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:

https://www.percona.com/blog/2017/08/08/avoiding-the-an-optimized-without-redo-logging-ddloperation-has-been-performed-error-with-percona-xtrabackup/

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?