Disk cache is memory used by the operating system to store copies of disk blocks temporarily. All file system based disk reads and writes usually pass through a disk cache. From an application's standpoint, all reads and writes involving disk caches are equivalent to actual disk operations.
Operating systems use two different methods to allocate memory to disk cache: fixed and dynamic. A preset amount of memory is used in a fixed allocation; usually a 10–15 percent memory allocation is set aside. The operating system usually manages this workspace using a LRU (least recently used) algorithm. For a dynamic allocation, the operating system determines the disk cache allocation as it is running. The goal is to keep as much memory in active use as possible, balancing the demand for real memory against the need for data from disk.
