Comment 5 for bug 1050355

You can check if I/O bandwidth is the issue by streaming the backup to /dev/null:

/root/backup/xtra/innobackupex --user=user --password=pwd --stream=tar --ibbackup=/path/to/xtrabackup_51 /tmp >/dev/null

xtrabackup will still copy the logfile to disk (to /tmp/xtrabackup_logfile with that command line), but the datafile will be streamed to /dev/null. If that succeeds, then it failed previously due to writes starvation.