often and less urgently, such as data kept for archiving or auditing purposes. Hot data should be stored in ways offering fast retrieval and modification, often accomplished by in-memory storage but not always. Cold data on the other hand can be stored in a more cost-effective way and is accepted that data access will likely be slower compared to hot data. While these descriptions are useful, "hot" and "cold" lack concrete definitions. Manufacturing efficiency provides another reason for selecting a combined in-memory/on-disk database system. Some device product lines, especially in consumer electronics, include some units with permanent storage, and others that rely on memory for storage (set-top boxes, for example). If such devices require a database system, a manufacturer can adopt a hybrid database system at lower and upper cost, and with less customization of code, rather than using separate in-memory and on-disk databases, respectively, for its disk-less and disk-based products. The first database engine to support both in-memory and on-disk tables in a single database, WebDNA, was released in 1995. Storage memory Another variation involves large amounts of nonvolatile memory in the server, for example, flash memory chips as addressable memory rather than structured as disk arrays. A database in this form of memory combines very fast access speed with persistence over reboots and power losses. See also Comparison of relational database management systems Distributed data store Embedded databases In-memory processing List of in-memory databases NoSQL NewSQL Notes References External links The Rebirth of the In-Memory Database Special Issue on Main-Memory Database Systems, IEEE Data Engineering Bulletin Computer memory Database management systems