{only when compiled with the |+persistent_undo| feature} When on, Vim automatically saves undo history to an undo file when writing a buffer to a file, and restores undo history from the same file on buffer read. The directory where the undo file is stored is specified by 'undodir'. For more information about this feature see |undo-persistence|. The undo file is not read when 'undoreload' causes the buffer from before a reload to be saved for undo. When 'undofile' is turned off the undo file is NOT deleted. NOTE: This option is reset when 'compatible' is set.