The first NVDIMM solutions were designed with supercapacitors instead of batteries for the backup power source. With this storage, IMDB can resume securely from its state upon reboot. Non-volatile random-access memory (NVRAM), usually in the form of static RAM backed up with battery power (battery RAM), or an electrically erasable programmable ROM (EEPROM). With this storage, the re-booting IMDB system can recover the data store from its last consistent state. High availability implementations that rely on database replication, with automatic failover to an identical standby database in the event of primary database failure. To protect against loss of data in the case of a complete system crash, replication of an IMDB is normally used in addition to one or more of the mechanisms listed above. Some IMDBs allow the database schema to specify different durability requirements for selected areas of the database thus, faster-changing data that can easily be regenerated or that has no meaning after a system shut-down would not need to be journaled for durability (though it would have to be replicated for high availability), whereas configuration information would be flagged as needing preservation. Hybrids with on-disk databases While storing data in-memory confers performance advantages, it is an expensive method of data storage. An approach to realising the benefits of in-memory storage while limiting its costs is to store the most frequently accessed data in-memory and the rest on disk. Since there is no hard distinction between which data should be stored in-memory and which should be stored on disk, some systems dynamically update where data is stored based on the data's usage. This approach is subtly different from caching, in which the most recently accessed data is cached, as opposed to the most frequently accessed data being stored in-memory. The flexibility of hybrid approaches allow a balance to be struck between: performance (which is enhanced by sorting, storing and retrieving specified data entirely in memory, rather than going to disk) cost, because a less costly hard disk can be substituted for more memory persistence form factor, because RAM chips cannot approach the density of a small hard drive In the cloud computing industry the terms "data temperature", or "hot data" and "cold data" have emerged to describe how data is stored in this respect. Hot data is used to describe mission-critical data that needs to be accessed frequently while cold data describes data that is needed less