 Otherwise, the page(s) are fetched from disk and requested modifications are done.A file that is created or opened in the page cache, but not written to, might result in a zero byte file at a later read.However, not all cached pages can be written to as program code is often mapped as read-only or copy-on-write; in the latter case, modifications to code will only be visible to the process itself and will not be written to disk.