'Sub-command that reads from srcpath and writes to dstpath.
Throttle disk I/O bandwidth used by a sub-command, such as \'dd\',
that reads from srcpath and writes to dstpath. The sub-command
must be executed with the generated prefix command.'

@contextlib.contextmanager
def subcommand(self, srcpath, dstpath):

    (yield {'prefix': self.prefix})
