public long getWritePoint() {
    if (writePos.bufferIndex == -1) {
      nextBufferToWrite();
    }
    return ((long)writePos.bufferIndex << wbSizeLog2) + writePos.offset;
  }
Compares the current readPosition of the buffer with the external byte array.
Does not modify readPoint.
serde/src/java/org/apache/hadoop/hive/serde2/WriteBuffers.java