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In computer storage, a disk buffer (often ambiguously called a disk cache or a 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 Mebibyte 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 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 , usually have normal cache memory of around 0.5–8 GiB.
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. Disks for Data Centers.
If several requested reads are on the same track (or close by on a spiral track), most unrequested data between them will be both read ahead and behind.
On some disks, this vulnerable period between signaling the write complete and fixing the data can be arbitrarily long, as the write can be deferred indefinitely by newly arriving requests. For this reason, the use of write acceleration can be controversial. Consistency can be maintained, however, by using a battery-backed memory system for caching data, although this is typically only found in high-end .
Alternatively, the caching can simply be turned off when the integrity of data is deemed more important than write performance. Another option is to send data to disk in a carefully managed order and to issue "cache flush" commands in the right places, which is usually referred to as the implementation of .
NCQ is usually used in combination with enabled write buffering. In case of a read/write command with Force Unit Access (FUA, see below) bit set to 0 and enabled write buffering, an operating system may see the write operation finished before the data is physically written to the media. In case of FUA bit set to 1 and enabled write buffering, write operation returns only after the data is physically written to the media.
An operating system will send FLUSH CACHE and STANDBY IMMEDIATE command to hard disk drives in the shutdown process.
Mandatory cache flushing is used in Linux for write barriers in some filesystems (for example, ext4), together with Force Unit Access write command for journal commit blocks.
FUA appeared in the SCSI command set, and was later adopted by SATA with NCQ. FUA is more fine-grained as it allows a single write operation to be forced to stable media and thus has smaller overall performance impact when compared to commands that flush the entire disk cache, such as the ATA FLUSH CACHE family of commands. (2025). 9780080553849, Morgan Kaufmann. ISBN 9780080553849
Windows (Vista and up) supports FUA as part of Transactional NTFS, but only for SCSI or Fibre Channel disks where support for FUA is common. It is not known whether a SATA drive that supports FUA write commands will actually honor the command and write data to disk platters as instructed; thus, Windows 8 and Windows Server 2012 instead send commands to flush the disk write cache after certain write operations.
Although the Linux kernel gained support for NCQ around 2007, SATA FUA remains disabled by default because of regressions that were found in 2012 when the kernel's support for FUA was tested. The Linux kernel supports FUA at the block layer level.
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