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In computer science, a cache is a collection of duplicate data, where the original data is expensive to fetch or compute (usually in terms of access time) relative to the cache. Future accesses to the data can be made by accessing the cached copy rather than refetching or recomputing the original data, so that the perceived average access time is lower.

The reason caches work at all is that many access patterns in typical computer applications have locality of reference. There are several sorts of locality, but we mainly mean that often the same data is accessed frequently or with accesses that are close together in time, or that data near to each other are accessed close together in time.

1 CPU caches

Main article: CPU cache

Small memories on or close to the CPU chip can be made faster than the much larger main memory. Most CPUs since the 1980s have used one or more caches, and modern general-purpose CPUs inside personal computers may have as many as half a dozen, each specialized to a different part of the problem of executing programs.

2 Disk buffer

(also known as Disk Cache or Cache Buffer)

Hard disks have historically often been packaged with embedded computers used for control and interface protocols. Since the late 1980s, nearly all disks sold have these embedded computers and either an ATA, SCSI, or Fibre Channel interface. The embedded computer usually has some small amount of memory which it uses to store the bits going to and coming from the disk platter.

The disk buffer is physically distinct from and is used differently than the page cache typically kept by the operating system in the computer's main memory. The disk buffer is controlled by the embedded computer in the disk drive, and the page cache is controlled by the computer to which that disk is attached. The disk buffer is usually quite small, 2 to 8 MB, and the page cache is generally all unused physical memory, which in a 2004 PC may be between 20 and 2000 MB. And while data in the page cache is reused multiple times, the data in the disk buffer is typically never reused. In this sense, the phrases disk cache and cache buffer are misnomers, and the embedded computer's memory is more appropriately called the disk buffer.

The disk buffer has multiple uses:

• Readahead / readbehind: When executing a read from the disk, the disk arm will move the read/write head to (or near) the correct track, and after some settling time the read head will begin to pick up bits. Usually, the first sectors to be read are not the ones that have been requested by the operating system. The disk's embedded computer will typically save these unrequested sectors in the disk buffer, in case the operating system requests them later.

• Speed matching: The speed of the disk's I/O interface to the computer almost never matches the speed at which the bits are transferred to and from the platter. The disk buffer is used so that both the I/O interface and the disk read/write head can operate at full speed.

• Write acceleration: The disk's embedded computer may signal the main computer that a disk write is complete immediately after receiving the write data, before the data is actually written to the platter. This early signal allows the main computer to continue working, but is somewhat dangerous because, if power is lost before the data is permanently fixed in the magnetic media, the data will be lost from the disk buffer, and the filesystem on the disk may be left in an inconsistent state. Write acceleration is controversial, and for this reason can usually be turned off. On some disks, this vulnerable period between signalling the write complete and fixing the data can be arbitrarily long, as the write can be deferred indefinitely by newly arriving requests. Write acceleration is generally never used on database servers or other machines where the integrity of the data on the disks is very important.

• Command queueing: Newer SATA and most SCSI disks can accept multiple commands while any one command is in operation. These commands are stored by the disk's embedded computer until they are completed. Should a read reference the data at the destination of a queued write, the write's data will be returned. Command queueing is different than write acceleration in that the main computer's operating system is notified when data is actually written onto the magnetic media. The O/S can use this information to keep the filesystem consistent through rescheduled writes.