Mariadb Galera Cluster using Xtradb backup is not working
In my experience, it seems one failed run can leave fragments of config that block future runs. 

Case in reference: Xbstream rather than tar is used, although the traceback is consistent in that it halts on writing to backup-my.cnf.

First run fails because my_print_defaults is reading from the wrong file, and this file was stored as the backup.

Once the --defaults-file flag is added to innobackupex, suddenly there is output on the other side of the xbstream.

xbstream: Can't create/write to file '././backup-my.cnf' (Errcode: 17 - File exists)
xbstream: failed to create file.


I believe this is an innobackupex safety feature and tar may be making the same checks as xbstream. I would have liked to comment on this rather than answer it but lack the reputation.