2 File storage and retrieval: Field: set of charactersRecord: set of fieldsFile: set of recordsData base: set of files
3 Mass storage (secondary storage) Like magnetic disk, CD, DVD, magnetic tapes and flash drivesAdvantages of mass storage over main memory:include less volatilelarge storage capacitylow costmany cases, the ability to remove the storage medium from the machine for archival purposesDisadvantages: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
4 On-line and Off-line:Means that the devices that can be either attached or de-attached from a machineOn-line: means that the device or information is connected readily available to machine without human interventionOff-line: means that human intervention is required before the devices or information can be accessed by the machine.
5 Magnetic system: It is significant to two kinds: sequential access secondary storageDirect access secondary storagesearching 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 )
6 Magnetic system: sequential access secondary storage 1 ) Reel Magnetic Tape:Used in mainframe computersIt 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, characters can be stored in 1 inchThe devices that reads the tape is called tape drive it consists of:Two reels one for source and the other for destinationR/W head which reads or writes the data on the stripControl unit which control in reading and writing operations
7 Magnetic system: Direct access secondary storage 2 ) magnetic diskNote: the capacity of disk storage system depends on the number of disk used and the density in which the tracks and sectors are placedLower-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 MBHigh capacity system like hard-disk (consist of rigid disks mounted on a common spindle )
8 Mass Storage: Magnetic Disk Systems Magnetic disks: the most widely used storage medium in computers todayData is stored by magnetizing particles on the storage medium.Capacity depends on # of disks & density of tracks and sector placed on
10 Mass Storage: Magnetic Disk Systems Two common types:Floppy disks: Diskette (low-capacity)Hard disks: (high-capacity)
11 Floppy Disks and Drives Floppy disk characteristicsSingle plastic flexible diskTypically 3½ inches in diameter.Typically hold 1.44 megabytes.Inserted into floppy disk drive to be read from or written to.
13 Floppy Disks and Drives, Cont’d. Disk is divided into tracks, sectors, and clusters.
14 Floppy Disks and Drives, Cont’d. Using floppy disksMust 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 drivesZip disks (750 MB)SuperDisks (240 MB)
15 Hard Disk System: Hard disk characteristics Usually (5-10) rigid disks mounted on a common spindle and combined into a disk packDisk drive - a device that allows data to be read from or written to a diskDisk drive for personal computers contained within computer housingCan be internal or external.Large computer systems may have several external disk drivesCapacity in gigabytesAccessing files much faster than accessing files on diskettes
17 Disk Packs Each disk has its own access arm with read/write head Most disk packs combine disks, access arms, and read/write head
18 Reading/Writing DataAccess arm moves read/write head over particular locationRead/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 destroyedData can be destroyed if head touches miniscule foreign matter on surface of disk
19 How Data Is Organized Track Sector Cluster Cylinder Organized into tracks, sectors, clusters, and cylindersTrackSectorClusterCylinder
20 TrackThe circular portion of the disk surface that passes under the read/write headFloppy diskette has 80 tracks on each surfaceHard disk may have 1,000 or more tracks on each surface of each diskReturn
21 SectorEach track is divided into sectors that hold a fixed number of bytesTypically 512 bytes per sectorZone recording assigns more sectors to tracks in outer zones than those in inner zonesUses storage space more fullyReturn
22 ClusterA fixed number of adjacent sectors that are treated as a unit of storageTypically two to eight sectors, depending on the operating systemReturn
23 CylinderThe track on each surface that is beneath the read/write head at a given position of the read/write headsWhen file is larger than the capacity of a single track, operating system will store it in tracks within the same cylinderReturn
24 Disk Access Speed Access time - the time needed to access data on disk Three factorsSeek timeHead switchingRotational delayOnce data found, next step is data transfer
25 Seek TimeThe time it takes the access arm to get into position over a particular trackAll access arms move as a unitAll simultaneously in position over a set of tracks that make up a cylinderReturn
26 Head SwitchingThe activation of a particular read/write head over a particular trackAll access arms move together, but only one read/write head can operate at any one timeReturn
27 Rotational DelayThe time it takes for the desired data on the track to rotate underneath the read/write headOn average, half the time for a complete revolution of the diskReturn
28 Data TransferThe process of transferring data between its location on the disk track and memoryMeasures of performanceAverage access timeAbout 10 milliseconds (in electric circuits in nanoseconds!)Data transfer rate - how fast data can be transferred once it has been foundStated in terms of megabytes per secondReturn
29 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 coatingThe 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.
30 Optical Systems: CD-DA: (Compact Disk Digital Audio): Which technology applied to audio recordingsInformation 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 dataNote: the distance around the spiraled track is greater from the outer edge of the disk than the inner portionNote: 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
31 Mass Storage: Optical Systems Provides inexpensive and compact storage with greater capacity ( approx. 5 inches diameter)Information is recorded by creating variation in reflective servicesInformation is retrieved by a laser beam that monitors irregularities on the reflective surfaceLaser scans disk and picks up light reflections from disk surface
32 Optical Systems: Compact Disks Categorized by read/write capability:CD-ROM - drive can only read data from CDsCD-ROM stores up to 700 MB per diskPrimary medium for software distributionCD-R - drive can write to disk onceDisk can be read by CD-ROM or CD-R driveCD-RW - drive can erase and record over data multiple timesSome compatibility problems trying to read CD-RW disks on CD-ROM drives
33 Optical Systems: Digital Versatile Disk (DVD) Constructed from multiple, semi-transparent layersDVD drive can read CD-ROMsCapacity up to 17GBAllows for full-length moviesSound is better than on audio CDsSeveral versions of writable and rewritable DVDs existWhich 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
34 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
35 Flash memory mediaNote: 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 applicationSince flash memory is not sensitive to physical shock, it is potential in portable applications is enticing
36 Flash memory mediaFlash drives, with capacities of up to a few GB, are available for general mass storage applicationsThese 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-lineCommonly used with digital cameras, digital music players, handheld PCs, notebook computers, smart phones, etc.
37 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.