public synchronized void instantiateBucket(Bucket b) {
      assert b.isUninstantiated() || b.isCompletelyFree();
      b.reconfigure(sizeIndex, bucketSizes, bucketCapacity);
      bucketList.put(b, b);
      freeBuckets.put(b, b);
      completelyFreeBuckets.put(b, b);
    }
Allocate a block in this bucket, return the offset representing the position in physical
space
@return the offset in the IOEngine
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/bucket/BucketAllocator.java