public long getFreeSize() {
    return this.totalSize - getUsedSize();
  }
Rebuild the allocator's data structures from a persisted map.
@param availableSpace capacity of cache
@param map            A map stores the block key and BucketEntry(block's meta data like offset,
                      length)
@param realCacheSize  cached data size statistics for bucket cache
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/bucket/BucketAllocator.java