Comment 2 for bug 975794

errno 2 is not a culprit, it is just an error propagation: wsrep_sst_xtrabackup script didn't create a magic file - because it failed previously due to tar error.

donor command line:
innobackupex --galera-info --tmpdir=/tmp --stream=tar /tmp 2>/var/lib/mysql/innobackup.backup.log | nc ec2-50-18-2-242.us-west-1.compute.amazonaws.com 4444

joiner command line:
nc -dl 4444 | tar xfi - -C /var/lib/mysql/

Looking into /var/lib/mysql/innobackup.backup.log reveals:
>>>>>>>>>>>>>>>>>>>
InnoDB Backup Utility v1.5.1-xtrabackup; Copyright 2003, 2009 Innobase Oy
and Percona Inc 2009-2012. All Rights Reserved.

This software is published under
the GNU GENERAL PUBLIC LICENSE Version 2, June 1991.

120407 10:40:00 innobackupex: Starting mysql with options: --unbuffered --
120407 10:40:00 innobackupex: Connected to database with mysql child process (pid=21850)
120407 10:40:06 innobackupex: Connection to database server closed
IMPORTANT: Please check that the backup run completes successfully.
           At the end of a successful backup run innobackupex
           prints "completed OK!".

innobackupex: Using mysql Ver 14.14 Distrib 5.5.21, for Linux (x86_64) using readline 5.1
innobackupex: Using mysql server version Copyright (c) 2000, 2011, Oracle and/or its affiliates. All rights reserved.

xtrabackup: Error: Please set parameter 'datadir'
innobackupex: fatal error: no 'mysqld' group in MySQL options
innobackupex: fatal error: OR no 'datadir' option in group 'mysqld' in MySQL options
<<<<<<<<<<<<<<<<<

However innobackupex does not seem to have a datadir option, so it has to be set in my.cnf for it to work. This may seem as a user configuration error, however MySQL does not require setting datadir variable in my.cnf to run. In other words, valid my.cnf should not break innobackupex.