long dataSize() {
            return (long) dpnSize + (long) indexSize + (long) extIndexSize + packSize;
        }
Store segment column by column.

<pre>
Segment storage structure:
|  IXRSEGxx | sementMeta | column0 | column1 ... | outerIndex0 | outerIndex0 ... |
            | <---                      sectionSize                        ----> |

Column structure:
|  dpn | rsIndex | extIndex | dataPack |
</pre>
indexr-segment/src/main/java/io/indexr/segment/storage/itg/IntegrateV1.java