Mass storage (secondary storage)

Mass storage Due to the volatility and limited size of a computer’s main memory, most computers have additional memory devices called mass storage (or secondary storage) system. example: magnetic disks, CDs, DVDs, magnetic tapes, and flash drivers.

Mass storage (secondary storage) Like magnetic disk, CD, DVD, magnetic tapes and flash drives Advantages of mass storage over main memory: include less volatile large storage capacity low cost many cases, the ability to remove the storage medium from the machine for archival purposes Disadvantages: A major disadvantage of the mass storage systems is that they typically require mechanical motion and therefore require significantly more time to store and retrieve data then a machine's main memory, where all activities are performed electronically

Mass Storage Systems Magnetic Systems Optical Systems Flash Drives Disk Tape Optical Systems CD DVD Flash Drives

On-line and Off-line: Means that the devices that can be either attached or de-attached from a machine On-line: means that the device or information is connected readily available to machine without human intervention Off-line: means that human intervention is required before the devices or information can be accessed by the machine.

Magnetic system: It is significant to two kinds: sequential access secondary storage Direct access secondary storage searching for a certain data means starting from the beginning of the file sequentially till the data is reached ( like magnetic reel tape and cassette ) Data can be reached directly without going through the stored data (like magnetic disk )

Magnetic system: sequential access secondary storage 1 ) Reel Magnetic Tape: Used in mainframe computers It has a plastic strip coated with one side with magnetized material (Chromium Dioxide) It is width 1/2 inch and it is length commonly 2400 feet (some reel with 300, 600 or 1200 feet ) The strip is divided into 7 or 9 tracks (9 common ) Strip with 9 tracks stores about 6250 characters per inch. In strip with 18 tracks, 38000 characters can be stored in 1 inch The devices that reads the tape is called tape drive it consists of: Two reels one for source and the other for destination R/W head which reads or writes the data on the strip Control unit which control in reading and writing operations

Magnetic tape An older from of mass storage Information is recorded on the magnetic coating of a thin tape that is wound on a reel. Taps is mounted in a device called tape drive (read/write). A major disadvantages of it, that moving between different position on tape can be very time-consuming

Figure 1.10 Magnetic tape storage

Magnetic system: Direct access secondary storage 2 ) magnetic disk Note: the capacity of disk storage system depends on the number of disk used and the density in which the tracks and sectors are placed Lower-capacity system consists of a single plastic disk known a diskette or floppy disk (off-line storage). 3 1/2 inch have a capacity of only 1.44 MB High capacity system like hard-disk (consist of 5-10 rigid disks mounted on a common spindle )

Mass Storage: Magnetic Disk Systems Magnetic disks: the most widely used storage medium in computers today Data is stored by magnetizing particles on the storage medium. Capacity depends on # of disks & density of tracks and sector placed on

Figure 1.9 A magnetic disk storage system

Magnetic systems Magnetic disk performance evaluation depends on four criteria's: Seek time, the time required to move the read/write head from track to another. Rotation delay (latency time), half -average calculation the time required for the disk to make complete rotation. Access time, the sum of the seek time and rotation delay. Transfer rate, the rate at which data can be transferred to/from the disk.

Mass Storage: Magnetic Disk Systems Two common types: Floppy disks: Diskette (low-capacity) Hard disks: (high-capacity)

Floppy Disks and Drives Floppy disk characteristics Single plastic flexible disk Typically 3½ inches in diameter. Typically hold 1.44 megabytes. Inserted into floppy disk drive to be read from or written to.

Floppy Disks and Drives, Cont’d. Disk is divided into tracks, sectors, and clusters.

Floppy Disks and Drives, Cont’d. Using floppy disks Must be inserted into the proper drive in the proper direction. Should not be removed when the disk is being accessed. High-capacity removable magnetic disks and drives Zip disks (750 MB) SuperDisks (240 MB)

Hard Disk System: Hard disk characteristics Usually (5-10) rigid disks mounted on a common spindle and combined into a disk pack Disk drive - a device that allows data to be read from or written to a disk Disk drive for personal computers contained within computer housing Can be internal or external. Large computer systems may have several external disk drives Capacity in gigabytes Accessing files much faster than accessing files on diskettes

Disk Packs Each disk has its own access arm with read/write head Most disk packs combine disks, access arms, and read/write head

Reading/Writing Data Access arm moves read/write head over particular location Read/write head hovers a few millionths of an inch above disk and doesn’t touch the surface of the disk. If head touches platter, a head crash occurs and data is destroyed Data can be destroyed if head touches miniscule foreign matter on surface of disk

How Data Is Organized Track Sector Cluster Cylinder Organized into tracks, sectors, clusters, and cylinders Track Sector Cluster Cylinder

Track The circular portion of the disk surface that passes under the read/write head Floppy diskette has 80 tracks on each surface Hard disk may have 1,000 or more tracks on each surface of each disk Return

Sector Each track is divided into sectors that hold a fixed number of bytes Typically 512 bytes per sector Zone recording assigns more sectors to tracks in outer zones than those in inner zones Uses storage space more fully Return

Cluster A fixed number of adjacent sectors that are treated as a unit of storage Typically two to eight sectors, depending on the operating system Return

Cylinder The track on each surface that is beneath the read/write head at a given position of the read/write heads When file is larger than the capacity of a single track, operating system will store it in tracks within the same cylinder Return

Disk Access Speed Access time - the time needed to access data on disk Three factors Seek time Head switching Rotational delay Once data found, next step is data transfer

Seek Time The time it takes the access arm to get into position over a particular track All access arms move as a unit All simultaneously in position over a set of tracks that make up a cylinder Return

Head Switching The activation of a particular read/write head over a particular track All access arms move together, but only one read/write head can operate at any one time Return

Rotational Delay The time it takes for the desired data on the track to rotate underneath the read/write head On average, half the time for a complete revolution of the disk Return

Data Transfer The process of transferring data between its location on the disk track and memory Measures of performance Average access time About 10 milliseconds (in electric circuits in nanoseconds!) Data transfer rate - how fast data can be transferred once it has been found Stated in terms of megabytes per second Return

Optical Systems: Like Compact Disk (CD): These disks are 12 cm (approximately 5 inches) in diameter and consist of reflective material covered with a clear protective coating The capacity of the CD about 600 to 700 MB. Information is recorded on them by crating variations in their reflective surfaces. This information can be retrieved by means of a laser beam. That monitors irregularities on the reflective surface of the CD.

Optical Systems: CD-DA: (Compact Disk Digital Audio): Which technology applied to audio recordings Information on these CDs is stored on a single track that spiral around the CD, this track divided into units called sectors. Each with its own identifying marking s and a capacity of 2KB of data Note: the distance around the spiraled track is greater from the outer edge of the disk than the inner portion Note: to maximize the capacity of a CD: information is stored at a uniform linear density over the entire spiraled track which means that more information is stored in a loop around the outer portion of spiral than in a loop around the inner portion

Mass Storage: Optical Systems Provides inexpensive and compact storage with greater capacity ( approx. 5 inches diameter) Information is recorded by creating variation in reflective services Information is retrieved by a laser beam that monitors irregularities on the reflective surface Laser scans disk and picks up light reflections from disk surface

Optical Systems: Compact Disks Categorized by read/write capability: CD-ROM - drive can only read data from CDs CD-ROM stores up to 700 MB per disk Primary medium for software distribution CD-R - drive can write to disk once Disk can be read by CD-ROM or CD-R drive CD-RW - drive can erase and record over data multiple times Some compatibility problems trying to read CD-RW disks on CD-ROM drives

Optical Systems: Digital Versatile Disk (DVD) Constructed from multiple, semi-transparent layers DVD drive can read CD-ROMs Capacity up to 17GB Allows for full-length movies Sound is better than on audio CDs Several versions of writable and rewritable DVDs exist Which are constructed from multiple layers that serve as distinct surfaces when viewed by precisely focused laser provide storage capacities of several GB. Such disks are storing lengthy multimedia presentations, including entire motion pictures

Mass Storage: Flash memory media Bits are stored by sending electronic signals directly to the storage medium where they causes electrons to be trapped in tiny chambers of silicon dioxide. Chip-based storage. This technology is suitable for off-line storage of data

Flash memory media Note: data stored in flash can be accessed in small byte-size units as in RAM applications. Repeated erasing slowly damages the silicon dioxide chambers, meaning that current flash memory technology is not suitable for general main memory application Since flash memory is not sensitive to physical shock, it is potential in portable applications is enticing

Flash memory media Flash drives, with capacities of up to a few GB, are available for general mass storage applications These units are packaged in small plastic cases approximately three inches long with the removable cap on one end to protected the unit's electrical connector when the drive is off-line Commonly used with digital cameras, digital music players, handheld PCs, notebook computers, smart phones, etc.

Flash memory media Flash memory media, cont’d. Common forms of flash memory: Flash memory sticks. Flash memory cards—CompactFlash, Secure Digital (SD), MiniSD, MultiMedia (MMC), SmartMedia, and xD Picture cards. Flash memory drives, such as USB flash drives.

File storage File: A unit of data stored in mass storage system data stored in a mass storage system is conceptually grouped into large unit called files Files stored on a magnetic disk must be manipulated by sector. A block of data conforming to the specific characteristics of a storage device is called a physical record. A large file stored in a mass storage will consists of many physical record. “the size of the sector”

File storage The nature f a file division is determined by the information represented in the file. File containing information regarding a company's employee would consists of multiple units. Each consists of information about one employee. These naturally producing “occurring” blocks of data are called logical records. Logical record: the nature divisions within the data

File storage

File storage Logical record consists of smaller units called fields. Employee name, address, employee ID, etc. Identifying a field by a key field. Buffer: A memory area used for the temporary storage of data (usually as a step in transferring the data)

File storage and retrieval: Field: set of characters Record: set of fields File: set of records Data base: set of files