Resize Amazon RDS storage

Question

We are currently working with a 200 GB database and we are running out of space, so we would like to increment the allocated storage.

We are using General Purpose (SSD) and a MySQL 5.5.53 database (without Multi-AZ deployment).

If I go to the Amazon RDS menu and change the Allocated storage to a bit more (from 200 to 500) I get the following " warnings ":

• Deplete the initial General Purpose (SSD) I/O credits, leading to longer conversion times: What does this mean?
• Impact instance performance until operation completes : And this is the most important question for me. Can I resize the instance with 0 downtime? I mean, I dont care if the queries are a bit slower if they work while it's resizing, but what I dont want to to is to stop all my production websites, resize the instance, and open them again (aka have downtime).

Thanks in advance.

Answer 1

You can expect degraded performance but you should really test the impact in a dev environment before running this on production so you're not caught off guard. If you perform this operation during off-peak hours you should be fine though.

To answer your questions: RDS instances can burst with something called I/O credits. Burst means its performance can go above the baseline performance to meet spikes in demand. It shouldn't be a big deal if you burn through them unless your instance relies on them (you can determine this from the rds instance metrics). Have a read through I/O Credits and Burst Performance .

Changing the disk size will not result in a complete rds instance outage, just performance degradation so it's better to do it during off-peak hours to minimise the impact as much as possible.

Answer 2

The technical answer is that AWS supports no downtime when scaling storage.

However, in the real world you need to factor how busy your current database is and how the "slowdown" will affect users. Consider the possibility that connections might timeout or the site may appear slower than usual for the duration of the scaling event.

In my experience, RDS storage resizing has been smooth without problems. However, we pick the best time of day (least busy) to implement this. We also go thru a backup procedure. We snapshot and bring up a standby server to switch over to manually just in case.

Answer 3

First according to RDS FAQs , there should be no downtime at all as long as you are only increasing storage size but not upgrading instance tier.

Q: Will my DB instance remain available during scaling?

The storage capacity allocated to your DB Instance can be increased while maintaining DB Instance availability.

Second, according to RDS documentation :

Baseline I/O performance for General Purpose SSD storage is 3 IOPS for each GiB, which means that larger volumes have better performance.... Volumes below 1 TiB in size also have ability to burst to 3,000 IOPS for extended periods of time (burst is not relevant for volumes above 1 TiB). Instance I/O credit balance determines burst performance.

I can not say for certain why but I guess when RDS increase the disk size, it may defragment the data or rearrange data blocks, which causes heavy I/O. If you server is under heavy usage during the resizing, it may fully consume the I/O credits and result in less I/O and longer conversion times. However given that you started with 200GB I suppose it should be fine.

Finally I would suggest you to use multi-az deployemnt if you are so worried about downtime or performance impact. During maintenance windows or snapshots, there will be a brief I/O suspension for a few seconds, which can be avoided with standby or read replicas.