如图，Mysql 5.7，在做Xtrabackup全备时候，执行了一条DDL（add column），备份结果说不是一个一致性备份，问下：
Xtrabackup全备过程中执行DDL，那么备份就完全不可用?可以用原理解释一下吗？

InnoDB: Last flushed lsn: 1591327780398 load_index lsn 1591437377075
InnoDB: An optimized (without redo logging) DDL operation has been performed. All modified pages may not have been flushed to the disk yet. 
PXB will not be able to make a consistent backup. Retry the backup operation