)A portion of RAM used to speed up access to data on a disk. The RAM can be part of the disk drive itself (sometimes called a hard disk cache or buffer) or it can be general-purpose RAM in the computer that is reserved for use by the disk drive (sometimes called a soft disk cache). Hard disk cachesare more effective, but they are also much more expensive, and therefore smaller. Nearly all modern disk drives include a small amount of internal cache.
A soft disk cache works by storing the most recently accessed data in the RAM cache. When a program needs to access new data, the operating system first checks to see if the data is in the cache before reading it from the disk. Because computers can access data from RAM much faster than from a disk, disk caching can significantly increase performance. Many cache systemsalso attempt to predict what data will be requested next so they can place that data in the cache ahead of time.
Although caching improves performance, there is some risk involved. If the computer crashes (due to a power failure, for example), the system may not have time to copy the cache back to the disk. In this case, whatever changes you made to the data will be lost. Usually, however, the cache system updates the disk frequently so that even if you lose some data, it will not be much. Caches that work in this manner are called write-back caches. Another type of disk cache, called a write-thru cache, removes the risk of losing data because it only caches data for read operations; writeoperations are always sent directly to the disk.
- disk compression
A type of data compression that works by storing compressed versions of files on the hard disk. A disk compression utility sits between the operating system and the disk drive. Whenever the operating system attempts to save a file to disk, the utility intercepts it and compresses it. Likewise, when the operating system attempts to […]
- disk controller
A chip and associated circuitry that is responsible for controlling a disk drive. There are different controllers for different interfaces. For example, an IDE interface requires an IDE controller and a SCSI interface requires a SCSI controller. On Macintosh computer systems, the disk controller is built into the motherboard. A controller for a hard disk […]
- Disk Drive
A disk drive is a randomly addressable and rewritable storage device. The term can be broadly interpreted to include optical drives and in earlier times, floppy drives. However, in popular usage, it has come to relate mainly to hard disk drives (HDDs). Disk drives can either be housed internally within a computer or housed in […]
- disk image
Also called a disk clone, an exact copy of a computer��s hard drive. The copy includes all the partition information, boot sectors, the file allocation table, operating system installation and application software. Disk images are used to transfer a hard drive��s contents during a hardware upgrade, to restore a hard drive��s contents during disaster recovery […]
- disk mirroring
A technique in which data is written to two duplicate disks simultaneously. This way if one of the disk drives fails, the system can instantly switch to the other disk without any loss of data or service. Disk mirroring is used commonly in on-line database systems where it’s critical that the data be accessible at […]