MySQL replication – adding additional slave

Master-slave replication is a common means to increase SQL performance. This post describes how to add an additional slave running on Ubuntu 14.04 Server. Replication of the master database (4.5G) is initially performed with mysqldump, and replication is started afterwards.

The scenario is a web-server setup where the capacity of the existing master-slave setup is maxed out. We'll add an extra slave to take some of the load; especially gaining extra SQL read capacity. The existing setup runs on stale Ubuntu 11.10 virtual machines – we'll take the plunge and add the new slave on Ubuntu 14.04 (LTS) in the same go.

Setup:

RoleBeforeAfter
Master Ubuntu 11.10, MySQL 5.1.69 Ubuntu 11.10, MySQL 5.1.69
Slave 1 Ubuntu 11.10, MySQL 5.1.69 Ubuntu 11.10, MySQL 5.1.69
Slave 2 Ubuntu 14.04, MySQL 5.5.40

Preparation

The new Ubuntu 14.04 server is setup with an OS disk and a data disk for easier future maintenance. The disk layout looks as follows:

slave: $ lsblk | grep vd  
vda    253:0    0    20G  0 disk  
├─vda1 253:1    0  19.5G  0 part /
├─vda2 253:2    0     1K  0 part 
└─vda5 253:5    0   510M  0 part [SWAP]
vdb    253:16   0    20G  0 disk  
└─vdb1 253:17   0    20G  0 part /var/www

MySQL databases are moved to the data disk in /var/www/mysql instead of the standard location in /var/lib/mysql – which is replaced with a symbolic link, minimizing the confusion of sysadmins accessing the server later on.

slave: $ mv /usr/lib/mysql /var/www/mysql  
slave: $ ln -s /var/www/mysql /usr/lib/mysql  

A common gotcha with placing the databases outside the common location is that AppArmor restricts MySQL access outside a predefined list of locations. MySQL is allowed access by adding relevant paths to the AppArmor profile for MySQL:

slave: $ cat >> /etc/apparmor.d/local/usr.sbin.mysqld <<EOF  
/var/www/mysql/ r,
/var/www/mysql/** rwk,
EOF  
slave: $ service apparmor reload  

In the replication master-slave setup servers identify themselves using the server-id property (a positive integer setup in the MySQL configuration). When the slave starts it opens a connection to the master which streams changes into the relay log. Unless the relay-log and relay-log-index are explicitly named in the configuration, the standard naming includes the hostname, which results in a number of problems (http://dev.mysql.com/doc/refman/5.7/en/slave-logs-relaylog.html)

In our setup, the existing master contains several databases but, only one is relevant for the new slave, namely the database named db; this is reflected in the configuration parameter replicate-do-db below. Note, however, that changes, to the master, is still sent and written to the slave in a relay log file.

The changes to the MySQL standard configuration are added:

slave: $ cat > /etc/mysql/conf.d/slave.cnf <<EOF  
[mysqld]
server-id               = 4  
relay-log               = mysql-relay-bin.log  
relay-log-index         = mysql-relay-bin.index  
replicate-do-db         = db  

Create the database and add a user. As an additional pre-caution the user is restricted to read-only access, since the slave should never be written to directly:

slave: $ mysql -u root -p  
mysql> CREATE DATABASE db;  
mysql> GRANT select ON db.* to 'user'@'localhost' identified by 'password';  

Load relevant timezone information into MySQL:

slave: $ mysql_tzinfo_to_sql /usr/share/zoneinfo/Europe/Copenhagen 'Europe/Copenhagen' | mysql -u root --password=$DB_PWD mysql  

Dump master database

The MySQL manual, describes how to setup an additional slave by replicating another slave: replication-howto-additionalslaves. The approach shuts down the slave database, while copying the data.

Another approach is to mysqldump or rsync the database from the master database, locking it for writes beforehand. This keeps the database slaves running, which in our setup is less obtrusive. Since we are crossing MySQL versions – 5.1 -> 5.5 – we'll use mysqldump to extract the data (I did not try rsync though).

On master:

master: $ mysql -u root -p  
mysql> use db;  
mysql> FLUSH TABLES WITH READ LOCK;  
mysql> SHOW MASTER STATUS;  
+------------------+----------+--------------+..
| File             | Position | Binlog_Do_DB | 
+------------------+----------+---------------..
| mysql-bin.004488 | 11131082 | db           |
+------------------+----------+--------------+..
-- do mysqldump in another session
mysql> UNLOCK TABLES;  
mysql> QUIT;  

In another SSH session on master:

master: $ mysqldump -u root -p db > db.sql  

Load database and initialize slave

After moving the database dump to the slave load it:

slave: $ mysql -u root -p db < db.sql  

Load the slave with connection and replication information about the master, and start it:

mysql> CHANGE MASTER TO  
  MASTER_HOST='10.2.10.2',
  MASTER_USER='repl',
  MASTER_PASSWORD='repl-password', 
  MASTER_LOG_FILE='mysql-bin.004488', 
  MASTER_LOG_POS=11131082;
mysql> START SLAVE;  

If the slave has access to the master database, relay files should now start to stream to the slave.

Debugging

Check the slave status to ensure things are working as expected:

slave: $ mysql -u root -p  
mysql> show slave status\G;  
*************************** 1. row ***************************
               Slave_IO_State: Waiting for master to send event
                  Master_Host: 10.2.10.2
                  Master_User: repl
                  Master_Port: 3306
                Connect_Retry: 60
              Master_Log_File: mysql-bin.004536
          Read_Master_Log_Pos: 69346276
               Relay_Log_File: mysql-relay-bin.000146
                Relay_Log_Pos: 69122883
        Relay_Master_Log_File: mysql-bin.004536
             Slave_IO_Running: Yes
            Slave_SQL_Running: Yes
              Replicate_Do_DB: db
...

Check the status of the MySQL I/O and SQL threads:

mysql> show processlist;  
+------+-------------+-----------+------+---------+--------+----------------------------------+
| Id   | User        | Host      | db   | Command | Time   | State                            |
+------+-------------+-----------+------+---------+--------+----------------------------------+
|    1 | system user |           | NULL | Connect |      8 | Slave has read all relay log     |
|    2 | system user |           | NULL | Connect | 552110 | Waiting for master to send event |
| 2264 | root        | localhost | NULL | Sleep   |  19896 |                                  |
| 2334 | root        | localhost | NULL | Query   |      0 | NULL                             |
+------+-------------+-----------+------+---------+--------+----------------------------------+
4 rows in set (0.00 sec)  

Conclusion

I can confirm that things are working smoothly with this setup. The next step is to upgrade the two stale Ubuntu 11.10 database servers.

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