@Override
  public Optional<Integer> getBlockSize(BlockCacheKey key) {
    Optional<Integer> l1Result = l1Cache.getBlockSize(key);
    return l1Result.isPresent() ? l1Result : l2Cache.getBlockSize(key);
  }
Returns the list of fully cached files
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/CombinedBlockCache.java