Xtrabackup备份报错
/backup> innobackupex --defaults-file=/etc/my.cnf --user=root --password='xxxxx' /backup/20170803
170421 18:15:39 innobackupex: Starting the backup operation

IMPORTANT: Please check that the backup run completes successfully.
At the end of a successful backup run innobackupex
prints "completed OK!".

170421 18:15:39 version_check Connecting to MySQL server with DSN 'dbi:mysql:;mysql_read_default_group=xtrabackup' as 'root' (using password: YES).
170421 18:15:39 version_check Connected to MySQL server
170421 18:15:39 version_check Executing a version check against the server...
170421 18:15:39 version_check Done.
170421 18:15:39 Connecting to MySQL server host: localhost, user: root, password: set, port: 0, socket: (null)
Failed to connect to MySQL server: Can't connect to local MySQL server through socket '/tmp/mysql.sock' (2).

仔细检查，发现输入的用户名和密码都是正确的，而且可以正常连接到数据库
/backup/MySQL_Backup/bin> mysql -uroot -p
Enter password:
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 12
Server version: 5.6.21-70.1-log Percona Server (GPL), Release 70.1, Revision 698
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
mysql>

报错原因：
在配置文件中没有配置socket
Xtrabackup在备份时，会使用本地的socket连接到数据库

解决方法：
在配置文件中增加socket选项

[root@localhost bin]# cat /etc/my.cnf
[mysqld]
basedir = /usr
datadir = /var/lib/mysql
log-error = /var/lib/mysql/localhost.localdomain.err
pid-file = /var/lib/mysql/localhost.localdomain.pid
#port = 3306
socket = /var/lib/mysql/mysql.sock

之后重启数据库，可以正常备份
/backup> innobackupex --defaults-file=/etc/my.cnf --user=backup --password='backup' /backup/20170803
170421 18:34:54 innobackupex: Starting the backup operation

IMPORTANT: Please check that the backup run completes successfully.
At the end of a successful backup run innobackupex
prints "completed OK!".

170421 18:34:54 version_check Connecting to MySQL server with DSN 'dbi:mysql:;mysql_read_default_group=xtrabackup;mysql_socket=/var/lib/mysql/mysql.sock' as 'backup' (using password: YES).
170421 18:34:54 version_check Connected to MySQL server
170421 18:34:54 version_check Executing a version check against the server...
170421 18:34:54 version_check Done.
170421 18:34:54 Connecting to MySQL server host: localhost, user: backup, password: set, port: 0, socket: /var/lib/mysql/mysql.sock
Using server version 5.6.21-70.1-log
innobackupex version 2.4.4 based on MySQL server 5.7.13 Linux (x86_64) (revision id: df58cf2)
xtrabackup: uses posix_fadvise().
xtrabackup: cd to /var/lib/mysql
xtrabackup: open files limit requested 0, set to 32768
xtrabackup: using the following InnoDB configuration:
xtrabackup: innodb_data_home_dir = .
xtrabackup: innodb_data_file_path = ibdata1:12M:autoextend
xtrabackup: innodb_log_group_home_dir = ./
xtrabackup: innodb_log_files_in_group = 2
xtrabackup: innodb_log_file_size = 50331648
InnoDB: Number of pools: 1
170421 18:34:54 >> log scanned up to (201239294)
xtrabackup: Generating a list of tablespaces
InnoDB: Allocated tablespace ID 45 for dmcdbTEST/com_numbering_plan, old maximum was 0
170421 18:34:55 >> log scanned up to (201239294)
170421 18:34:55 [01] Copying ./ibdata1 to /backup/20170803/2017-04-21_18-34-54/ibdata1
170421 18:34:56 >> log scanned up to (201239294)
170421 18:34:57 >> log scanned up to (201239294)