How to perform backup on quite active 500G+ MariaDB Galera Cluster?
Plan A:  Using just the nodes of the cluster:


take a node out of the cluster: SET GLOBAL wsrep_desync = ON; SET GLOBAL wsrep_on = OFF; (See wsrep_desync and wsrep_on.)
back it up via Xtrabackup (or whatever). 
add it back to cluster: SET GLOBAL wsrep_on = ON; SET GLOBAL wsrep_desync = OFF;
eventually IST will get that node in sync with the others.


Cons:  You lose a node for some time -- steps 2 and 4 are time consuming.
Need gcache to be big enough to allow for IST instead of SST.

Plan B:  Permanently add a slave to one of the nodes.  Take that Slave out of replication when you want to take a backup; then put it back into replication.

Con:  Requires another server.
Pros:  Not the Cons of Plan A.