. The head Drive Device moves the head parts in a radial direction at a very small equal distance to change the track.

Each disk on a hard disk has two sides, that is, the top and bottom sides. Generally, each side can be used to store data and become a valid disk, there are also a few hard disk faces. Each valid disk has a disk number, which is numbered from top to bottom from "0" in sequence. In the hard disk system, the disk number is also called the head number, because each valid disk has a corresponding read/write head. The disk group of the hard disk is 2 ~ 14 pieces, usually 2 ~ Three disks, so the disk number (head number) is 0 ~ 3 or 0 ~ 5.

2. Track

The disk is divided into many concentric circles during formatting. These concentric circle tracks are called tracks ). The track starts from 0 in sequence. Each disk on the hard disk has 300 ~ 1 024 channels, with more tracks on each side of the new large-capacity hard drive. Information is recorded in these tracks in the form of a pulse string. These concentric circles are not continuously recorded data, but are divided into arcs in the middle segment. These ARCs share the same angular velocity. Because the radial length is different, the line speed is also different. The line speed of the outer ring is higher than that of the inner ring. That is, at the same speed, the outer ring is in the same time period, the length of the arc to be crossed is larger than that of the arc to be crossed by the inner ring. Each section of an arc is called a slice. The Slice starts from "1" and the data in each slice is read or written as a unit at the same time. A standard 3.5in hard disk usually has hundreds to thousands of tracks. The track is invisible. It is only some of the magnetized areas on the disk surface that are magnetized in special forms. It has been planned during disk formatting.

 

3. cylindrical

A cylinder is a cylinder formed by the same track on all disks. The head of each cylinder starts from "0" from top to bottom. The read/write operations are performed by the cylinder. That is, when the head reads/writes data, the operation starts from the "0" head in the same cylinder, in turn, the operation is performed on different disks of the same cylinder, that is, the head is transferred to the next cylinder only after all the heads of the same cylinder have been read/written, because the selected head only needs to be switched electronically, and the selected cylinder must be switched mechanically. The electronic switching speed is much faster than that of the mechanical head moving towards the adjacent track. Therefore, data reading/writing is performed by the cylinder instead of by the disk. That is to say, after a track is full of data, it is written on the next disk of the same cylinder. After a cylinder is full, it is moved to the next sector to start writing data. This method also improves the read/write efficiency of the hard disk.

The number of columns on a hard drive (or the number of tracks on each disk) depends on the width and width of each track (also related to the size of the head ), it also depends on the gap between tracks determined by the positioning mechanism. For more details, see other books.

4. Sector

The operating system stores information on the hard disk in the form of a sector (sector). Each sector contains 512 bytes of data and other information. One slice has two main parts: the identifier of the location where the data is stored and the data segment where the data is stored, as shown in figure 1-9.

Figure 1-9 hard disk sector Composition

An identifier is the header of a sector. It consists of three digits that constitute the three-dimensional address of a sector: the head (or disk) of the sector, the track (or the cylinder number) and the position of the slice on the track, that is, the fan area number. The header also contains a field that shows whether the slice can store data reliably or whether a fault has been found and thus is not suitable for use. Some hard drive controllers also record indicators in the Sector Header, which can guide the disk to the replacement sector or track when an error occurs in the original sector. Finally, the Sector Header Mark ends with the cyclic redundancy check (CRC) value for the Controller to check the reading status of the Sector Header mark to ensure accuracy.

The second major part of the slice is the data segment that stores data, which can be divided into data and the Error Correction Code (ECC) that protects the data ). During the initial preparation, the computer fills in this section with 512 virtual information bytes (actual data storage location) and ECC numbers corresponding to these virtual information bytes.

The Sector Header mark contains a sector number that identifies the sector on the track. Interestingly, these partition numbers are physically not consecutively numbered and do not need to be specified in any specific order. The design of the Sector Header allows the fan area number to ranges from 1 to a maximum value, and in some cases, it can reach 255. The disk controller does not care about the number in the preceding range in which the slice header tag is arranged. In special cases, the slice can share the same number. The disk controller even reads the data it finds, or writes the data it needs to write, regardless of the size of the Data zone.

The simplest method for numbering slice is sequential numbers such as l, 2, 3, 4, 5, and 6. If the slice is sequentially numbered around the track, the Controller rotates the disk too far while processing the data of a slice, beyond the interval (this interval is very small ), the next sector to be read or written by the Controller has passed through the head, which may be a considerable distance. In this case, the disk controller can only wait until the disk is rotated for nearly a week to make the required sector under the head.

Obviously, it is unrealistic to increase the fan interval to solve this problem, which will waste a lot of disk space. Many years ago, an outstanding IBM engineer came up with a brilliant way to number a sector rather than using sequential numbers, rather than using an interleave. The cross factor is expressed by the ratio. For example, 3: 1 indicates that the first sector on the track is the first sector. If two sectors are skipped, that is, the second sector is the second sector, this process continues until a logical number is assigned to each physical sector. For example, if a disk with 17 sectors on each track is numbered by a crossover factor of 2: 1: l, the crossover factor numbers based on 3: 1 are: l. When the cross factor of 1: L is set, if the hard disk controller processes the information fast enough, it takes only one week to read all the sectors on the track; however, if the post-processing operation of the hard disk controller is not so fast, the number of turns on the disk is equal to the number of sectors on the same track to read all the data on each track. When the cross factor is set to 2: 1, the head needs to read all the data on the track, and the disk only needs to be converted to two weeks. If the cross factor of 2: 1 is still not slow enough, and the number of weeks of disk rotation is about the number of sectors of the track, you can adjust the cross factor to 3: 1, 1-10.

Figure 1-10 examples of different cross Factors

Figure 1-10 shows a typical MFM (modified frequency modulation, improved frequency modulation encoding) Hard Drive. Each track has 17 sectors, and three different sector crossover factor numbers are drawn. The sectors on the outermost track (cylinder 0) are numbered consecutively in a simple sequence, which is equivalent to the sector cross factor of 1: 1. The slice of the track 1 (cylindrical) is numbered by the cross factor of 2: 1, while that of the Track 2 is numbered by the cross factor of the slice of the 3: 1.

In the early hard disk management work, you need to set the cross factor yourself. When you use a low-level formatting program in the BIOS to perform low-level formatting on the hard disk, You need to specify the cross factor. Sometimes you need to set several different values to compare the performance, and then determine a better value, in order to improve the performance of the hard disk. The hard disk BIOS has already solved this problem by itself. Therefore, low-level formatting programs generally do not provide this option.

When the system stores files on a disk, it uses the cylindrical, Head, and sector method, that is, the first Magnetic Head of the 1st track (that is, the first Magnetic Track of the 1st disk) and then the next head of the same cylinder ,......, After a cylindrical storage is full, it is pushed to the next cylindrical until all the file content is written to the disk. The system also reads data in the same order. When reading data, the disk controller is notified to read the cylindrical number, head number, and fan area number of the slice (three parts of the physical address. The disk controller directly moves the head parts to the corresponding cylinder, selects the corresponding head, and waits for the required sector to move to the bottom of the head. When the slice arrives, the disk controller reads the header mark of each slice, compares the address information in the header mark with the head and the cylinder number to be detected (that is, seek), and then, find the required fan area number. When the disk controller finds this sector header, it determines whether to convert the write circuit or read data and tail records based on whether the task is to write a sector or read a sector. After a sector is found, the disk controller must post-process the information of the sector before continuing to search for the next sector. If the data is read, the Controller calculates the ECC code of the data and compares the ECC code with the recorded ECC code. If the data is written, the Controller calculates the ECC code of the data and stores it together with the data. The disk continues to rotate when the controller makes necessary processing of the data in this sector. Since post-processing of information takes some time, during this time, the disk has been converted to a considerable angle.

Determining the cross factor is a system-level problem. The cross factor of a specific hard drive depends on the speed of the disk controller, the clock speed of the motherboard, and the operation speed of the output bus connected to the Controller. If the cross factor value of the disk is too high, you need to spend more time waiting for data to be stored and read on the disk. If the cross factor value is too low, the disk performance will be greatly reduced.

As mentioned above, when the system writes information to a disk, it fills up one track and forwards it to the next head of the same cylinder. When the cylinder is full, it forwards it to the next one. From one track of the same cylinder to another, it takes time to convert from one cylinder to the next cylinder. During this period, the disk is always rotated, which brings about a problem: assuming that the system has just ended writing data to the first sector of a track and the best cross factor ratio has been set, and now the system is preparing to write data to the first sector of the next track, you must wait until the head is converted, re-Prepare the head part for positioning on the next part. If this operation takes longer than one point, the head will be delayed even though it is a cross-access operation. The solution to this problem is to move all the fan area numbers on the new track about one or several sectors based on the original track location. This is the head skew. The twisting angle of the head can be understood as the cross factor between the cylinder and the cylinder. It has been set up by the production plant, and users generally do not need to change it. It is difficult to change the head skew, but they only play a role when the file is long and exceeds the end of the track for reading and writing, incorrect skew settings cause much less time loss than incorrect slice cross factor values. Cross factor and head skew can be tested and changed with dedicated tools. More specific content is not described here. After all, many users have not seen these parameters.

The sector ID is stored in the slice header, and the slice crossover factor and head skew information are also stored here. Initially, the hard disk low-level formatting program only exercises the dedicated functions of the disk controller to complete the setting task. This process may destroy all the data on the low-level formatted track, and is rarely used.

The sector crossover factor is set by the number written to the Sector Header. Therefore, each track can have its own crossover factor. In most drives, all channels share the same crossover factor. However, due to operation, the track may have different sector crossover factors. For example, when the cross factor reset program works, the crossover factor of some magnetic channels may change due to power failure or human interruption, while that of other magnetic channels does not change. This inconsistency will not adversely affect computers, but the channels with the best cross factor will work faster than other channels.