Editing Computer data storage (section)

=== Secondary storage ===
<!-- Note that additional direct links point to this section by its name. -->''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 transfer the desired data to primary storage. Secondary storage is non-volatile (retaining data when its power is shut off). Modern computer systems typically have two orders of magnitude more secondary storage than primary storage because secondary storage is less expensive.

In modern computers, [[hard disk drive]]s (HDDs) or [[solid-state drive]]s (SSDs) are usually used as secondary storage. The [[access time]] per byte for HDDs or SSDs is typically measured in [[millisecond]]s (thousandths of a second), while the access time per byte for primary storage is measured in [[nanosecond]]s (billionths of a second). Thus, secondary storage is significantly slower than primary storage. Rotating [[Optical disc drive|optical storage]] devices, such as [[CD]] and [[DVD]] drives, have even longer access times. Other examples of secondary storage technologies include [[USB flash drive]]s, [[floppy disk]]s, [[magnetic-tape data storage|magnetic tape]], [[paper tape]], [[punched card]]s, and [[RAM drive|RAM disks]].

Once the [[disk read/write head]] on HDDs reaches the proper placement and the data, 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. 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 on sequential and block access. Another way to reduce the I/O bottleneck is to use multiple disks in parallel to increase the bandwidth between primary and secondary memory, for example, using [[RAID]].<ref>{{cite book|author=[[J. S. Vitter]]|url=http://www.ittc.ku.edu/~jsv/Papers/Vit.IO_book.pdf|url-status=live|title=Algorithms and data structures for external memory|archive-url=https://web.archive.org/web/20110104233254/http://www.ittc.ku.edu/~jsv/Papers/Vit.IO_book.pdf |archive-date=4 January 2011|series=Series on foundations and trends in theoretical computer science|publisher=now Publishers|location=Hanover, MA|year=2008|isbn=978-1-60198-106-6}}</ref>

Secondary storage is often formatted according to a [[file system]] format, which provides the abstraction necessary to organize data into [[Computer file|files]] and [[Directory (computing)|directories]], while also providing [[metadata]] describing the owner of a certain file, the access time, the access permissions, and other information.

Most computer [[operating system]]s use the concept of [[virtual memory]], allowing the 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 ([[Page (computer memory)|pages]]) to a swap file or page file on secondary storage, retrieving them later when needed. If a lot of pages are moved to slower secondary storage, the system performance is degraded.

The secondary storage, including [[Hard disk drive|HDD]], [[optical disc drive|ODD]] and [[Solid state drive|SSD]], are usually block-addressable.