External Memory Algorithms - Hierarchy of Storage - Secondary Storage

Secondary storage (also known as external memory or auxiliary storage), differs from primary storage in that it is not directly accessible by the CPU. The computer usually uses its input/output channels to access secondary storage and transfers the desired data using intermediate area in primary storage. Secondary storage does not lose the data when the device is powered down—it is non-volatile. Per unit, it is typically also two orders of magnitude less expensive than primary storage. Modern computer systems typically have two orders of magnitude more secondary storage than primary storage and data are kept for a longer time there.

In modern computers, hard disk drives are usually used as secondary storage. The time taken to access a given byte of information stored on a hard disk is typically a few thousandths of a second, or milliseconds. By contrast, the time taken to access a given byte of information stored in random-access memory is measured in billionths of a second, or nanoseconds. This illustrates the significant access-time difference which distinguishes solid-state memory from rotating magnetic storage devices: hard disks are typically about a million times slower than memory. Rotating optical storage devices, such as CD and DVD drives, have even longer access times. With disk drives, once the disk read/write head reaches the proper placement and the data of interest rotates under it, subsequent data on the track are very fast to access. To reduce the seek time and rotational latency, data are transferred to and from disks in large contiguous blocks.

When data reside on disk, block access to hide latency offers a ray of hope in designing efficient external memory algorithms. Sequential or block access on disks is orders of magnitude faster than random access, and many sophisticated paradigms have been developed to design efficient algorithms based upon sequential and block access. Another way to reduce the I/O bottleneck is to use multiple disks in parallel in order to increase the bandwidth between primary and secondary memory.

Some other examples of secondary storage technologies are: flash memory (e.g. USB flash drives or keys), floppy disks, magnetic tape, paper tape, punched cards, standalone RAM disks, and Iomega Zip drives.

The secondary storage is often formatted according to a file system format, which provides the abstraction necessary to organize data into files and directories, providing also additional information (called metadata) describing the owner of a certain file, the access time, the access permissions, and other information.

Most computer operating systems use the concept of virtual memory, allowing utilization of more primary storage capacity than is physically available in the system. As the primary memory fills up, the system moves the least-used chunks (pages) to secondary storage devices (to a swap file or page file), retrieving them later when they are needed. As more of these retrievals from slower secondary storage are necessary, the more the overall system performance is degraded.

Read more about this topic:  External Memory Algorithms, Hierarchy of Storage

Other articles related to "storage, secondary storage":

IBM Parallel Sysplex - Geographically Dispersed Parallel Sysplex
... Data is copied from the primary storage device to a secondary storage device ... In the event of a failure on the primary storage device, the system automatically makes the secondary storage device the primary, usually without ... In the event of a failure of a system or storage device, recovery can occur with limited or no data loss automatically ...
Memory Leak - Effects
... operating systems have both main memory which is physically housed in RAM microchips, and secondary storage such as a hard drive ... into main memory for fast access inactive pages are pushed out to secondary storage to make room, as needed ... it usually occupies more and more of main memory, pushing other programs out to secondary storage - usually significantly slowing performance of the ...
Portable Hard Drives
... A hard disk drive (HDD) is a data storage device used for storing and retrieving digital information using rapidly rotating discs (platters) coated with magnetic material ... IBM in 1956, HDDs became the dominant secondary storage device for general purpose computers by the early 1960s ... As of 2012, the primary competing technology for secondary storage is flash memory in the form of solid-state drives (SSDs) ...
Data Deduplication - Deduplication Overview - Deduplication Methods
... complete files are compared, which is called Single Instance Storage or SIS ... Primary storage and secondary storage ... By definition, primary storage systems are designed for optimal performance, rather than lowest possible cost ...

Famous quotes containing the words storage and/or secondary:

    Many of our houses, both public and private, with their almost innumerable apartments, their huge halls and their cellars for the storage of wines and other munitions of peace, appear to me extravagantly large for their inhabitants. They are so vast and magnificent that the latter seem to be only vermin which infest them.
    Henry David Thoreau (1817–1862)

    Cloud-clown, blue painter, sun as horn,
    Hill-scholar, man that never is,
    The bad-bespoken lacker,
    Ancestor of Narcissus, prince
    Of the secondary men. There are no rocks
    And stones, only this imager.
    Wallace Stevens (1879–1955)