the device before this offset us not touched, unless it is
used for storing metadata (Formats 1.1 and 1.2).
+ size
+ The amount of the device, after the offset, that can be used
+ for storage of data. This will normally be the same as the
+ component_size. This can be written while assembling an
+ array. If a value less than the current component_size is
+ written, component_size will be reduced to this value.
+
An active md device will also contain and entry for each active device
in the array. These are named