/**
    * Copyright The Apache Software Foundation
    *
    * 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.bucket;
  
  import java.io.File;
  import java.io.FileInputStream;
  import java.io.FileNotFoundException;
  import java.io.FileOutputStream;
  import java.io.IOException;
  import java.io.ObjectInputStream;
  import java.io.ObjectOutputStream;
  import java.io.Serializable;
  import java.nio.ByteBuffer;
  import java.util.ArrayList;
  import java.util.Comparator;
  import java.util.HashSet;
  import java.util.Iterator;
  import java.util.List;
  import java.util.Map;
  import java.util.NavigableSet;
  import java.util.PriorityQueue;
  import java.util.Set;
  import java.util.concurrent.ArrayBlockingQueue;
  import java.util.concurrent.BlockingQueue;
  import java.util.concurrent.ConcurrentHashMap;
  import java.util.concurrent.ConcurrentMap;
  import java.util.concurrent.ConcurrentSkipListSet;
  import java.util.concurrent.Executors;
  import java.util.concurrent.ScheduledExecutorService;
  import java.util.concurrent.TimeUnit;
  import java.util.concurrent.atomic.AtomicLong;
  import java.util.concurrent.locks.Lock;
  import java.util.concurrent.locks.ReentrantLock;
  import java.util.concurrent.locks.ReentrantReadWriteLock;
  
  import org.apache.commons.logging.Log;
  import org.apache.commons.logging.LogFactory;
  import org.apache.hadoop.hbase.classification.InterfaceAudience;
  import org.apache.hadoop.hbase.io.HeapSize;
  import org.apache.hadoop.hbase.io.hfile.BlockCache;
  import org.apache.hadoop.hbase.io.hfile.BlockCacheKey;
  import org.apache.hadoop.hbase.io.hfile.BlockCacheUtil;
  import org.apache.hadoop.hbase.io.hfile.BlockPriority;
  import org.apache.hadoop.hbase.io.hfile.BlockType;
  import org.apache.hadoop.hbase.io.hfile.CacheStats;
  import org.apache.hadoop.hbase.io.hfile.Cacheable;
  import org.apache.hadoop.hbase.io.hfile.CacheableDeserializer;
  import org.apache.hadoop.hbase.io.hfile.CacheableDeserializerIdManager;
  import org.apache.hadoop.hbase.io.hfile.CachedBlock;
  import org.apache.hadoop.hbase.io.hfile.HFileBlock;
  import org.apache.hadoop.hbase.util.EnvironmentEdgeManager;
  import org.apache.hadoop.hbase.util.HasThread;
  import org.apache.hadoop.hbase.util.IdReadWriteLock;
  import org.apache.hadoop.util.StringUtils;
  
  import com.google.common.annotations.VisibleForTesting;
  import com.google.common.util.concurrent.ThreadFactoryBuilder;
  
  /**
   * BucketCache uses {@link BucketAllocator} to allocate/free blocks, and uses
   * BucketCache#ramCache and BucketCache#backingMap in order to
   * determine if a given element is in the cache. The bucket cache can use on-heap or
   * off-heap memory {@link ByteBufferIOEngine} or in a file {@link FileIOEngine} to
   * store/read the block data.
   *
   * <p>Eviction is via a similar algorithm as used in
   * {@link org.apache.hadoop.hbase.io.hfile.LruBlockCache}
   *
   * <p>BucketCache can be used as mainly a block cache (see
   * {@link org.apache.hadoop.hbase.io.hfile.CombinedBlockCache}), combined with
   * combined with LruBlockCache to decrease CMS GC and heap fragmentation.
   *
   * <p>It also can be used as a secondary cache (e.g. using a file on ssd/fusionio to store
   * blocks) to enlarge cache space via
   * {@link org.apache.hadoop.hbase.io.hfile.LruBlockCache#setVictimCache}
   */
  @InterfaceAudience.Private
  public class BucketCache implements BlockCache, HeapSize {
    private static final Log LOG = LogFactory.getLog(BucketCache.class);
  
    /** Priority buckets */
   private static final float DEFAULT_SINGLE_FACTOR = 0.25f;
   private static final float DEFAULT_MULTI_FACTOR = 0.50f;
   private static final float DEFAULT_MEMORY_FACTOR = 0.25f;
   private static final float DEFAULT_EXTRA_FREE_FACTOR = 0.10f;
 
   private static final float DEFAULT_ACCEPT_FACTOR = 0.95f;
   private static final float DEFAULT_MIN_FACTOR = 0.85f;
 
   // Number of blocks to clear for each of the bucket size that is full
   private static final int DEFAULT_FREE_ENTIRE_BLOCK_FACTOR = 2;
 
   /** Statistics thread */
   private static final int statThreadPeriod = 5 * 60;
 
   final static int DEFAULT_WRITER_THREADS = 3;
   final static int DEFAULT_WRITER_QUEUE_ITEMS = 64;
 
   // Store/read block data
   final IOEngine ioEngine;
 
   // Store the block in this map before writing it to cache
   @VisibleForTesting
   final ConcurrentMap<BlockCacheKey, RAMQueueEntry> ramCache;
   // In this map, store the block's meta data like offset, length
   @VisibleForTesting
   ConcurrentMap<BlockCacheKey, BucketEntry> backingMap;
 
   /**
    * Flag if the cache is enabled or not... We shut it off if there are IO
    * errors for some time, so that Bucket IO exceptions/errors don't bring down
    * the HBase server.
    */
   private volatile boolean cacheEnabled;
 
   /**
    * A list of writer queues.  We have a queue per {@link WriterThread} we have running.
    * In other words, the work adding blocks to the BucketCache is divided up amongst the
    * running WriterThreads.  Its done by taking hash of the cache key modulo queue count.
    * WriterThread when it runs takes whatever has been recently added and 'drains' the entries
    * to the BucketCache.  It then updates the ramCache and backingMap accordingly.
    */
   @VisibleForTesting
   final ArrayList<BlockingQueue<RAMQueueEntry>> writerQueues =
       new ArrayList<BlockingQueue<RAMQueueEntry>>();
   @VisibleForTesting
   final WriterThread[] writerThreads;
 
   /** Volatile boolean to track if free space is in process or not */
   private volatile boolean freeInProgress = false;
   private final Lock freeSpaceLock = new ReentrantLock();
 
   private UniqueIndexMap<Integer> deserialiserMap = new UniqueIndexMap<Integer>();
 
   private final AtomicLong realCacheSize = new AtomicLong(0);
   private final AtomicLong heapSize = new AtomicLong(0);
   /** Current number of cached elements */
   private final AtomicLong blockNumber = new AtomicLong(0);
 
   /** Cache access count (sequential ID) */
   private final AtomicLong accessCount = new AtomicLong(0);
 
   private static final int DEFAULT_CACHE_WAIT_TIME = 50;
   // Used in test now. If the flag is false and the cache speed is very fast,
   // bucket cache will skip some blocks when caching. If the flag is true, we
   // will wait blocks flushed to IOEngine for some time when caching
   boolean wait_when_cache = false;
 
   private final BucketCacheStats cacheStats = new BucketCacheStats();
 
   private final String persistencePath;
   private final long cacheCapacity;
   /** Approximate block size */
   private final long blockSize;
 
   /** Duration of IO errors tolerated before we disable cache, 1 min as default */
   private final int ioErrorsTolerationDuration;
   // 1 min
   public static final int DEFAULT_ERROR_TOLERATION_DURATION = 60 * 1000;
 
   // Start time of first IO error when reading or writing IO Engine, it will be
   // reset after a successful read/write.
   private volatile long ioErrorStartTime = -1;
 
   /**
    * A ReentrantReadWriteLock to lock on a particular block identified by offset.
    * The purpose of this is to avoid freeing the block which is being read.
    */
   @VisibleForTesting
   final IdReadWriteLock offsetLock = new IdReadWriteLock();
 
   private final NavigableSet<BlockCacheKey> blocksByHFile =
       new ConcurrentSkipListSet<BlockCacheKey>(new Comparator<BlockCacheKey>() {
         @Override
         public int compare(BlockCacheKey a, BlockCacheKey b) {
           int nameComparison = a.getHfileName().compareTo(b.getHfileName());
           if (nameComparison != 0) {
             return nameComparison;
           }
 
           if (a.getOffset() == b.getOffset()) {
             return 0;
           } else if (a.getOffset() < b.getOffset()) {
             return -1;
           }
           return 1;
         }
       });
 
   /** Statistics thread schedule pool (for heavy debugging, could remove) */
   private final ScheduledExecutorService scheduleThreadPool = Executors.newScheduledThreadPool(1,
     new ThreadFactoryBuilder().setNameFormat("BucketCacheStatsExecutor").setDaemon(true).build());
 
   // Allocate or free space for the block
   private BucketAllocator bucketAllocator;
 
   public BucketCache(String ioEngineName, long capacity, int blockSize, int[] bucketSizes,
       int writerThreadNum, int writerQLen, String persistencePath) throws FileNotFoundException,
       IOException {
     this(ioEngineName, capacity, blockSize, bucketSizes, writerThreadNum, writerQLen,
       persistencePath, DEFAULT_ERROR_TOLERATION_DURATION);
   }
 
   public BucketCache(String ioEngineName, long capacity, int blockSize, int[] bucketSizes,
       int writerThreadNum, int writerQLen, String persistencePath, int ioErrorsTolerationDuration)
       throws FileNotFoundException, IOException {
     this.ioEngine = getIOEngineFromName(ioEngineName, capacity);
     this.writerThreads = new WriterThread[writerThreadNum];
     long blockNumCapacity = capacity / blockSize;
     if (blockNumCapacity >= Integer.MAX_VALUE) {
       // Enough for about 32TB of cache!
       throw new IllegalArgumentException("Cache capacity is too large, only support 32TB now");
     }
 
     this.cacheCapacity = capacity;
     this.persistencePath = persistencePath;
     this.blockSize = blockSize;
     this.ioErrorsTolerationDuration = ioErrorsTolerationDuration;
 
     bucketAllocator = new BucketAllocator(capacity, bucketSizes);
     for (int i = 0; i < writerThreads.length; ++i) {
       writerQueues.add(new ArrayBlockingQueue<RAMQueueEntry>(writerQLen));
     }
 
     assert writerQueues.size() == writerThreads.length;
     this.ramCache = new ConcurrentHashMap<BlockCacheKey, RAMQueueEntry>();
 
     this.backingMap = new ConcurrentHashMap<BlockCacheKey, BucketEntry>((int) blockNumCapacity);
 
     if (ioEngine.isPersistent() && persistencePath != null) {
       try {
         retrieveFromFile(bucketSizes);
       } catch (IOException ioex) {
         LOG.error("Can't restore from file because of", ioex);
       } catch (ClassNotFoundException cnfe) {
         LOG.error("Can't restore from file in rebuild because can't deserialise",cnfe);
         throw new RuntimeException(cnfe);
       }
     }
     final String threadName = Thread.currentThread().getName();
     this.cacheEnabled = true;
     for (int i = 0; i < writerThreads.length; ++i) {
       writerThreads[i] = new WriterThread(writerQueues.get(i));
       writerThreads[i].setName(threadName + "-BucketCacheWriter-" + i);
       writerThreads[i].setDaemon(true);
     }
     startWriterThreads();
 
     // Run the statistics thread periodically to print the cache statistics log
     // TODO: Add means of turning this off.  Bit obnoxious running thread just to make a log
     // every five minutes.
     this.scheduleThreadPool.scheduleAtFixedRate(new StatisticsThread(this),
         statThreadPeriod, statThreadPeriod, TimeUnit.SECONDS);
     LOG.info("Started bucket cache; ioengine=" + ioEngineName +
         ", capacity=" + StringUtils.byteDesc(capacity) +
       ", blockSize=" + StringUtils.byteDesc(blockSize) + ", writerThreadNum=" +
         writerThreadNum + ", writerQLen=" + writerQLen + ", persistencePath=" +
       persistencePath + ", bucketAllocator=" + this.bucketAllocator.getClass().getName());
   }
 
   /**
    * Called by the constructor to start the writer threads. Used by tests that need to override
    * starting the threads.
    */
   @VisibleForTesting
   protected void startWriterThreads() {
     for (WriterThread thread : writerThreads) {
       thread.start();
     }
   }
 
   @VisibleForTesting
   boolean isCacheEnabled() {
     return this.cacheEnabled;
   }
 
   public long getMaxSize() {
     return this.cacheCapacity;
   }
 
   public String getIoEngine() {
     return ioEngine.toString();
   }
 
   /**
    * Get the IOEngine from the IO engine name
    * @param ioEngineName
    * @param capacity
    * @return the IOEngine
    * @throws IOException
    */
   private IOEngine getIOEngineFromName(String ioEngineName, long capacity)
       throws IOException {
     if (ioEngineName.startsWith("file:") || ioEngineName.startsWith("files:")) {
       // In order to make the usage simple, we only need the prefix 'files:' in
       // document whether one or multiple file(s), but also support 'file:' for
       // the compatibility
       String[] filePaths =
           ioEngineName.substring(ioEngineName.indexOf(":") + 1).split(FileIOEngine.FILE_DELIMITER);
       return new FileIOEngine(capacity, filePaths);
     } else if (ioEngineName.startsWith("offheap"))
       return new ByteBufferIOEngine(capacity, true);
     else if (ioEngineName.startsWith("heap"))
       return new ByteBufferIOEngine(capacity, false);
     else
       throw new IllegalArgumentException(
           "Don't understand io engine name for cache - prefix with file:, heap or offheap");
   }
 
   /**
    * Cache the block with the specified name and buffer.
    * @param cacheKey block's cache key
    * @param buf block buffer
    */
   @Override
   public void cacheBlock(BlockCacheKey cacheKey, Cacheable buf) {
     cacheBlock(cacheKey, buf, false, false);
   }
 
   /**
    * Cache the block with the specified name and buffer.
    * @param cacheKey block's cache key
    * @param cachedItem block buffer
    * @param inMemory if block is in-memory
    * @param cacheDataInL1
    */
   @Override
   public void cacheBlock(BlockCacheKey cacheKey, Cacheable cachedItem, boolean inMemory,
       final boolean cacheDataInL1) {
     cacheBlockWithWait(cacheKey, cachedItem, inMemory, wait_when_cache);
   }
 
   /**
    * Cache the block to ramCache
    * @param cacheKey block's cache key
    * @param cachedItem block buffer
    * @param inMemory if block is in-memory
    * @param wait if true, blocking wait when queue is full
    */
   public void cacheBlockWithWait(BlockCacheKey cacheKey, Cacheable cachedItem, boolean inMemory,
       boolean wait) {
     if (LOG.isTraceEnabled()) LOG.trace("Caching key=" + cacheKey + ", item=" + cachedItem);
     if (!cacheEnabled) {
       return;
     }
 
     if (backingMap.containsKey(cacheKey)) {
       return;
     }
 
     /*
      * Stuff the entry into the RAM cache so it can get drained to the persistent store
      */
     RAMQueueEntry re =
         new RAMQueueEntry(cacheKey, cachedItem, accessCount.incrementAndGet(), inMemory);
     if (ramCache.putIfAbsent(cacheKey, re) != null) {
       return;
     }
     int queueNum = (cacheKey.hashCode() & 0x7FFFFFFF) % writerQueues.size();
     BlockingQueue<RAMQueueEntry> bq = writerQueues.get(queueNum);
     boolean successfulAddition = false;
     if (wait) {
       try {
         successfulAddition = bq.offer(re, DEFAULT_CACHE_WAIT_TIME, TimeUnit.MILLISECONDS);
       } catch (InterruptedException e) {
         Thread.currentThread().interrupt();
       }
     } else {
       successfulAddition = bq.offer(re);
     }
     if (!successfulAddition) {
       ramCache.remove(cacheKey);
       cacheStats.failInsert();
     } else {
       this.blockNumber.incrementAndGet();
       this.heapSize.addAndGet(cachedItem.heapSize());
       blocksByHFile.add(cacheKey);
     }
   }
 
   /**
    * Get the buffer of the block with the specified key.
    * @param key block's cache key
    * @param caching true if the caller caches blocks on cache misses
    * @param repeat Whether this is a repeat lookup for the same block
    * @param updateCacheMetrics Whether we should update cache metrics or not
    * @return buffer of specified cache key, or null if not in cache
    */
   @Override
   public Cacheable getBlock(BlockCacheKey key, boolean caching, boolean repeat,
       boolean updateCacheMetrics) {
     if (!cacheEnabled) {
       return null;
     }
     RAMQueueEntry re = ramCache.get(key);
     if (re != null) {
       if (updateCacheMetrics) {
         cacheStats.hit(caching, key.isPrimary(), key.getBlockType());
       }
       re.access(accessCount.incrementAndGet());
       return re.getData();
     }
     BucketEntry bucketEntry = backingMap.get(key);
     if (bucketEntry != null) {
       long start = System.nanoTime();
       ReentrantReadWriteLock lock = offsetLock.getLock(bucketEntry.offset());
       try {
         lock.readLock().lock();
         // We can not read here even if backingMap does contain the given key because its offset
         // maybe changed. If we lock BlockCacheKey instead of offset, then we can only check
         // existence here.
         if (bucketEntry.equals(backingMap.get(key))) {
           int len = bucketEntry.getLength();
           if (LOG.isTraceEnabled()) {
             LOG.trace("Read offset=" + bucketEntry.offset() + ", len=" + len);
           }
           ByteBuffer bb = ByteBuffer.allocate(len);
           int lenRead = ioEngine.read(bb, bucketEntry.offset());
           if (lenRead != len) {
             throw new RuntimeException("Only " + lenRead + " bytes read, " + len + " expected");
           }
           CacheableDeserializer<Cacheable> deserializer =
             bucketEntry.deserializerReference(this.deserialiserMap);
           Cacheable cachedBlock = deserializer.deserialize(bb, true);
           long timeTaken = System.nanoTime() - start;
           if (updateCacheMetrics) {
             cacheStats.hit(caching, key.isPrimary(), key.getBlockType());
             cacheStats.ioHit(timeTaken);
           }
           bucketEntry.access(accessCount.incrementAndGet());
           if (this.ioErrorStartTime > 0) {
             ioErrorStartTime = -1;
           }
           return cachedBlock;
         }
       } catch (IOException ioex) {
         LOG.error("Failed reading block " + key + " from bucket cache", ioex);
         checkIOErrorIsTolerated();
       } finally {
         lock.readLock().unlock();
       }
     }
     if (!repeat && updateCacheMetrics) {
       cacheStats.miss(caching, key.isPrimary(), key.getBlockType());
     }
     return null;
   }
 
   @VisibleForTesting
   void blockEvicted(BlockCacheKey cacheKey, BucketEntry bucketEntry, boolean decrementBlockNumber) {
     bucketAllocator.freeBlock(bucketEntry.offset());
     realCacheSize.addAndGet(-1 * bucketEntry.getLength());
     blocksByHFile.remove(cacheKey);
     if (decrementBlockNumber) {
       this.blockNumber.decrementAndGet();
     }
   }
 
   @Override
   public boolean evictBlock(BlockCacheKey cacheKey) {
     if (!cacheEnabled) {
       return false;
     }
     RAMQueueEntry removedBlock = ramCache.remove(cacheKey);
     if (removedBlock != null) {
       this.blockNumber.decrementAndGet();
       this.heapSize.addAndGet(-1 * removedBlock.getData().heapSize());
     }
     BucketEntry bucketEntry = backingMap.get(cacheKey);
     if (bucketEntry == null) {
       if (removedBlock != null) {
         cacheStats.evicted(0, cacheKey.isPrimary());
         return true;
       } else {
         return false;
       }
     }
     ReentrantReadWriteLock lock = offsetLock.getLock(bucketEntry.offset());
     try {
       lock.writeLock().lock();
       if (backingMap.remove(cacheKey, bucketEntry)) {
         blockEvicted(cacheKey, bucketEntry, removedBlock == null);
       } else {
         return false;
       }
     } finally {
       lock.writeLock().unlock();
     }
     cacheStats.evicted(bucketEntry.getCachedTime(), cacheKey.isPrimary());
     return true;
   }
 
   /*
    * Statistics thread.  Periodically output cache statistics to the log.
    */
   private static class StatisticsThread extends Thread {
     private final BucketCache bucketCache;
 
     public StatisticsThread(BucketCache bucketCache) {
       super("BucketCacheStatsThread");
       setDaemon(true);
       this.bucketCache = bucketCache;
     }
 
     @Override
     public void run() {
       bucketCache.logStats();
     }
   }
 
   public void logStats() {
     long totalSize = bucketAllocator.getTotalSize();
     long usedSize = bucketAllocator.getUsedSize();
     long freeSize = totalSize - usedSize;
     long cacheSize = getRealCacheSize();
     LOG.info("failedBlockAdditions=" + cacheStats.getFailedInserts() + ", " +
         "totalSize=" + StringUtils.byteDesc(totalSize) + ", " +
         "freeSize=" + StringUtils.byteDesc(freeSize) + ", " +
         "usedSize=" + StringUtils.byteDesc(usedSize) +", " +
         "cacheSize=" + StringUtils.byteDesc(cacheSize) +", " +
         "accesses=" + cacheStats.getRequestCount() + ", " +
         "hits=" + cacheStats.getHitCount() + ", " +
         "IOhitsPerSecond=" + cacheStats.getIOHitsPerSecond() + ", " +
         "IOTimePerHit=" + String.format("%.2f", cacheStats.getIOTimePerHit())+ ", " +
         "hitRatio=" + (cacheStats.getHitCount() == 0 ? "0," :
           (StringUtils.formatPercent(cacheStats.getHitRatio(), 2)+ ", ")) +
         "cachingAccesses=" + cacheStats.getRequestCachingCount() + ", " +
         "cachingHits=" + cacheStats.getHitCachingCount() + ", " +
         "cachingHitsRatio=" +(cacheStats.getHitCachingCount() == 0 ? "0," :
           (StringUtils.formatPercent(cacheStats.getHitCachingRatio(), 2)+ ", ")) +
         "evictions=" + cacheStats.getEvictionCount() + ", " +
         "evicted=" + cacheStats.getEvictedCount() + ", " +
         "evictedPerRun=" + cacheStats.evictedPerEviction());
     cacheStats.reset();
   }
 
   public long getRealCacheSize() {
     return this.realCacheSize.get();
   }
 
   private long acceptableSize() {
     return (long) Math.floor(bucketAllocator.getTotalSize() * DEFAULT_ACCEPT_FACTOR);
   }
 
   private long singleSize() {
     return (long) Math.floor(bucketAllocator.getTotalSize()
         * DEFAULT_SINGLE_FACTOR * DEFAULT_MIN_FACTOR);
   }
 
   private long multiSize() {
     return (long) Math.floor(bucketAllocator.getTotalSize() * DEFAULT_MULTI_FACTOR
         * DEFAULT_MIN_FACTOR);
   }
 
   private long memorySize() {
     return (long) Math.floor(bucketAllocator.getTotalSize() * DEFAULT_MEMORY_FACTOR
         * DEFAULT_MIN_FACTOR);
   }
 
   /**
    * Return the count of bucketSizeinfos still needf ree space
    */
   private int bucketSizesAboveThresholdCount(float minFactor) {
     BucketAllocator.IndexStatistics[] stats = bucketAllocator.getIndexStatistics();
     int fullCount = 0;
     for (int i = 0; i < stats.length; i++) {
       long freeGoal = (long) Math.floor(stats[i].totalCount() * (1 - minFactor));
       freeGoal = Math.max(freeGoal, 1);
       if (stats[i].freeCount() < freeGoal) {
         fullCount++;
       }
     }
     return fullCount;
   }
 
   /**
    * This method will find the buckets that are minimally occupied
    * and are not reference counted and will free them completely
    * without any constraint on the access times of the elements,
    * and as a process will completely free at most the number of buckets
    * passed, sometimes it might not due to changing refCounts
    *
    * @param completelyFreeBucketsNeeded number of buckets to free
    **/
   private void freeEntireBuckets(int completelyFreeBucketsNeeded) {
     if (completelyFreeBucketsNeeded != 0) {
       // First we will build a set where the offsets are reference counted, usually
       // this set is small around O(Handler Count) unless something else is wrong
       Set<Integer> inUseBuckets = new HashSet<Integer>();
       for (BucketEntry entry : backingMap.values()) {
         inUseBuckets.add(bucketAllocator.getBucketIndex(entry.offset()));
       }
 
       Set<Integer> candidateBuckets = bucketAllocator.getLeastFilledBuckets(
           inUseBuckets, completelyFreeBucketsNeeded);
       for (Map.Entry<BlockCacheKey, BucketEntry> entry : backingMap.entrySet()) {
         if (candidateBuckets.contains(bucketAllocator
             .getBucketIndex(entry.getValue().offset()))) {
           evictBlock(entry.getKey());
         }
       }
     }
   }
 
   /**
    * Free the space if the used size reaches acceptableSize() or one size block
    * couldn't be allocated. When freeing the space, we use the LRU algorithm and
    * ensure there must be some blocks evicted
    * @param why Why we are being called
    */
   private void freeSpace(final String why) {
     // Ensure only one freeSpace progress at a time
     if (!freeSpaceLock.tryLock()) {
       return;
     }
     try {
       freeInProgress = true;
       long bytesToFreeWithoutExtra = 0;
       // Calculate free byte for each bucketSizeinfo
       StringBuffer msgBuffer = LOG.isDebugEnabled()? new StringBuffer(): null;
       BucketAllocator.IndexStatistics[] stats = bucketAllocator.getIndexStatistics();
       long[] bytesToFreeForBucket = new long[stats.length];
       for (int i = 0; i < stats.length; i++) {
         bytesToFreeForBucket[i] = 0;
         long freeGoal = (long) Math.floor(stats[i].totalCount() * (1 - DEFAULT_MIN_FACTOR));
         freeGoal = Math.max(freeGoal, 1);
         if (stats[i].freeCount() < freeGoal) {
           bytesToFreeForBucket[i] = stats[i].itemSize() * (freeGoal - stats[i].freeCount());
           bytesToFreeWithoutExtra += bytesToFreeForBucket[i];
           if (msgBuffer != null) {
             msgBuffer.append("Free for bucketSize(" + stats[i].itemSize() + ")="
               + StringUtils.byteDesc(bytesToFreeForBucket[i]) + ", ");
           }
         }
       }
       if (msgBuffer != null) {
         msgBuffer.append("Free for total=" + StringUtils.byteDesc(bytesToFreeWithoutExtra) + ", ");
       }
 
       if (bytesToFreeWithoutExtra <= 0) {
         return;
       }
       long currentSize = bucketAllocator.getUsedSize();
       long totalSize = bucketAllocator.getTotalSize();
       if (LOG.isDebugEnabled() && msgBuffer != null) {
         LOG.debug("Free started because \"" + why + "\"; " + msgBuffer.toString() +
           " of current used=" + StringUtils.byteDesc(currentSize) + ", actual cacheSize=" +
           StringUtils.byteDesc(realCacheSize.get()) + ", total=" + StringUtils.byteDesc(totalSize));
       }
 
       long bytesToFreeWithExtra = (long) Math.floor(bytesToFreeWithoutExtra
           * (1 + DEFAULT_EXTRA_FREE_FACTOR));
 
       // Instantiate priority buckets
       BucketEntryGroup bucketSingle = new BucketEntryGroup(bytesToFreeWithExtra,
           blockSize, singleSize());
       BucketEntryGroup bucketMulti = new BucketEntryGroup(bytesToFreeWithExtra,
           blockSize, multiSize());
       BucketEntryGroup bucketMemory = new BucketEntryGroup(bytesToFreeWithExtra,
           blockSize, memorySize());
 
       // Scan entire map putting bucket entry into appropriate bucket entry
       // group
       for (Map.Entry<BlockCacheKey, BucketEntry> bucketEntryWithKey : backingMap.entrySet()) {
         switch (bucketEntryWithKey.getValue().getPriority()) {
           case SINGLE: {
             bucketSingle.add(bucketEntryWithKey);
             break;
           }
           case MULTI: {
             bucketMulti.add(bucketEntryWithKey);
             break;
           }
           case MEMORY: {
             bucketMemory.add(bucketEntryWithKey);
             break;
           }
         }
       }
 
       PriorityQueue<BucketEntryGroup> bucketQueue = new PriorityQueue<BucketEntryGroup>(3);
 
       bucketQueue.add(bucketSingle);
       bucketQueue.add(bucketMulti);
       bucketQueue.add(bucketMemory);
 
       int remainingBuckets = 3;
       long bytesFreed = 0;
 
       BucketEntryGroup bucketGroup;
       while ((bucketGroup = bucketQueue.poll()) != null) {
         long overflow = bucketGroup.overflow();
         if (overflow > 0) {
           long bucketBytesToFree = Math.min(overflow,
               (bytesToFreeWithoutExtra - bytesFreed) / remainingBuckets);
           bytesFreed += bucketGroup.free(bucketBytesToFree);
         }
         remainingBuckets--;
       }
 
       // Check and free if there are buckets that still need freeing of space
       if (bucketSizesAboveThresholdCount(DEFAULT_MIN_FACTOR) > 0) {
         bucketQueue.clear();
         remainingBuckets = 3;
 
         bucketQueue.add(bucketSingle);
         bucketQueue.add(bucketMulti);
         bucketQueue.add(bucketMemory);
 
         while ((bucketGroup = bucketQueue.poll()) != null) {
           long bucketBytesToFree = (bytesToFreeWithExtra - bytesFreed) / remainingBuckets;
           bytesFreed += bucketGroup.free(bucketBytesToFree);
           remainingBuckets--;
         }
       }
 
       // Even after the above free we might still need freeing because of the
       // De-fragmentation of the buckets (also called Slab Calcification problem), i.e
       // there might be some buckets where the occupancy is very sparse and thus are not
       // yielding the free for the other bucket sizes, the fix for this to evict some
       // of the buckets, we do this by evicting the buckets that are least fulled
       freeEntireBuckets(DEFAULT_FREE_ENTIRE_BLOCK_FACTOR *
           bucketSizesAboveThresholdCount(1.0f));
 
       if (LOG.isDebugEnabled()) {
         long single = bucketSingle.totalSize();
         long multi = bucketMulti.totalSize();
         long memory = bucketMemory.totalSize();
         if (LOG.isDebugEnabled()) {
           LOG.debug("Bucket cache free space completed; " + "freed="
             + StringUtils.byteDesc(bytesFreed) + ", " + "total="
             + StringUtils.byteDesc(totalSize) + ", " + "single="
             + StringUtils.byteDesc(single) + ", " + "multi="
             + StringUtils.byteDesc(multi) + ", " + "memory="
             + StringUtils.byteDesc(memory));
         }
       }
 
     } catch (Throwable t) {
       LOG.warn("Failed freeing space", t);
     } finally {
       cacheStats.evict();
       freeInProgress = false;
       freeSpaceLock.unlock();
     }
   }
 
   // This handles flushing the RAM cache to IOEngine.
   @VisibleForTesting
   class WriterThread extends HasThread {
     private final BlockingQueue<RAMQueueEntry> inputQueue;
     private volatile boolean writerEnabled = true;
 
     WriterThread(BlockingQueue<RAMQueueEntry> queue) {
       super("BucketCacheWriterThread");
       this.inputQueue = queue;
     }
 
     // Used for test
     @VisibleForTesting
     void disableWriter() {
       this.writerEnabled = false;
     }
 
     public void run() {
       List<RAMQueueEntry> entries = new ArrayList<RAMQueueEntry>();
       try {
         while (cacheEnabled && writerEnabled) {
           try {
             try {
               // Blocks
               entries = getRAMQueueEntries(inputQueue, entries);
             } catch (InterruptedException ie) {
               if (!cacheEnabled) break;
             }
             doDrain(entries);
           } catch (Exception ioe) {
             LOG.error("WriterThread encountered error", ioe);
           }
         }
       } catch (Throwable t) {
         LOG.warn("Failed doing drain", t);
       }
       LOG.info(this.getName() + " exiting, cacheEnabled=" + cacheEnabled);
     }
 
     /**
      * Flush the entries in ramCache to IOEngine and add bucket entry to backingMap.
      * Process all that are passed in even if failure being sure to remove from ramCache else we'll
      * never undo the references and we'll OOME.
      * @param entries Presumes list passed in here will be processed by this invocation only. No
      * interference expected.
      * @throws InterruptedException
      */
     @VisibleForTesting
     void doDrain(final List<RAMQueueEntry> entries) throws InterruptedException {
       if (entries.isEmpty()) {
         return;
       }
       // This method is a little hard to follow. We run through the passed in entries and for each
       // successful add, we add a non-null BucketEntry to the below bucketEntries.  Later we must
       // do cleanup making sure we've cleared ramCache of all entries regardless of whether we
       // successfully added the item to the bucketcache; if we don't do the cleanup, we'll OOME by
       // filling ramCache.  We do the clean up by again running through the passed in entries
       // doing extra work when we find a non-null bucketEntries corresponding entry.
       final int size = entries.size();
       BucketEntry[] bucketEntries = new BucketEntry[size];
       // Index updated inside loop if success or if we can't succeed. We retry if cache is full
       // when we go to add an entry by going around the loop again without upping the index.
       int index = 0;
       while (cacheEnabled && index < size) {
         RAMQueueEntry re = null;
         try {
           re = entries.get(index);
           if (re == null) {
             LOG.warn("Couldn't get entry or changed on us; who else is messing with it?");
             index++;
             continue;
           }
           BucketEntry bucketEntry =
             re.writeToCache(ioEngine, bucketAllocator, deserialiserMap, realCacheSize);
           // Successfully added.  Up index and add bucketEntry. Clear io exceptions.
           bucketEntries[index] = bucketEntry;
           if (ioErrorStartTime > 0) {
             ioErrorStartTime = -1;
           }
           index++;
         } catch (BucketAllocatorException fle) {
           LOG.warn("Failed allocation for " + (re == null ? "" : re.getKey()) + "; " + fle);
           // Presume can't add. Too big? Move index on. Entry will be cleared from ramCache below.
           bucketEntries[index] = null;
           index++;
         } catch (CacheFullException cfe) {
           // Cache full when we tried to add. Try freeing space and then retrying (don't up index)
           if (!freeInProgress) {
             freeSpace("Full!");
           } else {
             Thread.sleep(50);
           }
         } catch (IOException ioex) {
           // Hopefully transient. Retry. checkIOErrorIsTolerated disables cache if problem.
           LOG.error("Failed writing to bucket cache", ioex);
           checkIOErrorIsTolerated();
         }
       }
 
       // Make sure data pages are written on media before we update maps.
       try {
         ioEngine.sync();
       } catch (IOException ioex) {
         LOG.error("Failed syncing IO engine", ioex);
         checkIOErrorIsTolerated();
         // Since we failed sync, free the blocks in bucket allocator
         for (int i = 0; i < entries.size(); ++i) {
           if (bucketEntries[i] != null) {
             bucketAllocator.freeBlock(bucketEntries[i].offset());
             bucketEntries[i] = null;
           }
         }
       }
 
       // Now add to backingMap if successfully added to bucket cache.  Remove from ramCache if
       // success or error.
       for (int i = 0; i < size; ++i) {
         BlockCacheKey key = entries.get(i).getKey();
         // Only add if non-null entry.
         if (bucketEntries[i] != null) {
           backingMap.put(key, bucketEntries[i]);
         }
         // Always remove from ramCache even if we failed adding it to the block cache above.
         RAMQueueEntry ramCacheEntry = ramCache.remove(key);
         if (ramCacheEntry != null) {
           heapSize.addAndGet(-1 * entries.get(i).getData().heapSize());
         } else if (bucketEntries[i] != null){
           // Block should have already been evicted. Remove it and free space.
           ReentrantReadWriteLock lock = offsetLock.getLock(bucketEntries[i].offset());
           try {
             lock.writeLock().lock();
             if (backingMap.remove(key, bucketEntries[i])) {
               blockEvicted(key, bucketEntries[i], false);
             }
           } finally {
             lock.writeLock().unlock();
           }
         }
       }
 
       long used = bucketAllocator.getUsedSize();
       if (used > acceptableSize()) {
         freeSpace("Used=" + used + " > acceptable=" + acceptableSize());
       }
       return;
     }
   }
 
   /**
    * Blocks until elements available in <code>q</code> then tries to grab as many as possible
    * before returning.
    * @param recepticle Where to stash the elements taken from queue. We clear before we use it
    * just in case.
    * @param q The queue to take from.
    * @return <code>receptical laden with elements taken from the queue or empty if none found.
    */
   @VisibleForTesting
   static List<RAMQueueEntry> getRAMQueueEntries(final BlockingQueue<RAMQueueEntry> q,
       final List<RAMQueueEntry> receptical)
   throws InterruptedException {
     // Clear sets all entries to null and sets size to 0. We retain allocations. Presume it
     // ok even if list grew to accommodate thousands.
     receptical.clear();
     receptical.add(q.take());
     q.drainTo(receptical);
     return receptical;
   }
 
   private void persistToFile() throws IOException {
     assert !cacheEnabled;
     FileOutputStream fos = null;
     ObjectOutputStream oos = null;
     try {
       if (!ioEngine.isPersistent()) {
         throw new IOException("Attempt to persist non-persistent cache mappings!");
       }
       fos = new FileOutputStream(persistencePath, false);
       oos = new ObjectOutputStream(fos);
       oos.writeLong(cacheCapacity);
       oos.writeUTF(ioEngine.getClass().getName());
       oos.writeUTF(backingMap.getClass().getName());
       oos.writeObject(deserialiserMap);
       oos.writeObject(backingMap);
     } finally {
       if (oos != null) oos.close();
       if (fos != null) fos.close();
     }
   }
 
   @SuppressWarnings("unchecked")
   private void retrieveFromFile(int[] bucketSizes) throws IOException, BucketAllocatorException,
       ClassNotFoundException {
     File persistenceFile = new File(persistencePath);
     if (!persistenceFile.exists()) {
       return;
     }
     assert !cacheEnabled;
     FileInputStream fis = null;
     ObjectInputStream ois = null;
     try {
       if (!ioEngine.isPersistent())
         throw new IOException(
             "Attempt to restore non-persistent cache mappings!");
       fis = new FileInputStream(persistencePath);
       ois = new ObjectInputStream(fis);
       long capacitySize = ois.readLong();
       if (capacitySize != cacheCapacity)
         throw new IOException("Mismatched cache capacity:"
             + StringUtils.byteDesc(capacitySize) + ", expected: "
             + StringUtils.byteDesc(cacheCapacity));
       String ioclass = ois.readUTF();
       String mapclass = ois.readUTF();
       if (!ioEngine.getClass().getName().equals(ioclass))
         throw new IOException("Class name for IO engine mismatch: " + ioclass
             + ", expected:" + ioEngine.getClass().getName());
       if (!backingMap.getClass().getName().equals(mapclass))
         throw new IOException("Class name for cache map mismatch: " + mapclass
             + ", expected:" + backingMap.getClass().getName());
       UniqueIndexMap<Integer> deserMap = (UniqueIndexMap<Integer>) ois
           .readObject();
       ConcurrentHashMap<BlockCacheKey, BucketEntry> backingMapFromFile =
           (ConcurrentHashMap<BlockCacheKey, BucketEntry>) ois.readObject();
       BucketAllocator allocator = new BucketAllocator(cacheCapacity, bucketSizes,
         backingMapFromFile, realCacheSize);
       bucketAllocator = allocator;
       deserialiserMap = deserMap;
       backingMap = backingMapFromFile;
     } finally {
       if (ois != null) ois.close();
       if (fis != null) fis.close();
       if (!persistenceFile.delete()) {
         throw new IOException("Failed deleting persistence file "
             + persistenceFile.getAbsolutePath());
       }
     }
   }
 
   /**
    * Check whether we tolerate IO error this time. If the duration of IOEngine
    * throwing errors exceeds ioErrorsDurationTimeTolerated, we will disable the
    * cache
    */
   private void checkIOErrorIsTolerated() {
     long now = EnvironmentEdgeManager.currentTime();
     if (this.ioErrorStartTime > 0) {
       if (cacheEnabled && (now - ioErrorStartTime) > this.ioErrorsTolerationDuration) {
         LOG.error("IO errors duration time has exceeded " + ioErrorsTolerationDuration +
           "ms, disabling cache, please check your IOEngine");
         disableCache();
       }
     } else {
       this.ioErrorStartTime = now;
     }
   }
 
   /**
    * Used to shut down the cache -or- turn it off in the case of something broken.
    */
   private void disableCache() {
     if (!cacheEnabled) return;
     cacheEnabled = false;
     ioEngine.shutdown();
     this.scheduleThreadPool.shutdown();
     for (int i = 0; i < writerThreads.length; ++i) writerThreads[i].interrupt();
     this.ramCache.clear();
     if (!ioEngine.isPersistent() || persistencePath == null) {
       // If persistent ioengine and a path, we will serialize out the backingMap.
       this.backingMap.clear();
     }
   }
 
   private void join() throws InterruptedException {
     for (int i = 0; i < writerThreads.length; ++i)
       writerThreads[i].join();
   }
 
   @Override
   public void shutdown() {
     disableCache();
     LOG.info("Shutdown bucket cache: IO persistent=" + ioEngine.isPersistent()
         + "; path to write=" + persistencePath);
     if (ioEngine.isPersistent() && persistencePath != null) {
       try {
         join();
         persistToFile();
       } catch (IOException ex) {
         LOG.error("Unable to persist data on exit: " + ex.toString(), ex);
       } catch (InterruptedException e) {
         LOG.warn("Failed to persist data on exit", e);
       }
     }
   }
 
   @Override
   public CacheStats getStats() {
     return cacheStats;
   }
 
   public BucketAllocator getAllocator() {
     return this.bucketAllocator;
   }
 
   @Override
   public long heapSize() {
     return this.heapSize.get();
   }
 
   @Override
   public long size() {
     return this.realCacheSize.get();
   }
 
   @Override
   public long getFreeSize() {
     return this.bucketAllocator.getFreeSize();
   }
 
   @Override
   public long getBlockCount() {
     return this.blockNumber.get();
   }
 
   @Override
   public long getCurrentSize() {
     return this.bucketAllocator.getUsedSize();
   }
 
   /**
    * Evicts all blocks for a specific HFile.
    * <p>
    * This is used for evict-on-close to remove all blocks of a specific HFile.
    *
    * @return the number of blocks evicted
    */
   @Override
   public int evictBlocksByHfileName(String hfileName) {
     Set<BlockCacheKey> keySet = blocksByHFile.subSet(
         new BlockCacheKey(hfileName, Long.MIN_VALUE), true,
         new BlockCacheKey(hfileName, Long.MAX_VALUE), true);
 
     int numEvicted = 0;
     for (BlockCacheKey key : keySet) {
       if (evictBlock(key)) {
           ++numEvicted;
       }
     }
 
     return numEvicted;
   }
 
   /**
    * Item in cache. We expect this to be where most memory goes. Java uses 8
    * bytes just for object headers; after this, we want to use as little as
    * possible - so we only use 8 bytes, but in order to do so we end up messing
    * around with all this Java casting stuff. Offset stored as 5 bytes that make
    * up the long. Doubt we'll see devices this big for ages. Offsets are divided
    * by 256. So 5 bytes gives us 256TB or so.
    */
   static class BucketEntry implements Serializable {
     private static final long serialVersionUID = -6741504807982257534L;
 
     // access counter comparator, descending order
     static final Comparator<BucketEntry> COMPARATOR = new Comparator<BucketCache.BucketEntry>() {
 
       @Override
       public int compare(BucketEntry o1, BucketEntry o2) {
         return Long.compare(o2.accessCounter, o1.accessCounter);
       }
     };
 
     private int offsetBase;
     private int length;
     private byte offset1;
     byte deserialiserIndex;
     private volatile long accessCounter;
     private BlockPriority priority;
     /**
      * Time this block was cached.  Presumes we are created just before we are added to the cache.
      */
     private final long cachedTime = System.nanoTime();
 
     BucketEntry(long offset, int length, long accessCounter, boolean inMemory) {
       setOffset(offset);
       this.length = length;
       this.accessCounter = accessCounter;
       if (inMemory) {
         this.priority = BlockPriority.MEMORY;
       } else {
         this.priority = BlockPriority.SINGLE;
       }
     }
 
     long offset() { // Java has no unsigned numbers
       long o = ((long) offsetBase) & 0xFFFFFFFFL; //This needs the L cast otherwise it will be sign extended as a negative number.
       o += (((long) (offset1)) & 0xFF) << 32; //The 0xFF here does not need the L cast because it is treated as a positive int.
       return o << 8;
     }
 
     private void setOffset(long value) {
       assert (value & 0xFF) == 0;
       value >>= 8;
       offsetBase = (int) value;
       offset1 = (byte) (value >> 32);
     }
 
     public int getLength() {
       return length;
     }
 
     protected CacheableDeserializer<Cacheable> deserializerReference(
         UniqueIndexMap<Integer> deserialiserMap) {
       return CacheableDeserializerIdManager.getDeserializer(deserialiserMap
           .unmap(deserialiserIndex));
     }
 
     protected void setDeserialiserReference(
         CacheableDeserializer<Cacheable> deserializer,
         UniqueIndexMap<Integer> deserialiserMap) {
       this.deserialiserIndex = ((byte) deserialiserMap.map(deserializer
           .getDeserialiserIdentifier()));
     }
 
     /**
      * Block has been accessed. Update its local access counter.
      */
     public void access(long accessCounter) {
       this.accessCounter = accessCounter;
       if (this.priority == BlockPriority.SINGLE) {
         this.priority = BlockPriority.MULTI;
       }
     }
 
     public BlockPriority getPriority() {
       return this.priority;
     }
 
     public long getCachedTime() {
       return cachedTime;
     }
   }
 
   /**
    * Used to group bucket entries into priority buckets. There will be a
    * BucketEntryGroup for each priority (single, multi, memory). Once bucketed,
    * the eviction algorithm takes the appropriate number of elements out of each
    * according to configuration parameters and their relative sizes.
    */
   private class BucketEntryGroup implements Comparable<BucketEntryGroup> {
 
     private CachedEntryQueue queue;
     private long totalSize = 0;
     private long bucketSize;
 
     public BucketEntryGroup(long bytesToFree, long blockSize, long bucketSize) {
       this.bucketSize = bucketSize;
       queue = new CachedEntryQueue(bytesToFree, blockSize);
       totalSize = 0;
     }
 
     public void add(Map.Entry<BlockCacheKey, BucketEntry> block) {
       totalSize += block.getValue().getLength();
       queue.add(block);
     }
 
     public long free(long toFree) {
       Map.Entry<BlockCacheKey, BucketEntry> entry;
       long freedBytes = 0;
       while ((entry = queue.pollLast()) != null) {
         evictBlock(entry.getKey());
         freedBytes += entry.getValue().getLength();
         if (freedBytes >= toFree) {
           return freedBytes;
         }
       }
       return freedBytes;
     }
 
     public long overflow() {
       return totalSize - bucketSize;
     }
 
     public long totalSize() {
       return totalSize;
     }
 
     @Override
     public int compareTo(BucketEntryGroup that) {
       return Long.compare(this.overflow(), that.overflow());
     }
 
     @Override
     public boolean equals(Object that) {
       return this == that;
     }
 
   }
 
   /**
    * Block Entry stored in the memory with key,data and so on
    */
   @VisibleForTesting
   static class RAMQueueEntry {
     private BlockCacheKey key;
     private Cacheable data;
     private long accessCounter;
     private boolean inMemory;
 
     public RAMQueueEntry(BlockCacheKey bck, Cacheable data, long accessCounter,
         boolean inMemory) {
       this.key = bck;
       this.data = data;
       this.accessCounter = accessCounter;
       this.inMemory = inMemory;
     }
 
     public Cacheable getData() {
       return data;
     }
 
     public BlockCacheKey getKey() {
       return key;
     }
 
     public void access(long accessCounter) {
       this.accessCounter = accessCounter;
     }
 
     public BucketEntry writeToCache(final IOEngine ioEngine,
         final BucketAllocator bucketAllocator,
         final UniqueIndexMap<Integer> deserialiserMap,
         final AtomicLong realCacheSize) throws CacheFullException, IOException,
         BucketAllocatorException {
       int len = data.getSerializedLength();
       // This cacheable thing can't be serialized
       if (len == 0) return null;
       long offset = bucketAllocator.allocateBlock(len);
       BucketEntry bucketEntry = new BucketEntry(offset, len, accessCounter, inMemory);
       bucketEntry.setDeserialiserReference(data.getDeserializer(), deserialiserMap);
       try {
         if (data instanceof HFileBlock) {
           // If an instance of HFileBlock, save on some allocations.
           HFileBlock block = (HFileBlock)data;
           ByteBuffer sliceBuf = block.getBufferReadOnly();
           ByteBuffer metadata = block.getMetaData();
           if (LOG.isTraceEnabled()) {
             LOG.trace("Write offset=" + offset + ", len=" + len);
           }
           ioEngine.write(sliceBuf, offset);
           ioEngine.write(metadata, offset + len - metadata.limit());
         } else {
           ByteBuffer bb = ByteBuffer.allocate(len);
           data.serialize(bb);
           ioEngine.write(bb, offset);
         }
       } catch (IOException ioe) {
         // free it in bucket allocator
         bucketAllocator.freeBlock(offset);
         throw ioe;
       }
 
       realCacheSize.addAndGet(len);
       return bucketEntry;
     }
   }
 
   /**
    * Only used in test
    * @throws InterruptedException
    */
   void stopWriterThreads() throws InterruptedException {
     for (WriterThread writerThread : writerThreads) {
       writerThread.disableWriter();
       writerThread.interrupt();
       writerThread.join();
     }
   }
 
   @Override
   public Iterator<CachedBlock> iterator() {
     // Don't bother with ramcache since stuff is in here only a little while.
     final Iterator<Map.Entry<BlockCacheKey, BucketEntry>> i =
         this.backingMap.entrySet().iterator();
     return new Iterator<CachedBlock>() {
       private final long now = System.nanoTime();
 
       @Override
       public boolean hasNext() {
         return i.hasNext();
       }
 
       @Override
       public CachedBlock next() {
         final Map.Entry<BlockCacheKey, BucketEntry> e = i.next();
         return new CachedBlock() {
           @Override
           public String toString() {
             return BlockCacheUtil.toString(this, now);
           }
 
           @Override
           public BlockPriority getBlockPriority() {
             return e.getValue().getPriority();
           }
 
           @Override
           public BlockType getBlockType() {
             // Not held by BucketEntry.  Could add it if wanted on BucketEntry creation.
             return null;
           }
 
           @Override
           public long getOffset() {
             return e.getKey().getOffset();
           }
 
           @Override
           public long getSize() {
             return e.getValue().getLength();
           }
 
           @Override
           public long getCachedTime() {
             return e.getValue().getCachedTime();
           }
 
           @Override
           public String getFilename() {
             return e.getKey().getHfileName();
           }
 
           @Override
           public int compareTo(CachedBlock other) {
             int diff = this.getFilename().compareTo(other.getFilename());
             if (diff != 0) return diff;
 
             diff = Long.compare(this.getOffset(), other.getOffset());
             if (diff != 0) return diff;
             if (other.getCachedTime() < 0 || this.getCachedTime() < 0) {
               throw new IllegalStateException("" + this.getCachedTime() + ", " +
                 other.getCachedTime());
             }
             return Long.compare(other.getCachedTime(), this.getCachedTime());
           }
 
           @Override
           public int hashCode() {
             return e.getKey().hashCode();
           }
 
           @Override
           public boolean equals(Object obj) {
             if (obj instanceof CachedBlock) {
               CachedBlock cb = (CachedBlock)obj;
               return compareTo(cb) == 0;
             } else {
               return false;
             }
           }
         };
       }
 
       @Override
       public void remove() {
         throw new UnsupportedOperationException();
       }
     };
   }
 
   @Override
   public BlockCache[] getBlockCaches() {
     return null;
   }
 }