Comment 10 for bug 1428133

MySQL cluster is controlled by Pacemaker. When you introduce backup script pacemaker should know how to treat in such cases. I would suggest you to backport OCF script as there were some changes to handle such cases

http://docs.mirantis.com/openstack/fuel/fuel-6.0/operations.html#howto-backport-galera-pacemaker-ocf-script

Also, I would recommend to to read

http://docs.mirantis.com/openstack/fuel/fuel-6.0/operations.html#openstack-database-backup-and-restore-with-percona-xtrabackup