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.

(default off)

only when compiled with the +persistent_undo feature