Before overwriting a file system block, write the old contents of the block to
an undo file. This undo file can be used with e2undo(8) to restore the old
contents of the file system should something go wrong. If the empty string is
passed as the undo_file argument, the undo file will be written to a file named
e2undo-device.e2undo in the directory specified via the
E2FSPROGS_UNDO_DIR environment variable.
WARNING: The undo file cannot be used to recover from a power or system crash.