A common problem that is encountered when adding new replication slaves is that the new slave fails with a series of warning and error messages like these:

071118 16:44:10 [Warning] Neither --relay-log nor --relay-log-index were used; so
replication may break when this MySQL server acts as a slave and has his hostname
changed!! Please use '--relay-log=new_slave_hostname-relay-bin' to avoid this problem.
071118 16:44:10 [ERROR] Failed to open the relay log './old_slave_hostname-relay-bin.003525'
(relay_log_pos 22940879)
071118 16:44:10 [ERROR] Could not find target log during relay log initialization
071118 16:44:10 [ERROR] Failed to initialize the master info structure

This is due to the fact that, if the --relay-log option is not specified, the relay log files contain the host name as part of their file names. (This is also true of the relay log index file if the --relay-log-index option is not used. See Section 17.1.3, “Replication and Binary Logging Options and Variables”, for more information about these options.)

To avoid this problem, use the same value for --relay-log on the new slave that was used on the existing slave. (If this option was not set explicitly on the existing slave, use existing_slave_hostname-relay-bin.) If this is not feasible, copy the existing slave's relay log index file to the new slave and set the --relay-log-index option on the new slave to match what was used on the existing slave. (If this option was not set explicitly on the existing slave, use existing_slave_hostname-relay-bin.index.) Alternatively—if you have already tried to start the new slave (after following the remaining steps in this section) and have encountered errors like those described previously—then perform the following steps: