code: public SegmentIndexBuffer openSegmentIndexBuffer(int segId) {
          SegmentIndexBuffer sib = _sibLookup.get(segId);
          if(sib == null) {
              sib = new SegmentIndexBuffer();
              sib.setSegmentId(segId);
              _sibLookup.put(segId, sib);
          }
          return sib;
      }
docs: Opens a segment index buffer for the specified <code>segId</code>.
  
  @param segId - the segment Id
  @return the segment index buffer which is not <code>null</code>.