taklwu commented on a change in pull request #3684:
URL: https://github.com/apache/hbase/pull/3684#discussion_r710335335



##########
File path: 
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/IndexOnlyLruBlockCache.java
##########
@@ -0,0 +1,50 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.hadoop.hbase.io.hfile;
+
+import org.apache.hadoop.conf.Configuration;
+import org.apache.yetus.audience.InterfaceAudience;
+
+/**
+ * An on heap block cache implementation extended LruBlockCache and only cache 
index block.
+ * This block cache should be only used by
+ * {@link org.apache.hadoop.hbase.client.ClientSideRegionScanner} that 
normally considers to be
+ * used by client resides out of the region server, e.g. a container of a map 
reduce job.
+ **/
[email protected]
+public class IndexOnlyLruBlockCache extends LruBlockCache {
+
+  public IndexOnlyLruBlockCache(long maxSize, long blockSize, boolean 
evictionThread,
+    Configuration conf) {
+    super(maxSize, blockSize, evictionThread, conf);
+  }
+
+  /**
+   * Cache only index block with the specified name and buffer
+   * @param cacheKey block's cache key
+   * @param buf      block buffer
+   * @param inMemory if block is in-memory
+   */
+  @Override
+  public void cacheBlock(BlockCacheKey cacheKey, Cacheable buf, boolean 
inMemory) {
+    boolean isIndexBlock = buf.getBlockType().getCategory() != 
BlockType.BlockCategory.DATA;

Review comment:
       ack and done




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