Chapter-5a(types Of Storage Device)

Chapter-5A Types of Storage Devices

How data is stored on a disk

Tracks & sectors Any disk must be formatted before being used. Formatting means that imaginary tracks and sectors are marked on the disk surface.

Tracks & Sectors Tracks are concentric circles where data is recorded. They are numbered from outermost to innermost starting with 0. Each track contains the same number of bytes, therefore the amount of data is limited by the size of the innermost track. Sectors are imaginary pie slices

How OS Find Data on a Disk Booting FAT Master Folder

Booting To load the first piece of software that starts a computer. Because the operating system is essential for running all other programs, it is usually the first piece of software loaded during the boot process. Boot is short for bootstrap, which in olden days was a strap attached to the top of your boot that you could pull to help get your boot on. Hence, the expression "pull oneself up by the bootstraps." Similarly, bootstrap utilities help the computer get started. Short for bootstrap, the starting-up of a computer, which involves loading the operating system and other basic software. A cold boot is when you turn the computer on from an off position. A warm boot is when you reset a computer that is already on.

FAT A table that the operating system uses to locate files on a disk. Due to fragmentation, a file may be divided into many sections that are scattered around the disk. The FAT keeps track of all these pieces.

Fragmentation Refers to the condition of a disk in which files are divided into pieces scattered around the disk. Fragmentation occurs naturally when you use a disk frequently, creating, deleting, and modifying files. At some point, the operating system needs to store parts of a file in noncontiguous clusters. This is entirely invisible to users, but it can slow down the speed at which data is accessed because the disk drive must search through different parts of the disk to put together a single file.

Cluster A group of disk sectors.

Master Folder An organizational unit, or container, used to organize folders and files into a hierarchical structure. Directories contain bookkeeping information about files that are, figuratively speaking, beneath them in the hierarchy. You can think of a directory as a file cabinet that contains folders that contain files. Many graphical user interfaces use the term folder instead of directory. Computer manuals often describe directories and file structures in terms of an inverted tree. The files and directories at any level are contained in the directory above them. To access a file, you may need to specify the names of all the directories above it. You do this by specifying a path. The topmost directory in any file is called the root directory. A directory that is below another directory is called a subdirectory. A directory above a subdirectory is called the parent directory. Under DOS and Windows, the root directory is a back slash (\). To read information from, or write information into, a directory, you must use an operating system command. You cannot directly edit directory files. For example, the DIR command in DOS reads a directory file and displays its contents.

Master Folder/Directory

Hard Disk. The hard disk or hard drive is an essential hardware that every computer desktop and server contain. And its functionality, is to store vast amount of digital data in a nonvolatile form, so that data can be retained when the computer is powered off. Data is again accessed when the power is on.

Read Write Head The RW head is the key component that performs the reading and writing functions. It is placed on a slider which is in term connected to an actuator arm which allow the RW head to access various parts of the platter during data IO functions by sliding across the spinning platter. The sliding motion is derived by passing a current through the coil which is part of the actuatorassembly. As the coil is placed between two magnets, the forward or backward sliding motion is hence derived by simple current reversal. This location of the platter (just like the landmark along the road) is identified and made possible by the embedded servo code written on the platter.

Read Write Function of Disk As the head writes data onto the disk, it changes its magnetic polarization to induce either a one or zero value. During a read request, data is interpreted when the magnetic fields on the platter brings about an electrical change (as a result of change in electrical resistance of some special material property) in the read-head that passes over it. These electrical fields are then encoded and transmitted to the CPU to be processed and read by the system.

Parking of RW Head When the computer is switched off, the head is usually pulled to a safe parking zone to prevent the head from scratching against the data zone on platter. This process is called a parking and different techniques have been implemented in various hard disks to handle the take offs and landings. In a Ramp load/unload design, a lifting mechanism parks the head outside of the platter onto a "parking bay" prior to a shutdown. It then automatically unpacks and relocates itself

There're two popular ways to measure the performance of a hard disk:

DataRate Data rate is also known as data transfer rate or throughput. It refers to the speed with which the data can be transmitted from one device to another. Data rates are commonly measured in megabytes (MB) per second. The data rates for a typically hard disk ranges between 5 to 40 megabytes per second. Seek Rate Seek time refers to the total span of time between a file request by the CPU to the point when the first byte of the file is sent to the CPU. This is one of the several delays associated with reading or writing data on a computer's disk drive. In order to read or write data in a particular place on the disk, the read/write head needs to maneuver to the precise location of the disk platter. This process is known as "seeking", and the time it takes for the head to move to the right place is referred to the "seek time". A typical seek time for a hard disk is about 9 to 15 ms.

S.M.A.R.T

Majority of all hard disk today support a technology known as S.M.A.R.T. (Self-Monitoring, Analysis, and Reporting Technology) which helps to predict imminent disk failures so that users can be alerted to take preventive actions before the disk fails completely. Generally, there are two classes of failures, namely Non-Predictable failures and Predictable failure. Non-predictable failure associates with abrupt and unpredictable damages such as a sudden power surge that burns an electronic component. On the other hand, Predictable failure monitors mechanical wear and aging which takes place over time. In a nutshell, S.M.A.R.T functions like a monitoring device that reads and analyses the performance of a disk. If a gradual defection is observed (which can lead to a likely disk failure), a message will notify you to backup your data and and replace your disk

CD-ROM The definition of a CD-ROM CD-ROM describes a drive that is able to read digital data stored on a compact disk. Like an audio compact+ disk player, the CDROM uses a laser pickup mechanism to read the stored data. A laser beam shines on the compact disk, and if it is deflected by the disk to a sensor, the computer registers this as a "1". If no light is deflected, it is assumed as a "0". Note that these drives are only able to read data, but not able write data. Up to 680MB (ISO 9660) of digital data can be stored on a single compact disk. The basic CD-ROM spins the compact disk at the same rotational speed that a normal audio compact disk player spins at. This allows for a data transfer rate of 150Kb/s. What is a 2x CD-ROM? This is the same as the basic CD-ROM, but the compact disk has a rotational speed twice that of a normal audio compact disk player. This allows double the amount of information to be read from the disk per second - 300Kb/s.

DVD Definition of a DVD-ROM A Digital Video Disk (DVD) is similar to a normal compact disk, but it is able to store more than 4Gb of digital data on a single disk. The drive has a different frequency laser from normal CDROMs and hence DVD media cannot be used on a normal CDROM. What is a 2x DVD-ROM? This is the same as the basic DVD-ROM, but the Digital Video Disk has a rotational speed twice that of a normal DVD-ROM. This allows two times the amount of information to be read from the disk per second. The same goes for 5x etc.

DVD-RAM A DVD format wherein DVDRAM discs can be recorded and erased repeatedly but are only compatible with devices manufactured by the companies that support the DVD-RAM format