Storage increase seems to be taking days

In most cases scaling storage occurs in a short amount of time, however for a small set of legacy volumes it is not possible to modify the volume in place. In these cases the duration of the scale storage depends on the size of the original volume, which is quite large in your case.
In your case I see you restored from your prod instance, this new instance you are testing is a physical copy of that prod instance.

The good part is that the instance is available for the duration of the scaling operation. And when this scaling operation completes, this instance will not require the legacy mechanism going forward, once complete any future scaling will use the faster method.

If you absolutly cannot run the scale storage on your prod instance, an alternative wouldbe to create a read replica, then scale the read replica and promote this to be the new prod db.

Hope that helps.

-Phil

Hi Phil,

So we did exactly this solution. We ran a few tests and everything went well so we waited until the holidays were over before running this on production. As you can likely see if you take a look, the replica lag is huge and growing slowly everyday. I've doubled the instance type and double checked all of the relevant flags and parameter group options I could think of to boost performance but can't seem to get anywhere.

Any ideas?