long getLogUsedCapacity() {
    ConcurrentSkipListMap<Offset, IndexSegment> indexSegments = validIndexSegments;
    return getAbsolutePositionInLogForOffset(getCurrentEndOffset(indexSegments), indexSegments);
  }
@return the currently capacity used of the log abstraction. Includes "wasted" space at the end of
{@link LogSegment} instances that are not fully filled.
ambry-store/src/main/java/com/github/ambry/store/PersistentIndex.java