@Override
  public Iterator<CachedBlock> iterator() {
    return new Iterator<CachedBlock>() {
      @Override
      public boolean hasNext() {
        return false;
      }

      @Override
      public CachedBlock next() {
        throw new NoSuchElementException("MemcachedBlockCache can't iterate over blocks.");
      }

      @Override
      public void remove() {

      }
    };
  }
This method does nothing so that memcached can handle all evictions.
hbase-external-blockcache/src/main/java/org/apache/hadoop/hbase/io/hfile/MemcachedBlockCache.java