may be used to overwrite the source files but isn't guaranteed to be
effective on all filesystems and storage devices.
-- Direct I/O is not supported on encrypted files. Attempts to use
- direct I/O on such files will fall back to buffered I/O.
-
- The fallocate operations FALLOC_FL_COLLAPSE_RANGE,
FALLOC_FL_INSERT_RANGE, and FALLOC_FL_ZERO_RANGE are not supported
on encrypted files and will fail with EOPNOTSUPP.