Interface Types, Physical Structure and Logical Structure of Hard Disk

This is the second part of hard disk introduction. In Hard Disk 1, we absorb the information about the hard disk types, technology, basic parameters, manufactures, and sizes. Here in the article below, other information about hard disk will be shown for you.

Types of Interface

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IDE (Integrated Drive Electronics)
It is commonly known as PATA parallel port.

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ATA (Advanced Technology Attachment)
It uses the traditional 40-pin parallel port cable to connect the motherboard and hard disk. The maximum speed of external interface is up to 133 MB/s. Because of the poor anti-jamming and bad computer cooling, parallel line will be gradually replaced by SATA.

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SATA (Serial ATA)
It adopts serial connection mode. The Serial ATA bus uses embedded clock signal, and it has stronger ability of error correction. Compared with the previous ones, it can check the transferring instruction and correct errors automatically.

Unlike SATA, SATA II's external transfer rate has been improved from 150 MB/s to 300 MB/s. What's more, it also owns other features like NCQ (Native Command Queuing), Port Multiplier, Staggered Spin-up and so on. But not all of the SATA hard disks can support NCQ technology. Not only the hard drive, but also the SATA controller of mainboard chipset should support NCQ.

SATA III (SATARevision3.0)
The transmission speed has been doubled to 6 Gbps. It is compatible with "SATARevision2.6" (or SATA3Gbps). The interface and data line don't get changed.

SAS (Serial Attached SCSI)
It adopts the serial technology to gain higher transmission speed. And it improves the internal space by shortening the link line. This interface is designed to optimize the efficiency of storage system, availability and expandability. And it also provides compatibility with SATA hard disk.

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SCSI (Small Computer System Interface)
It is completely different from IDE (ATA). The IDE Interface is ordinary PC standard Interface. And SCSI is not specially designed for hard disk. It is a high speed data transmission technology widely used in small machines. SCSI interface has many advantages like wide application range, multitasking, large bandwidth, low CPU usage and hot plug. Due to the high price, it is hard to get popularized. Therefore SCSI is mainly applied in medium or high-end servers and high grade workstations.

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Fiber Channel

It is developed for improving the speed and flexibility of the hard disk storage systems. It is developed to greatly improve the communication speed of multi-hard disk system. Its main features are hot plug, high-speed bandwidth, remote connection, and support on large quantity equipments.

Fiber channel is designed for server hard disk system. It can meet the needs for high data transmission in high-end workstations, servers, mass storage sub-network, and peripherals through hubs, switches and point-to-point connection between two-way or serial data communication system.

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Hard drive crashes easily, no matter what kind of interface it has (IDE, ATA, SATA, SCSI, etc.). Therefore, we think it’s necessary to show you how to recover important data after hard drive crash.

Physical Structure

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Magnetic Head
Heads are not only the most expensive part of hard disk but also the most crucial link in hard drive technology. Traditional head is electromagnetic induction head which is a combination of reading and writing. But the operations on reading and writing are totally different from each other, which lead to the limitation in the hard disk design.

Magneto resistive heads use a separate head structure: write head still adopts the traditional induction head and read head uses the new type MR head. In addition, through the changes of resistance, the MR heads can induct the changes in signal amplitude. It is quite sensitive to signal changes, and also can increase the data reading accuracy. Reading signal amplitude has nothing to do with the track width, so the track can be made narrow so as to improve the density (to 200MB per cubic inches) of the disk. MR heads have been widely used. The GMR (Giant Magneto resistive) heads which use the multi-layer structure and materials (with better magnetic resistance effect) also began to spread.

On a hard disk drive, the circular part which can be used without moving the drive's access arm is called track. There are concentric circles in the disk surface and information is stored in them. Each circle equals to a track. A few hundred to a few thousand tracks exist on a disk range (a modern hard disk even has tens of thousands of tracks on each platter).

A disk track can be divided into many arcs, and these arcs are called disk sector. Data can be located by knowing the number of the sector and the concentric track that passes through that sector.

Hard disk usually consists of a set of disk platters, and each of them is divided into equal number of tracks. When the head for one surface is on a track, the heads for the other surfaces are also on the corresponding tracks. All the corresponding tracks taken together are called a cylinder. A cylinder, composed by tracks, is a division of data in disk. The amount of disk cylinders is the same as the amount of tracks.

Logical Structure

3D Disk Geometry
Many years ago, people used structure which similar to that of the floppy disk drive to produce hard disks. It means that each track of the hard disk has the same number of sectors. Then there comes the 3D Disk Geometry, namely the Heads (255 to the largest number-with 8 binary storage), Cylinders (1023 to the largest number-with 10 binary storage), Sectors (63 to the largest number-with six binary storage) and the corresponding way of addressing.

BIOS interrupt call 13hex
It is used to transfer interrupt information in the process of inputting and outputting. It can complete many functions like disk reset, reading and writing, calibration, positioning, diagnosis, formatting and so on. It adopts the CHS addressing mode, so it can only access 8 GB hard drive at most (1mb = 1048576 bytes).

In order to break the limitation of 8 GB, West Phoenix and Digital Technologies jointly launched the EDD standard (BIOS Enhanced Disk Drive Services) to support 64-bit LBA. It also can support the removable media (such as active hard Disk).

Modern Structure
In the old fashioned hard disk, each track shares with the number of sectors. The recording density of the outer track is much lower than inner track which will waste a lot of disk space (like the floppy disks). In order to solve this problem to improve the hard drive capacity, people changed the density structure. That is to say, the number of outer track sectors is larger than that of inner track. After that, the hard drive no longer owns the 3D Disk Geometry. And the addressing mode is also changed to linear addressing (or using sector for addressing).

In order to be compatible with the old software which uses the 3D Disk Geometry, an address translator (responsible for translating the old 3D Disk Geometry into new linear parameters) is installed into the hard disk controller.

Since a RAID card may employ different types of interface, so here we give a brief introduction RAID.

Advantages of RAID

1. Fast transmission rate. It can make many disk drives transmit data at the same time in some RAID mode. And these disk drives are logically grouped in a disk drive. Thus RAID can achieve a rate that is much faster than single disk drives. The speed of the CPU is growing rapidly but disk data transmission can not be significantly improved, so you need to have a plan to solve the contradiction between them. 
2. Higher security. Compared with ordinary disk drives, RAID mode provides a variety of data repair functions. When a disk drive suffered severe fault, you can use other disk drive in RAID to restore the data.

RAID 0 is no redundant check disk array. Its data is distributed in each disk at the same time. There is no fault tolerance function in it. And the read-write speed is the fastest in RAID. Disk damage can make the whole RAID system failure, so the safety coefficient is lower than a single disk. It is commonly used in playing big games or editing graphics/image, which ask for a low data safety but a high read-write speed. This mode needs at least 2 disks, and more disks can provide more efficient effect in data transmission.