Primary and secondary are usually peers, where the secondary takes over when the primary isn't functioning. Which isn't the same relationship, as the master/slave terminology indicates that if the master fails, the slave will also fail.
Parent/child is probably a better way to describe this kind of relationship.
Parent/child is probably a better way to describe this kind of relationship.
Not quite as child presumes a ultimate reliance on a parent.
In master/slave (harddrive configurations) you can promote EITHER drive to master and use the other as slave. It doesn't matter which is master. Just that they both configured correctly in regards to the other.
Parent/Child doesn't really accommodate for that quirk.