Secondary storage

Secondary storage requires the computer to use its input/output channels to access the information, and is used for long-term storage of persistent information. However most computer operating systems also use secondary storage devices as virtual memory - to artificially increase the apparent amount of main memory in the computer. Secondary storage is also known as "mass storage", as shown in the diagram above. Secondary or mass storage is typically of much greater capacity than primary storage (main memory), but it is also much slower. In modern computers, hard disks are usually used for mass 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 thousand-millionths of a second, or nanoseconds. This illustrates the very significant speed 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, are typically even slower than hard disks, although their access speeds are likely to improve with advances in technology. Therefore, the use of virtual memory, which is millions of times slower than "real" memory, significantly degrades the performance of any computer. Virtual memory is implemented by many operating systems using terms like swap file or "cache file". The main historical advantage of virtual memory was that it was much less expensive than real memory. That advantage is less relevant today, yet surprisingly most operating systems continue to implement it, despite the significant performance penalties.

Off-line storage is a system where the storage medium can be easily removed from the storage device. Off-line storage is used for data transfer and archival purposes. In modern computers, CDs, DVDs, memory cards, flash memory devices including "USB drives", floppy disks, Zip disks and magnetic tapes are commonly used for off-line mass storage purposes. "Hot-pluggable" USB hard disks are also available. Off-line storage devices used in the past include punched cards, microforms, and removable Winchester disk drums.