“mke2fs -z undo_file”选项有什么用途？

Question

 -z undo_file
          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
          mke2fs-device.e2undo  in  the  directory  specified  via the E2FSPROGS_UNDO_DIR environment variable or the
          undo_dir directive in the configuration file.

          WARNING: The undo file cannot be used to recover from a power or system crash.