private long getDataSize() throws IOException {
        return ((SeekableInputStream) data).getSize();
    }
This is an implementation of <code>SegmentInputStream</code> that uses random
access files for the data and index.
<p>
Also new handles working within a windows on the underlying data. With this
mode the segments are logical (i.e. they start at 0 regardless of the
window).
stroom-data/stroom-data-store-impl-fs/src/main/java/stroom/data/store/impl/fs/RASegmentInputStream.java