long getDirectMemoryUsage() {
      if (config.storeIndexMemState == IndexMemState.IN_DIRECT_MEM) {
        return serEntries.capacity();
      } else {
        return 0;
      }
    }
@return The direct memory usage for this Index Segment in bytes.
ambry-store/src/main/java/com/github/ambry/store/IndexSegment.java