| Hard Disc Drive Fundamentals | | | | The host sends the READ command to the hard |
| | | | disk, it gets transferred from logical to physical, then |
| Abstract: | | | | hard disk drive send back the specified block data to |
| This article describes the main technical fundamentals | | | | read. At the time of writing in the memory, the host |
| of Hard Disc Drive and its operation. I focus my | | | | sends the WRITE command to the HDD. After |
| attraction on the particular perspective of describing | | | | receiving the acknowledgement, the data is sent |
| Physical and Logical view of HDD, Reading and | | | | through the bus to write in the specified block. |
| Writing in the HDD, How does data get stored in that | | | | HDD Interface: |
| and touched some information about HDD interfaces. | | | | A shared boundary between two, or more, functional |
| This article also brings out the technology wise | | | | units, defined by specific attributes, such as functional |
| company working on this hard disc drives and this | | | | characteristics (Phases, states, protocol), common |
| related technology. | | | | physical interconnection characteristics (cable, plugs), |
| Starting of HDD: | | | | and signal characteristics (Polarity, voltage range, |
| Hard disc drive, it is the magnetic device used to | | | | timing). This is called Interface. There are two main |
| store soft copy of data in the form of 0's and 1's. | | | | ways to interface the HDD with the host. They are, |
| Specific drivers are used to retrieve back the data in | | | | ATA (Advanced Technology Attachment) |
| a specified format. Magnetic hard disks are used from | | | | PATA (Parallel Advanced Technology Attachment) |
| 1956 onwards. On that day, the size of the hard | | | | SATA (Serial Advanced Technology Attachment) |
| disks is very huge for a small amount of storage and | | | | SCSI (Small Computer System Interface) |
| it also takes more time to store and retrieve. But | | | | ATA (PATA): |
| today we are using very compact, high capacity, high | | | | ATA is a common interface used in many personal |
| latency hard disks for our systems. | | | | computers before the emergence of SATA (Serial |
| Physical representation of HDD: | | | | Advanced Technology Attachment). The initial version |
| The user data / information are stored on a disc's | | | | of ATA is a Parallel one. So after emergence of |
| surface media. The media is a specially coated | | | | SATA, it is usually called as PATA (Parallel Advanced |
| surface. Each media surface is divided into tracks and | | | | Technology Attachment). It is the least expensive of |
| sectors. These sectors are often called "blocks". | | | | the interfaces used in the system in older days. |
| Tracks are often referred to as cylinders. The Track, | | | | PATA can support data transfer up to 133 MB/s. |
| Sector separations of the hard disk are shown below. | | | | Nowadays SATA replaces this PATA in all areas. But |
| The Cylinder, Head, Sector (CHS) gives a physical | | | | the recent version UDMA133 is used in some |
| location. | | | | subsystems depends upon the customer requirement. |
| Logical representation of HDD: | | | | SATA: |
| Hard disks use circular hard platters to store data on. | | | | Serial Advanced Technology Attachment is |
| Each platter has two surfaces. Both sides of the | | | | commonly known as SATA. It is the very common |
| media are used to store information. If each surface | | | | hard disk interface used in all personal systems. One |
| had 4 tracks and 4 sectors means there would be 32 | | | | SATA port permits one device to connect to it. The |
| sectors, or blocks. Each surface requires a "head" to | | | | main disadvantage of this is, it does not support the |
| read/write the information. Surfaces are often | | | | older systems without the use of additional |
| referred to as the "head". A cylinder is the area that | | | | components |
| a head can write to or read from without being | | | | SCSI: |
| positioned to another track. | | | | It is abbreviated like Small Computer System |
| Take a hard disk has two surfaces/head; four | | | | Interface. SCSI is commonly used in servers, and |
| sectors/blocks then the logical representation of that | | | | more in industrial applications than home uses. It is |
| disk will be like this figure 3. In that figure, the two | | | | the famous storage subsystem component used in all |
| heads of that disk represented as 0 and 1. The four | | | | medium and small level computer interfaces. A |
| cylinders/tracks and sectors are represented by 0 to | | | | mechanism called 'Cable Sharing' used here. It can |
| 3. So for, each surface has 16 logical sectors/blocks | | | | support the data transfer rate up to 320 MB/s. |
| that is named as Logical block Addressing (LBA). | | | | HDD - Enterprise Information: |
| The Host and the file system using this LBA only for | | | | In the Enterprise level, the companies are |
| HDD operation like reading, writing the data, searching | | | | differentiated the hard disks using their spindle |
| and seek etc., But for heads and media, the hard disk | | | | revolution per minute (RPM) like 4,200 RPM HDDs, |
| driver circuits convert the LBA to CHS (Cylinder, | | | | 5,400 RPM HDDs, 7,200 RPM HDDs, 10,000 RPM |
| Head, Sector). | | | | HDDs, 15,000 RPM HDDs. These are all the main |
| Reading and Writing in HDD: | | | | manufactures and companies have working with Hard |
| Hard Disk Drive is a digital data storage device that | | | | Disk Drives and Marketing. |
| reads and writes data via magnetization changes of a | | | | Audavi,Dane-Elec Memory, Dataslide, DTS, Dynamic |
| magnetic storage disk. This includes one or more | | | | Network Factory, EDGE Tech, Excelstor Technology, |
| randomly accessible rotatable storage media, or disks. | | | | EZQuest Fantom Drives, Formation, Freecom |
| In a magnetic disk drive, the data is encoded as bits | | | | Technologies, Fujitsu, Hitachi, Imation, I/OMagic, |
| of information using magnetic field reversals grouped | | | | Iomega,LaCie, Maxell, Miltope,NEC, Olixir Technologies, |
| in tracks on the magnetic hard surface of rotating | | | | PNY Technologies, ProStor Systems, Quantum, |
| disks. It also includes a transducer supported by a | | | | Samsung Electronics, SDK,Seagate, Shining |
| hydrodynamic bearing which flies above each | | | | Technology, Sony, SOYO, Toshiba,Verbatim, |
| magnetic disk. The transducer head supported by an | | | | Western Digital |
| actuator arm is used to read data from or write data | | | | Hard Drive Failures: |
| to the disks. Hard disk drives are an indispensable | | | | Hard drive crashes are one of the most frustrating |
| component in most modern computing/data handling | | | | experiences with a computer. The inability to read |
| systems. | | | | data from the hard drive can render a computer |
| Hard disk drives are commonly utilized to store | | | | useless. Even if the OS can run, the data may be |
| relatively large amounts of data in today's computer | | | | inaccessible or damaged. The only way to recover |
| systems like desk. If the capacity of the disk is | | | | from such a failure is to typically restore data from a |
| more, the computer can store and access more data. | | | | backup onto a new drive with all the software |
| In a computer, a hard disk drive is connected to the | | | | installed from scratch. If no backup is available, then |
| central processing unit (CPU) by means of the | | | | the data is either lost or will cost a lot for recovery |
| system bus. Computer architectures have a | | | | services to retrieve. |
| motherboard which includes a central processing unit | | | | Common drive failures |
| and the system bus to which various peripherals, | | | | The most common failure for a hard drive is |
| including a hard disk drive, are connected. | | | | something called a head crash. A head crash is any |
| In a hard disk drive, data signals are read from and | | | | instance where the drive head manages to touch a |
| written to a rotating disk by a head, which is moved | | | | platter. When this happens, the magnetic media will |
| substantially with respect to the disk. The head | | | | be etched off of the platter by the head and render |
| assembly facilitates reading and writing of information | | | | both the data and the drive head inoperable. There is |
| on a surface of a rotating magnetic disk. The | | | | no clean recovery from such a failure. |
| interconnect assembly includes a plurality of | | | | Another common failure comes from imperfections |
| transmission elements, such as wires or traces, for | | | | on the magnetic media. Any time that a sector on |
| transmitting data to and from the head assembly. | | | | the disk fails to properly hold the magnetic alignment |
| The suspension assembly positions the head | | | | will cause data to be inaccessible. |
| assembly at a generally constant distance away from | | | | Mobile systems tended to be prone to platters that |
| the moving surface of the rotating disk. When the | | | | shattered. This was due to the fact that most hard |
| electric power is turned off to end the use of a | | | | drive platters are made of glass and was susceptible |
| computer, a driving voltage to a spindle motor also | | | | to shock. |
| stops so that the speed of the hard disk rotated by | | | | MTBF: |
| the spindle motor gradually decreases. At this point, | | | | In order for consumers to get a good idea of the |
| the magnetic head in the state of flying above the | | | | lifespan of a hard drive; a drive was rated by MTBF. |
| hard disk by the flow of air descends onto the | | | | This term stands for Mean Time Between Failure and |
| surface of the hard disk. | | | | is used to represent the length of time that 50 |
| The servo control system generally performs two | | | | percent of drives would fail before and 50 percent |
| distinct functions: | | | | would fail after. It is used to give an idea to a buyer |
| • Seek control | | | | as to the average amount of time the device will |
| • Track following | | | | function for. The "Mean Time Between Failures" |
| The seek control function comprises controllably | | | | (MTBF) for hard disc drives had been going up fairly |
| moving the actuator arm such that the transducer | | | | steadily from around 200,000 hours in the 1990's to |
| head is moved from an initial position to a target | | | | nearly 1,200,000 hours today. |
| track position. The read/write head is typically a part | | | | Recovery Software: |
| of or affixed to a larger body that flies over the disk | | | | At the time of hard drive failures, user wants to |
| and is typically referred to as a slider. The slider has a | | | | recover all data from that failure hard disk. Various |
| lower surface referred to as the air-bearing surface. | | | | companies provide the recovery software for hard |
| The air-bearing surface typically comprises one or | | | | disk data recovery. Some of them would be free |
| more rails which generally generate a positive air | | | | too. |
| pressure | | | | |