the dirty bit allows for a performance optimization. a page on disk that is paged in to physical memory, then read from, and subsequently paged out again does not need to be written back to disk, since the page has not changed. however, if the page was written to after it is paged in, its dirty bit will be set, indicating that the page must be written back to the backing store. this strategy requires that the backing store retain a copy of the page after it is paged in to memory. when a dirty bit is not used, the backing store need only be as large as the instantaneous total size of all paged-out pages at any moment. when a dirty bit is used, at all times some pages will exist in both physical memory and the backing store.