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What are they doing during those 20 minutes?

Define "maintenance window" - the DB becomes read-only? Or unreachable? This sounds insane to me.

Actions that require database downtime/restarts can happen during the maintenance window. For example, if you change a DB parameter that requires a database restart, you can tell AWS to restart the DB during the next maintenance window. Same goes for DB engine upgrades and the like.

If you're running anything production-worthy, you'll be using one of AWS's multi-AZ/failover solutions for your production RDS instances. In that case, the database will perform a failover in those instances so there isn't downtime.

That depends on the parameter being changed; not all parameter changes are compatible with block level replication. Such as the MySQL InnoDB log file size, something that needs to be changed on every instance.

> In that case, the database will perform a failover in those instances so there isn't downtime.

Have you done that? I have and there's been downtime every time (~5m).

Not usually, but it sometimes will go offline for 5 or 10 minutes or more. It fails over essentially.

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