MySQL - master - slave

What about that?

You can install mysqlnd extension and mysqlnd_ms plugin. It will transparently query slave(s) when needed so no code change is required in your app or CI.

More info here: http://php.net/manual/en/book.mysqlnd-ms.php

yes I know that but in CI active record there is no option to add comments in query? Is there any option for this?

What do you understand under Mysql Master - Slave ?

If its a Replication MYSQL service handles this relation.

If its some kind of duplication with 2 instances you need to specify 2nd DB connection inside your app..

you just do not get it