synchronized (mutex) {
        return sortedMap(delegate().headMap(toKey), mutex);
      }
    }

    @Override
    public K lastKey() {
      synchronized (mutex) {
        return delegate().lastKey();
      }
    }

    @Override
    public SortedMap<K, V> subMap(K fromKey, K toKey) {
      synchronized (mutex) {

      synchronized (mutex) {
        return sortedMap(delegate().headMap(toKey), mutex);
      }
    }

    @Override
    public K lastKey() {
      synchronized (mutex) {
        return delegate().lastKey();
      }
    }

    @Override
    public SortedMap<K, V> subMap(K fromKey, K toKey) {
      synchronized (mutex) {

  @Override
  public Comparator<? super K> comparator() {
    return keySet().comparator();
  }

  @Override
  public K firstKey() {
    return keySet().first();
  }

  @Override
  public K lastKey() {
    return keySet().last();
  }

  private ImmutableSortedMap<K, V> getSubMap(int fromIndex, int toIndex) {
    if (fromIndex == 0 && toIndex == size()) {
      return this;

			done := globalScannerMetrics.time(scannerMetricTierObjSweep)
			globalExpiryState.enqueueFreeVersion(oi)
			done()
		}

		// These are rather expensive. Skip if nobody listens.
		if globalTrace.NumSubscribers(madmin.TraceScanner) > 0 {
			if len(fivs.FreeVersions) > 0 {
				res["free-versions"] = strconv.Itoa(len(fivs.FreeVersions))
			}

			if sizeS.versions > 0 {