Secondary Storage Devices

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Secondary Storage Used to hold data or information permanently. Also called auxiliary storage , external storage , or backing store . Lies outside the CPU. Non-volatile: data is not lost when power goes off. High capacity (Terabytes). Cost-effective compared to main memory. Reusable as data can be erased and stored any time the user requires. Can be sequential or random/direct access. Examples: Magnetic Tapes, Magnetic Disks, Optical disks, Flash memory, Memory cards. Secondary Storage Devices Characteristics Non-volatile: Data is not lost when power goes off. Capacity: The capacity of these devices is very high. It goes to Terabytes. Cost: They are cost effective as compared to the main memory. Reusability: They are reusable as data can be erased and stored any time the user requires. Access Method: Depending on their characteristics, these are sequential and random or direct access. Difference Between Sequential Access and Direct Access Storage Sequential Access Storage Direct Access Storage Storage mechanism where data is stored and read in a fixed or linear order. Direct access, also called Random access, is a storage mechanism where the data is stored and read directly from storage devices. Some types of sequential access are unavoidable, such as when playing back a cassette. In this system all data items are addressed independently. It is usually less expensive storage mechanism. It is more expensive storage mechanism. It is less efficient to access particular piece of data. It is more efficient to access particular piece of data. In sequential access storage devices, retrieval of information is indirect i.e., Scanning is involved to access record. In direct access storage devices, retrieval of information is direct without having to scan a series of records. Magnetic tape, audio or video cassette have sequential access mechanism. Magnetic disks, compact disks and all latest memory devices have direct access storage mechanism. Types of Secondary Storage Devices Magnetic Tapes Used for data storage for over 50 years. Sequential access storage device. Consists of a thin tape with a coating of fine magnetic material. Used for recording analog or digital data. Magnetic tapes generally have a slower data access rate but higher storage capacity (up to 30 Terabytes). Magnetic Disks Most common type of secondary storage devices. Made of either flexible plastic material (Floppy Disks) or rigid metal (Hard Disks). Magnetic disks are coated with a magnetic substance. A single disk can have multiple platters mounted on each other. Each surface has at least one read-write head. Common types: Floppy Disks and Hard Disks. Floppy Disks and Zip Disks Old type of magnetic storage. Popular during the 1980s and early 1990s. Now-a-days the floppy disks are obsolete. Hard Disk Drive (HDD) Non-volatile, random access large backing storage device. Consists of several inflexible, circular disks (platters), circuits, and read/write heads. Houses in a sealed unit to keep out contaminants. Data is stored magnetically. Data transfer speed improved with interfaces like SATA. Platters rotate at high speeds (4,200 to 15,000 rpm). Read-and-write head moves radially across the platter. Tracks: Concentric circles on the surface of a platter. Sectors: Divisions of tracks, holding a block of data. Inner tracks have more sectors than outer tracks. Hard Disk Drive Schematic Read/Write Head Actuator Spindle Platters Tracks