Editing Database engine (section)

===Database storage hierarchy===
A database, while in operation, resides simultaneously in several types of storage, forming a [[storage hierarchy]]. Inside of a contemporary computer hosting a DBMS, most of the "database" part resides, partially replicated, in [[Volatile memory|volatile storage]]. Data that are actively being processed and manipulated reside inside the [[Processor (computing)|processor]], possibly in [[CPU cache|processor's caches]]. These data are read from and written to memory, typically through a computer [[Bus (computing)|bus]], which is usually a volatile storage component. Computer memory communicates data to and from external storage, typically through standard storage interfaces or networks (e.g., [[fibre channel]], [[iSCSI]]). 

A [[Disk array|storage array]], a common external storage unit, typically has storage hierarchy of its own. A fast cache, typically consisting of volatile and fast [[Dynamic random-access memory|DRAM]], is connected (via standard interfaces) to drives. These drives may have different speeds, like [[USB flash drive|flash drives]] and [[Non-volatile memory|non-volatile]] magnetic [[disk drive]]s. Speed and price are generally correlated. The drives may be connected to [[magnetic tape]]s, on which the least active parts of a large database may reside. This may also be where [[Backup|backups]] are located.