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Hard Disc Drive Fundamentals

Hard Disc Drive Fundamentals
By: Muthu Senthil Kumar (mskumar@tataelxsi.co.in / mskumar86@myway.com)

Abstract:

This article describes the main technical fundamentals of Hard Disc Drive and its operation. I focus my attraction on the particular perspective of describing Physical and Logical view of HDD, Reading and Writing in the HDD, How does data get stored in that and touched some information about HDD interfaces. This article also brings out the technology wise company working on this hard disc drives and this related technology.

Starting of HDD:

Hard disc drive, it is the magnetic device used to store soft copy of data in the form of 0's and 1's. Specific drivers are used to retrieve back the data in a specified format. Magnetic hard disks are used from 1956 onwards. On that day, the size of the hard disks is very huge for a small amount of storage and it also takes more time to store and retrieve. But today we are using very compact, high capacity, high latency hard disks for our systems.

Physical representation of HDD:

The user data / information are stored on a disc's surface media. The media is a specially coated surface. Each media surface is divided into tracks and sectors. These sectors are often called "blocks". Tracks are often referred to as cylinders. The Track, Sector separations of the hard disk are shown below. The Cylinder, Head, Sector (CHS) gives a physical location.

Logical representation of HDD:

Hard disks use circular hard platters to store data on. Each platter has two surfaces. Both sides of the media are used to store information. If each surface had 4 tracks and 4 sectors means there would be 32 sectors, or blocks. Each surface requires a "head" to read/write the information. Surfaces are often referred to as the "head". A cylinder is the area that a head can write to or read from without being positioned to another track.

Take a hard disk has two surfaces/head; four sectors/blocks then the logical representation of that disk will be like this figure 3. In that figure, the two heads of that disk represented as 0 and 1. The four cylinders/tracks and sectors are represented by 0 to 3. So for, each surface has 16 logical sectors/blocks that is named as Logical block Addressing (LBA).

The Host and the file system using this LBA only for HDD operation like reading, writing the data, searching and seek etc., But for heads and media, the hard disk driver circuits convert the LBA to CHS (Cylinder, Head, Sector).

Reading and Writing in HDD:

Hard Disk Drive is a digital data storage device that reads and writes data via magnetization changes of a magnetic storage disk. This includes one or more randomly accessible rotatable storage media, or disks. In a magnetic disk drive, the data is encoded as bits of information using magnetic field reversals grouped in tracks on the magnetic hard surface of rotating disks. It also includes a transducer supported by a hydrodynamic bearing which flies above each magnetic disk. The transducer head supported by an actuator arm is used to read data from or write data to the disks. Hard disk drives are an indispensable component in most modern computing/data handling systems.

Hard disk drives are commonly utilized to store relatively large amounts of data in today's computer systems like desk. If the capacity of the disk is more, the computer can store and access more data. In a computer, a hard disk drive is connected to the central processing unit (CPU) by means of the system bus. Computer architectures have a motherboard which includes a central processing unit and the system bus to which various peripherals, including a hard disk drive, are connected.

In a hard disk drive, data signals are read from and written to a rotating disk by a head, which is moved substantially with respect to the disk. The head assembly facilitates reading and writing of information on a surface of a rotating magnetic disk. The interconnect assembly includes a plurality of transmission elements, such as wires or traces, for transmitting data to and from the head assembly. The suspension assembly positions the head assembly at a generally constant distance away from the moving surface of the rotating disk. When the electric power is turned off to end the use of a computer, a driving voltage to a spindle motor also stops so that the speed of the hard disk rotated by the spindle motor gradually decreases. At this point, the magnetic head in the state of flying above the hard disk by the flow of air descends onto the surface of the hard disk.

The servo control system generally performs two distinct functions:
• Seek control
• Track following

The seek control function comprises controllably moving the actuator arm such that the transducer head is moved from an initial position to a target track position. The read/write head is typically a part of or affixed to a larger body that flies over the disk and is typically referred to as a slider. The slider has a lower surface referred to as the air-bearing surface. The air-bearing surface typically comprises one or more rails which generally generate a positive air pressure

The host sends the READ command to the hard disk, it gets transferred from logical to physical, then hard disk drive send back the specified block data to read. At the time of writing in the memory, the host sends the WRITE command to the HDD. After receiving the acknowledgement, the data is sent through the bus to write in the specified block.

HDD Interface:

A shared boundary between two, or more, functional units, defined by specific attributes, such as functional characteristics (Phases, states, protocol), common physical interconnection characteristics (cable, plugs), and signal characteristics (Polarity, voltage range, timing). This is called Interface. There are two main ways to interface the HDD with the host. They are,

ATA (Advanced Technology Attachment)

PATA (Parallel Advanced Technology Attachment)

SATA (Serial Advanced Technology Attachment)
SCSI (Small Computer System Interface)

ATA (PATA):
ATA is a common interface used in many personal computers before the emergence of SATA (Serial Advanced Technology Attachment). The initial version of ATA is a Parallel one. So after emergence of SATA, it is usually called as PATA (Parallel Advanced Technology Attachment). It is the least expensive of the interfaces used in the system in older days. PATA can support data transfer up to 133 MB/s. Nowadays SATA replaces this PATA in all areas. But the recent version UDMA133 is used in some subsystems depends upon the customer requirement.

SATA:
Serial Advanced Technology Attachment is commonly known as SATA. It is the very common hard disk interface used in all personal systems. One SATA port permits one device to connect to it. The main disadvantage of this is, it does not support the older systems without the use of additional components

SCSI:
It is abbreviated like Small Computer System Interface. SCSI is commonly used in servers, and more in industrial applications than home uses. It is the famous storage subsystem component used in all medium and small level computer interfaces. A mechanism called 'Cable Sharing' used here. It can support the data transfer rate up to 320 MB/s.

HDD - Enterprise Information:

In the Enterprise level, the companies are differentiated the hard disks using their spindle revolution per minute (RPM) like 4,200 RPM HDDs, 5,400 RPM HDDs, 7,200 RPM HDDs, 10,000 RPM HDDs, 15,000 RPM HDDs. These are all the main manufactures and companies have working with Hard Disk Drives and Marketing.

Audavi,Dane-Elec Memory, Dataslide, DTS, Dynamic Network Factory, EDGE Tech, Excelstor Technology, EZQuest Fantom Drives, Formation, Freecom Technologies, Fujitsu, Hitachi, Imation, I/OMagic, Iomega,LaCie, Maxell, Miltope,NEC, Olixir Technologies, PNY Technologies, ProStor Systems, Quantum, Samsung Electronics, SDK,Seagate, Shining Technology, Sony, SOYO, Toshiba,Verbatim, Western Digital

Hard Drive Failures:

Hard drive crashes are one of the most frustrating experiences with a computer. The inability to read data from the hard drive can render a computer useless. Even if the OS can run, the data may be inaccessible or damaged. The only way to recover from such a failure is to typically restore data from a backup onto a new drive with all the software installed from scratch. If no backup is available, then the data is either lost or will cost a lot for recovery services to retrieve.
Common drive failures

The most common failure for a hard drive is something called a head crash. A head crash is any instance where the drive head manages to touch a platter. When this happens, the magnetic media will be etched off of the platter by the head and render both the data and the drive head inoperable. There is no clean recovery from such a failure.

Another common failure comes from imperfections on the magnetic media. Any time that a sector on the disk fails to properly hold the magnetic alignment will cause data to be inaccessible.

Mobile systems tended to be prone to platters that shattered. This was due to the fact that most hard drive platters are made of glass and was susceptible to shock.

MTBF:

In order for consumers to get a good idea of the lifespan of a hard drive; a drive was rated by MTBF. This term stands for Mean Time Between Failure and is used to represent the length of time that 50 percent of drives would fail before and 50 percent would fail after. It is used to give an idea to a buyer as to the average amount of time the device will function for. The "Mean Time Between Failures" (MTBF) for hard disc drives had been going up fairly steadily from around 200,000 hours in the 1990's to nearly 1,200,000 hours today.

Recovery Software:

At the time of hard drive failures, user wants to recover all data from that failure hard disk. Various companies provide the recovery software for hard disk data recovery. Some of them would be free too.

About the Author

Muthu Senthil Kumar M - Design Engineer, Tata Elxsi Limited, ITPB Road, Whitefield, Bangalore - 560048, India - Mail: mskumar@tataelxsi.co.in - Phone: +91 9916516728.

can a scsi scanner be used with a usb/firewire adapter/converter?

have an 'all in one' gateway 610 computer hence cant install a scsi interface ....will a scsi scanner work using a scsi to usb or scsi to firewire converter/adapter?

IF you can find the right converter cable, yes. ~
http://castle.pricewatch.com/s/search.asp?s=USB+to+scsi
Looks like you can get an USB scanner for the price of the converter.

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