protected ArrayList<HStoreFile> checkMinFilesCriteria(ArrayList<HStoreFile> candidates,
    int minFiles) {
    if (candidates.size() < minFiles) {
      if (LOG.isDebugEnabled()) {
        LOG.debug("Not compacting files because we only have " + candidates.size()
          + " files ready for compaction. Need " + minFiles + " to initiate.");
      }
      candidates.clear();
    }
    return candidates;
  }
@param candidates pre-filtrate
@return filtered subset forget the compactionSelection if we don't have enough files
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/compactions/SortedCompactionPolicy.java