private Bucket grabGlobalCompletelyFreeBucket() {
    for (BucketSizeInfo bsi : bucketSizeInfos) {
      Bucket b = bsi.findAndRemoveCompletelyFreeBucket();
      if (b != null) return b;
    }
    return null;
  }
Allocate a block with specified size. Return the offset
@param blockSize size of block
@return the offset in the IOEngine
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/bucket/BucketAllocator.java