In a computer, storage is the place where data is held in an electromagnetic or optical form for access by a computer processor. There are two general usages.
1) Storage is frequently used to mean the devices and data connected to the computer through input/output operations - that is, hard disk and tape systems and other forms of storage that don't include computer memory and other in-computer storage. For the enterprise, the options for this kind of storage are of much greater variety and expense than that related to memory. This meaning is probably more common in the IT industry than meaning 2.
2) In a more formal usage, storage has been divided into: (1) primary storage, which holds data in memory (sometimes called random access memory or RAM) and other "built-in" devices such as the processor's L1 cache, and (2) secondary storage, which holds data on hard disks, tapes, and other devices requiring input/output operations.
Primary storage is much faster to access than secondary storage because of the proximity of the storage to the processor or because of the nature of the storage devices. On the other hand, secondary storage can hold much more data than primary storage.
In addition to RAM, primary storage includes read-only memory (ROM) and L1 and L2 cache memory. In addition to hard disks, secondary storage includes a range of device types and technologies, including diskettes, Zip drives, redundant array of independent disks (RAID) systems, and holographic storage. Devices that hold storage are collectively known as storage media.
A somewhat antiquated term for primary storage is main storage and a somewhat antiquated term for secondary storage is auxiliary storage. Note that, to add to the confusion, there is an additional meaning for primary storage that distinguishes actively used storage from backup storage.
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