Can't create Multi-AZ Aurora RDS Instance

Question

When I restore a MySQL snapshot, I\'m given the option to make the new instance Multi-AZ. However for some reason when I restore to Aurora, the \"Multi-AZ Deployment\" selection

Answer 1

Multi-AZ doesn't mean the same thing in Aurora as it does for MySQL and MariaDB. With Aurora, any replica in the cluster can take over for the master on failure (though the selection is based on priority, so it isn't just a randomly selected replica, if there is more than one replica), and the storage is already/always multi-AZ in Aurora because that's a core part of the Aurora design. So, Aurora can be "converted" to Multi-AZ.

Apparently, that's the only way it is done when creating an Aurora instance from a snapshot, based on this:

You can migrate a DB snapshot of an Amazon RDS MySQL DB instance to create an Aurora DB cluster.

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You can migrate either a manual or automated DB snapshot. After the DB cluster is created, you can then create optional Aurora Replicas.

http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/Aurora.Migrate.RDSMySQL.html

If you create a multi-AZ Aurora cluster without a snapshot, that's all that happens -- a master and a replica are created. In that case, you can actually delete the replica and make it non-multi-AZ, which is a different process than what's involved with MySQL or MariaDB.

"Create a cluster" -- as used in the quote, above -- is a potentially confusing term, since you would naturally assume a cluster means two or more, but in fact an Aurora cluster can technically be a "cluster" of just one instance. Every Aurora instance is part of exactly one cluster. One instance is the primary (master) and any additional instances are replicas.

If the DB cluster doesn't contain any Aurora Replicas, then the primary instance is recreated during a failure event.

http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/Aurora.Managing.html#Aurora.Managing.FaultTolerance