 High-end hardware disk controllers of the type found in servers include a small battery back-up unit on their cache memory so that they may offer the performance gains of write caching while mitigating the risk of unintended shutdowns.The battery back-up unit keeps the memory powered even during a shutdown so that when the computer is powered back up, it can quickly complete any writes it has previously committed.With such a controller, the operating system may request four writes (1-2-3-4) in that order, but the controller may decide the quickest way to write them is 4-3-1-2.