Mysql – Do MySQL limitations apply to MariaDB as well on AWS RDS

Both DBAs are right, in a way. There are no limits on the number of schemas, except those imposed by the filesystem, as in MySQL/MariaDB, a schema is basically a directory, producing absolutely no difference for both MyISAM and InnoDB if a table is part of one schema or another in terms of performance or features (you can join 2 tables in different schemas as fast or as slow as if they were in the same schema). I've managed instances with hundreds of thousands of schemas, and the multiple directories were not the performance bottleneck. It is a pure identifier thing. While you can run into some issues, those are indirect to having more or less schemas: for example, in 5.6, a slave can apply changes in parallel only for rows from different schemas.

Why do I say that both are right, in a way? Because having too many objects in the database is a very common source of issues. When you have lots of databases, you usually have 10x or more the number of tables, and that may mean thousands of file descriptors open (that requires memory), thousands of data dictionary cache entries, millions of open table cache entries, etc., which can lead truly to a CPU and above all, memory, bottleneck.

Changing Instance Class:

From the documentation "Modifying a DB Instance and Using the Apply Immediately Parameter", changing the instance class and setting "Apply Immediately" to false will result in:

• The change being applied during the next maintenance window.
• The change causing an outage to occur.

Since the instance is Multi-AZ and you're changing the instance class, your outage will be minimized to the amount of time it takes to failover to the standby replica. That isn't very long; likely worst-case a minute or two. You won't lose data that is already in the DB. Any existing connections will be dropped during failover. You should verify that applications connected to your database can withstand these conditions, or alternatively take them down for maintenance during this time.

Identifying RDS instance storage capacity:

You can identify how much of the RDS storage is currently in use by either:

1) Simply selecting your RDS instance from the RDS web console and viewing the "Storage" bar under the "Monitoring" header.

2) Viewing the "FreeStorageSpace" RDS cloudwatch metric for your RDS instance.

Further Reading:

• RDS Documentation - Viewing DB Instance Metrics