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Concept
Disk buffer
Summary
In computer storage, disk buffer (often ambiguously called disk cache or cache buffer) is the embedded memory in a hard disk drive (HDD) or solid state drive (SSD) acting as a buffer between the rest of the computer and the physical hard disk platter or flash memory 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 MB to 4 GB, 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 a disk's controller executes a physical read, the actuator moves the read/write head to (or near to) the correct cylinder. After some settling and possibly fine-actuating the read head begins to pick up track data, and all is left to do is wait until platter rotation brings the requested data.
The data read ahead of request during this wait is unrequested but free, so typically saved in the disk buffer in case it is requested later.
Similarly, data can be read for free behind the requested one if the head can stay on track because there is no other read to execute or the next actuating can start later and still complete in time.
Official source
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