MySQL replication


If you’re a heavy user of Autotest and its reporting/graphing functionality its possible that you’ve experienced slow downs that database slave(s) could mitigate. There are lots of guides on the internet for doing MySQL replication. This presents just one possible way to set it up.

Notes on replication:

  • Only read-only operations can go through the slave. At the moment, only the new TKO interface supports splitting read-only and read-write traffic up between servers.
  • MySQL replicates by replaying SQL statements. This means that it is possible to construct SQL statements that will execute non-deterministically on replicas. None of the commands Autotest runs should have this problem, but you need to know it’s possible. This also means that you might want to verify the consistency of the slave database once in a while.
  • MySQL replication happens in one thread. In highly parallelizable, write heavy workloads, the slave will probably fall behind. In practice this is pretty much never an issue.
  • ...there’s lots of other caveats. If you’re still reading, you might want to check out

Preparing the Master

First of all, you’re going to need to set up the binary log. All queries which might affect the database (i.e. not SELECTs) will be written to this log. Replication threads will then read the file and send updates to the database slaves. Because it’s in a file, this also means that if a slave goes off line for a while (under the limit we’ll set in a moment), it can easily re-sync later.

Open the /etc/mysql/my.cnf file with root permissions (so probably with sudo).

Uncomment out (or add) the following lines in the [mysqld] section of the file.

server-id               = SOMETHING_UNIQUE
log_bin                 = /var/log/mysql/mysql-bin.log
expire_logs_days        = 10
max_binlog_size         = 100M

The server-id needs to be an unique 32 bit int but otherwise doesn’t matter. The log_bin says to use binary logging and specifies the prefix used for log files. The log files are rotated when they become max_binlog_size and are kept for expire_logs_days days.

Restart the mysql server and log into the prompt with the mysql command. Now create a user for replication:

GRANT REPLICATION SLAVE ON *.* TO 'slave_user'@'%' IDENTIFIED BY 'some_password';

Creating a Snapshot

MySQL has a built in command to sync a slave to a master without any existing data, but this isn’t useful in a production environment because it locks all the tables on the master for an extended period of time. The following is a good compromise of downtime (it’ll lock things for a couple minutes) and ease of use. If you can’t have any down time, consult other resources and good luck. :-)

The following command will dump all databases to a file called /tmp/backup.sql. It uses extended inserts which cuts down on the file size, but makes the file (a bit) less portable. The –master-data tells it to write what the current bin-log location is to the beginning of the file and causes the database to be read-only locked during the duration.

mysqldump -uroot -p --all-databases --master-data --extended-insert > /tmp/backup.sql

Setting up the Slave

On the database slave, simply copy over the SQL dump you created in the last step and (assuming the dump is in /tmp/backup.sql):

mysql -uroot -p < /tmp/backup.sql

Now edit your /etc/mysql/my.cnf. Add the following lines under the [mysqld] section:

server-id               = SOMETHING_UNIQUE
log_bin                 = /var/log/mysql/mysql-bin.log
expire_logs_days        = 10
max_binlog_size         = 100M
read_only               = 1

The read_only parameter makes it so that only DB slave processes and those with SUPER access can modify the database. The log_bin turns on the binary logging so that other servers can be chained off of this replica.

If you’re using a debian based distro, you’ll need to copy over the login data from the /etc/mysql/debian.cnf of the master to the slave.

Stop and start mysql.

sudo /etc/init.d/mysql stop
sudo /etc/init.d/mysql start

Out of the SQL dump we loaded earlier, get the master position via

grep  'CHANGE MASTER' /tmp/backup.sql | head -n1

Open up a mysql root prompt and run the following command (modified for your local setup). After that, start the slave thread and show the current status.

CHANGE MASTER TO MASTER_HOST='', MASTER_USER='slave_user', MASTER_PASSWORD='some_password', MASTER_LOG_FILE='from the output above', MASTER_LOG_POS=ditto;

On your database master, you can run SHOW MASTER STATUS;’ and verify that the slave is up to date (or is currently catching up).