Concurrent DDL can cause xtrabackup to see the following error in MariaDB 10.2:
[FATAL] InnoDB: An optimized(without redo logging) DDLoperation has been performed. All modified pages may not have been flushed to the disk yet.
This problem is described here:
This also seems to effect xtrabackup-v2 SSTs. Should the SST script use the --lock-ddl-per-table option in 10.2 to avoid this?