Correct the comment of caching index & filter block with additional o…

…ptions to improve performance

More internal details could refer to https://github.com/facebook/rocksdb/wiki/Block-Cache#caching-index-filter-and-compression-dictionary-blocks

bookkeeper-server/src/main/java/org/apache/bookkeeper/bookie/storage/ldb/KeyValueStorageRocksDB.java

@@ -149,8 +149,12 @@ public KeyValueStorageRocksDB(String basePath, String subPath, DbConfigType dbCo
                     tableOptions.setFilterPolicy(new BloomFilter(bloomFilterBitsPerKey, false));
                 }
 
-                // Options best suited for HDDs
+                // Cache index & filter in block cache to limit the memory usage
                 tableOptions.setCacheIndexAndFilterBlocks(true);
+                // Options below are tune to mitigate the performance regression when limiting the memory usage
+                tableOptions.setCacheIndexAndFilterBlocksWithHighPriority(true);
+                tableOptions.setPinL0FilterAndIndexBlocksInCache(true);
+
                 options.setLevelCompactionDynamicLevelBytes(true);
 
                 options.setTableFormatConfig(tableOptions);