Disk buffer

In computer storage, disk buffer (often ambiguously called disk cache or cache buffer) is the embedded memory in a hard disk drive (HDD) acting as a buffer between the rest of the computer and the physical hard disk platter that is used for storage. Modern hard disk drives come with 8 to 256 MiB of such memory, and solid-state drives come with up to 4 GB of cache memory. In computer storage, disk buffer (often ambiguously called disk cache or cache buffer) is the embedded memory in a hard disk drive (HDD) acting as a buffer between the rest of the computer and the physical hard disk platter that is used for storage. Modern hard disk drives come with 8 to 256 MiB of such memory, and solid-state drives come with up to 4 GB of cache memory. Since the late 1980s, nearly all disks sold have embedded microcontrollers and either an ATA, Serial ATA, SCSI, or Fibre Channel interface. The drive circuitry usually has a small amount of memory, used to store the data going to and coming from the disk platters. The disk buffer is physically distinct from and is used differently from the page cache typically kept by the operating system in the computer's main memory. The disk buffer is controlled by the microcontroller in the hard disk drive, and the page cache is controlled by the computer to which that disk is attached. The disk buffer is usually quite small, ranging between 8 and 256 MiB, and the page cache is generally all unused main memory. While data in the page cache is reused multiple times, the data in the disk buffer is rarely reused. In this sense, the terms disk cache and cache buffer are misnomers; the embedded controller's memory is more appropriately called disk buffer. Note that disk array controllers, as opposed to disk controllers, usually have normal cache memory of around 0.5–8 GiB. When executing a read from the disk, the disk arm moves the read/write head to (or near) the correct track, and after some settling time the read head begins 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 typically saves these unrequested sectors in the disk buffer, in case the operating system requests them later. 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 hard disk platter. The disk buffer is used so that both the I/O interface and the disk read/write head can operate at full speed.