How can we set a different wsrep_sst_method?

Issue

• By default Galera on the controllers is configured with wsrep_sst_method = rsync on Red Hat Openstack. In the past we had issues with this, as when 1 database server was restoring, both the donor and the receptor were blocked for transactions so all load came to the third controller. In a highly loaded environment that was too much so the third controller would fail.

• By setting the wsrep_sst_method to mariabackup, the donor will not be blocked during sst replication so we don't have the risk of the control plane going down. However there is no way as far as I can see to configure the wsrep_sst_method via the heat templates / environment files. How should we do this?

• Business impact can be that when a control node needs to resync its database, both the donor and the receptor are blocked and all load comes to the third controller which could then break as it can't handle that load.