+Start I/O at the provided offset in the file, given as either a fixed size or a
+percentage. If a percentage is given, the next \fBblockalign\fR-ed offset will
+be used. Data before the given offset will not be touched. This effectively
+caps the file size at (real_size - offset). Can be combined with \fBsize\fR to
+constrain the start and end range of the I/O workload.