Page cache

In computing, a page cache, sometimes also called disk cache,^[1] is a transparent cache for the pages originating from a secondary storage device such as a hard disk drive (HDD) or a solid-state drive (SSD). The operating system keeps a page cache in otherwise unused portions of the main memory (RAM), resulting in quicker access to the contents of cached pages and overall performance improvements. A page cache is implemented in kernels with the paging memory management, and is mostly transparent to applications.

Usually, all physical memory not directly allocated to applications is used by the operating system for the page cache. Since the memory would otherwise be idle and is easily reclaimed when applications request it, there is generally no associated performance penalty and the operating system might even report such memory as "free" or "available".

When compared to main memory, hard disk drive read/writes are slow and random accesses require expensive disk seeks; as a result, larger amounts of main memory bring performance improvements as more data can be cached in memory.^[2] Separate disk caching is provided on the hardware side, by dedicated RAM or NVRAM chips located either in the disk controller (in which case the cache is integrated into a hard disk drive and usually called disk buffer^[3]), or in a disk array controller, such memory should not be confused with the page cache. The operating system may also use some of main memory as filesystem write buffer, it may be called page buffer.^[4]