Some output from the log file when this is set wrong,  adding for bug history purposes


150727 15:18:48 [Note] WSREP: Flow-control interval: [23, 23]
150727 15:18:48 [Note] WSREP: New cluster view: global state: 02e60dd2-20fb-11e5-b5d1-b668c5066ff4:6805821, view# 2: Primary, number of nodes: 2, my index: 0, protocol version 3
150727 15:18:48 [Note] WSREP: wsrep_notify_cmd is not defined, skipping notification.
150727 15:18:48 [Note] WSREP: REPL Protocols: 7 (3, 2)
150727 15:18:48 [Note] WSREP: Service thread queue flushed.
150727 15:18:48 [Note] WSREP: Assign initial position for certification: 6805821, protocol version: 3
150727 15:18:48 [Note] WSREP: Service thread queue flushed.
150727 15:18:49 [Note] WSREP: Member 1.0 (605021-infra02_galera_container-d43cfc6c) requested state transfer from '*any*'. Selected 0.0 (605023-infra03_galera_container-bd4b49ab)(SYNCED) as donor.
150727 15:18:49 [Note] WSREP: Shifting SYNCED -> DONOR/DESYNCED (TO: 6805826)
150727 15:18:49 [Note] WSREP: wsrep_notify_cmd is not defined, skipping notification.
150727 15:18:49 [Note] WSREP: Running: 'wsrep_sst_xtrabackup --role 'donor' --address '172.27.236.238:4444/xtrabackup_sst' --auth 'root:5cd96ef20bdc302a5c1d0e965bfe5b1a' --socket '/var/run/mysqld/mysqld.sock' --datadir '/var/lib/mysql/' --defaults-file '/etc/mysql/my.cnf' --gtid '02e60dd2-20fb-11e5-b5d1-b668c5066ff4:6805826''
150727 15:18:49 [Note] WSREP: sst_donor_thread signaled with 0
WSREP_SST: [INFO] Streaming with tar (20150727 15:18:49.302)
WSREP_SST: [INFO] Using socat as streamer (20150727 15:18:49.304)
WSREP_SST: [INFO] Streaming the backup to joiner at 172.27.236.238 4444 (20150727 15:18:49.313)
WSREP_SST: [INFO] Evaluating innobackupex --defaults-file=/etc/mysql/my.cnf $INNOEXTRA --galera-info --stream=$sfmt ${TMPDIR} 2>${DATA}/innobackup.backup.log | socat -u stdio TCP:172.27.236.238:4444; RC=( ${PIPESTATUS[@]} ) (20150727 15:18:49.316)
WSREP_SST: [ERROR] innobackupex finished with error: 13.  Check /var/lib/mysql//innobackup.backup.log (20150727 15:18:49.511)
WSREP_SST: [ERROR] Cleanup after exit with status:22 (20150727 15:18:49.514)
150727 15:18:49 [ERROR] WSREP: Failed to read from: wsrep_sst_xtrabackup --role 'donor' --address '172.27.236.238:4444/xtrabackup_sst' --auth 'root:5cd96ef20bdc302a5c1d0e965bfe5b1a' --socket '/var/run/mysqld/mysqld.sock' --datadir '/var/lib/mysql/' --defaults-file '/etc/mysql/my.cnf' --gtid '02e60dd2-20fb-11e5-b5d1-b668c5066ff4:6805826'
150727 15:18:49 [ERROR] WSREP: Process completed with error: wsrep_sst_xtrabackup --role 'donor' --address '172.27.236.238:4444/xtrabackup_sst' --auth 'root:5cd96ef20bdc302a5c1d0e965bfe5b1a' --socket '/var/run/mysqld/mysqld.sock' --datadir '/var/lib/mysql/' --defaults-file '/etc/mysql/my.cnf' --gtid '02e60dd2-20fb-11e5-b5d1-b668c5066ff4:6805826': 22 (Invalid argument)
150727 15:18:49 [ERROR] WSREP: Command did not run: wsrep_sst_xtrabackup --role 'donor' --address '172.27.236.238:4444/xtrabackup_sst' --auth 'root:5cd96ef20bdc302a5c1d0e965bfe5b1a' --socket '/var/run/mysqld/mysqld.sock' --datadir '/var/lib/mysql/' --defaults-file '/etc/mysql/my.cnf' --gtid '02e60dd2-20fb-11e5-b5d1-b668c5066ff4:6805826'
150727 15:18:49 [Warning] WSREP: 0.0 (605023-infra03_galera_container-bd4b49ab): State transfer to 1.0 (605021-infra02_galera_container-d43cfc6c) failed: -22 (Invalid argument)