@Override
  public void cacheBlock(BlockCacheKey cacheKey, Cacheable buf, boolean inMemory) {
    // This is the inclusive part of the combined block cache.
    // Every block is placed into both block caches.
    l1Cache.cacheBlock(cacheKey, buf, inMemory);

    // This assumes that insertion into the L2 block cache is either async or very fast.
    l2Cache.cacheBlock(cacheKey, buf, inMemory);
  }
@param cacheKey The block's cache key.
@param buf      The block contents wrapped in a ByteBuffer.
@param inMemory Whether block should be treated as in-memory. This parameter is only useful for
                the L1 lru cache.
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/InclusiveCombinedBlockCache.java