AWS RDS: Invalid max storage size for engine name postgres and storage type gp2: 198

First, note that you may be looking at the incorrect operation -- you describe that you want to change storage size, but have quoted documentation describing storage type. This is an important distinction: RDS advises that you won't experience an outage for changing storage size, but that you will experience an outage for changing storage type.

Expect degraded performance for changing storage size, the duration and impact of which will depend on several factors:

• Your RDS instance type
• Configuration
• Will this occur during maintenance?
• Will these changes occur first on your Multi-AZ slave, and then failover?
• Current database size
• Candidate database size
• AWS capacity to handle this request at your requested time of day, at your requested availability zone, in your requested region
• Engine type (for Amazon Aurora users, storage additions are managed by RDS as-needed in 10 GB increments, so this discussion is moot)

With this in mind, you would be better served by testing this yourself, in your environment, and on your terms. Try experimenting with the following:

• Restoring a new RDS instance from a snapshot of your existing instance, and performing this operation on the new clone.
• With this clone:
  • Increase the size at different times of day, when you would expect a different load on AWS.
  • Increase to different sizes.
  • Try it with multi-AZ. See if your real downtime changes as compared to not enabling multi-AZ.
  • Try it during a maintenance window, and compare it with applying the change immediately.

This will cost a bit more (it doesn't have to... you could do most of that in 1-3 instance-hours), but you will get a much cleaner answer than peddling for our experiences in a myriad of different RDS environments.

If you're still looking for a "ballpark" answer, I would advise to plan for at least performance degradation in the scope of minutes, not seconds -- again dependent very much on your environment and configuration.

For reference, I most recently applied this exact operation to add 10GB to a 40GB db.m1.small type instance on a Saturday afternoon (in EST). The instance remained in a "modifying" state for approximately 17 minutes. Note that the modifying state does not describe real downtime, but rather the duration that the operation is being applied. You won't be able to apply additional changes to the actual instance (although you can still access the DB itself) and this is also the duration that you can expect any performance degradation to occur.

Note : If you're only planning on changing the storage size an outage is unexpected, but note that it can occur if this change is made in conjunction with other operations like changing the instance identifier/class, or storage type.

Add a CNAME record in your DNS for db.example.com that points to your RDS endpoint (without the port, i.e. my-name.ck4k21dvamqbq9.eu-west-1.rds.amazonaws.com).