@Override
    public String toString() {
      return MoreObjects.toStringHelper(this.getClass()).add("sizeIndex", sizeIndex)
        .add("bucketSize", bucketSizes[sizeIndex]).toString();
    }
Find a bucket to allocate a block
@return the offset in the IOEngine
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/bucket/BucketAllocator.java