Given the Following

+<------+
|       ^
V       |
M1      M2
|       ^
V       |
+-------+

and You Want the Following

      +<------+
      |       ^
      V       |
S1<---M1      M2
      |       ^
      V       |
      +-------+

Given Master IP is 10.1.2.30

Logon to the Slave and run the mysqldump like this

MAS_IP=10.1.2.30
SLV_IP=localhost
MYSQL_MAS_CONN="${MAS_IP} -uroot -ppassword"
MYSQL_SLV_CONN="${SLV_IP} -uroot -ppassword"
MYSQLDUMP_OPTIONS="--single-transaction --master-data=1 --flush-privileges"
MYSQLDUMP_OPTIONS="${MYSQLDUMP_OPTIONS} --routines --triggers"
MYSQLDUMP_OPTIONS="${MYSQLDUMP_OPTIONS} --add-drop-database --all-databases"
mysql ${MYSQL_MAS_CONN} -e"FLUSH LOGS;"
echo "STOP SLAVE;" > MySQLData.sql
mysqldump ${MYSQL_MAS_CONN} ${MYSQLDUMP_OPTIONS} >> MySQLData.sql
echo "START SLAVE;" >> MySQLData.sql
mysql ${MYSQL_MAS_CONN} < MySQLData.sql

If you have already performed this, let me address your comments.

But upon adding the third DB server and starting slave on it, it throws Cannot add or update a child row. foreign key constraint fails error.

Why would foreign key problems happen if you copied the data from a Master to a Slave? The auto_increment ids on the Slave simply did not match that of the Master. This is true even if you the Slave's Master is where you got the data from. Why?

auto_increment_increment is set to 2 in both masters. auto_increment_offset is set to 1 in Master 1 and 2 in Master 2. We have mostly innoDB tables and some MyISAM tables.

You have to make sure the Slave has the exact same auto_increment_increment and auto_increment_offset as the Master you are replicating from.

Goto the Master and run

SHOW VARIABLES LIKE 'auto_increment%';

Put those values into the Slave's my.cnf and restart mysql.

REMEMBER : A Slave's auto_increment behavior needs to be identical to that of its master.

Give it a Try !!!

UPDATE 2013-04-06 17:26

Here is the problem: Let's say you have the setup

      +<------+
      |       ^
      V       |
S1<---M1      M2
      |       ^
      V       |
      +-------+

If you did the following:

• mysqldump data from M1
• load dump it S1
• enabled Replication from M1 to S1
• M1 and S1 have identical auto_increment_offset
• M1 and S1 have identical auto_increment_increment

there should never be foreign key violations.

What you may need to do is cleanup every server's view of the auto increment values

Here is something to try

• Set these values on M1's /etc/my.cnf
  • auto_increment_increment = 10
  • auto_increment_offset = 1
• Set these values on M2's /etc/my.cnf
  • auto_increment_increment = 10
  • auto_increment_offset = 1
• Set these values on S1's /etc/my.cnf
  • auto_increment_increment = 10
  • auto_increment_offset = 1

Restart mysql on all three servers

Load Data like this:

• mysqldump from M1 to
• Run SET GLOBAL sql_log_bin = 0; on M1
• Load the mysqldump into M1
• Run SET GLOBAL sql_log_bin = 1; on M1
• Run SET GLOBAL sql_log_bin = 0; on M2
• Load the mysqldump into M2
• Run SET GLOBAL sql_log_bin = 1; on M2
• Load the mysqldump into S1
• Reset Replication amongst the threee server

If nothing breaks, you can then do this

Set these values on M2's /etc/my.cnf - auto_increment_increment = 10 - auto_increment_offset = 2

Restart mysql on M2

Everything should be clean now. From here, INSERTs should be properly handled.

Give it a Try !!1

This is a guess, but please note that the option server-id cannot be activated in a hot way. In my experience, this is a common cause of problems when setting up the replication for the first time, specially because the master accepts the following syntax without complaining:

mysql> SET GLOBAL server_id = 1;
Query OK, 0 rows affected (0.04 sec)

And the server id seems to have taken effect:

mysql> SHOW GLOBAL VARIABLES like 'server\_id';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| server_id     | 1     |
+---------------+-------+
1 row in set (0.02 sec)

But it doesn't work, (it does not have effect until restart and read from the my.cnf or passed as a parameter). The slave will complain until the master is restarted.

The official documentation is misleading, while it says it is dynamic in reality it is not (or at least it is not when setup for the first time, or setup with the default, etc.). See this bug for details, mentioning the misleading documentation.

In summary, activation of the binary logs and server id setup requires a server restart, so I always recommend to do it on installation time. Change it on the [mysqld]section of your configuration file and restart the service.