What is the size of your RDS backups and snapshots?

According to the Amazon documentation, many users never pay for automated backups and DB Snapshots

There is no additional charge for backup storage, up to 100% of your total database storage for a region. (Based upon our experience as database administrators, the vast majority of databases require less raw storage for a backup than for the primary dataset, meaning that most customers will never pay for backup storage.)

But what if you are not one a lucky one? How do you know if you can increase the retention period of a RDS instance without ending up paying for extra storage? Or how much do you need to reduce your retention period to use only the (free) quota?

Here it gets tricky: there is no API to find out the total size of your backups or snapshots and there is nothing available in the console either.

As snapshots are incremental, it is really down to the activity of your database and it might be quite hard to predict the storage extra you are going to pay at the end of the month if you have a fixed retention or some business requirements on manual snapshots.

Sometime it might even be better to allocate more storage to the RDS instances in the region – and have access as well to higher IOPS on standard SSD storage – than simply pay for the extra storage of the snapshots.

But how can you figure out the best balance to take advantage of the free storage or forecast your backup storage? There is no direct way to find it out but you can estimate the cost – and so indirectly the size –  by running a billing report.

Billing report and backup usage information

You can get a billing report from the console going to the Billing & Cost Management and:

  • select “Reports”
  • choose “AWS Usage Report”
  • select “Amazon RDS Service”
  • specify time period and  granularity
  • download report in XML or CVS format.
  • open the report and check the “TotalBackupUsage” and “ChargedBackupUsage”

Knowing now the cost and the AWS S3 pricing for your region, you can now determine the storage.

How can I automate the process?

You can generate and have a Hourly Cost Allocation Report delivered automatically to a S3 bucket and process the information and create your alarms and logic accordingly.

For more information see the Monthly Cost Allocation Report page


Triggering a failover when running out of credits on db.t2

Since 3 years ago, RDS offers the option of running a MySQL database using the T2 instance type, currently from db.t2.micro to db.t2.large. These are low-cost standard instances that provide a baseline level of CPU performance – according to the size — with the ability to burst above the baseline using a credit approach.

You can find more information on the RDS Burst Capable Current Generation (db.t2) here and on CPU credits here.

What happens when you run out of credits?

There are two metrics you should monitor in CloudWatch, the CpuCreditUsage and the CpuCreditBalance. When your CpuCreditBalance approaches zero, your CPU usage is going to be capped and you will start having issues on your database.

It is usually better to have some alarms in place to prevent that, either checking for a minimum number of credits or spotting a significant drop in a time interval. But what can you do when you hit the bottom and your instance remains at the baseline performance level?

How to recover from a zero credit scenario?

The most obvious approach is to increase the instance class (for example from db.t2.medium to db.t2.large) or switch to a different instance type, for example a db.m4 or db.c3 instance. But it might not be the best approach when your end users are suffering: if you are running a Multi-AZ database in production, this is likely not the fastest option to recover, as it first requires a change of instance type on the passive master and then a failover to the new master.


You can instead try a simple reboot with failover: the credit you see on CloudWatch is based on current active host and usually your passive master has still credits available as it is usually less loaded than the master one. As for the sceenshot above, you might gain 80 credits without any cost and with a simple DNS change that minimizes the downtime.

Do I still need to change the instance class?

Yes. Performing a reboot with failover is simply a way to reduce your recovery time when having issues related to the capped CPUs and gain some time. It is not a long term solution as you will most likely run out of credits again if you do not change your instance class soon.

To summarize, triggering a failover on a Multi-AZ RDS running on T2 is usually a faster way to gain credits than modifying immediately the instance class.

Percona Live: Do Not Press That Button

PLD-17-01.pngIf I had to mention a single technical blog that I always find informative and I have followed for many years, I would say without doubts the Database Performance Blog from Percona. That is why I am so keen to attend this year the Percona Live Open Source Database Conference in Dublin and present a lighting talk Do Not Press That Button” on September 26th. You can find more about my short session on RDS here. Looking forward to Dublin!

My RDS is running out of IOPS. What can I do?

One of the hardest challenges to handle with RDS is running out of IOPS.

How RDS storage works

If you are not already familiar with the topic, there is a very detailed Storage for Amazon RDS page that covers the different storage options. The  GP2 volumes have the base performance 3 times of their allocated size. For example a 200GB RDS will have a baseline of 600 IOPS, a 1TB RDS will have a baseline of 3000 IOPS. In case you temporary need more IOPS, the GP2 volumes will give you a Burst Balance up to a maximum 3000 IOPS. When the Burt Balance is empty, you go down to the base performance (for example 600 IOPS).

How do I know how long I can burst?

Congratulations! Your application is finally successful, you have a nice increase in traffic and you go over your IOPS baseline. The very first challenge is to decide if you can handle the peak in traffic with the available burst of if you need to provide more IOPS to the RDS instance.

Not long ago AWS announced the Burst Balance Metric for EC2’s General Purpose SSD (gp2) Volumes but unfortunately as for today there is no such metric available in RDS to check the IOPS Burst Balance, it is available only for the EBS volumes attached to a EC2 instance. So after a back-of-the-envelope calculation (AWS provides a formula to calculate how long you can burst), you decide the burst balance is sadly not enough (your application is really successful!) and you need to increase as soon as possible your baseline.

What is the safest and quickest approach to handle the increased IOPS for a RDS Instance?

Let’s immediately discard the option of changing the instance type. Unless you are currently running a micro or small t2 instance, the change doesn’t usually have any effect the IOPS performance.

You are now left with the two standard options: increase the allocated storage on the gp2 volume (for example from 200GB to 400GB doubling the IOPS from 600 to 1200) or rely on Provisioned IOPS (allocating 1000 or 2000 PIOPS for the 200GB volume). Note that RDS doesn’t allow you to reduce the storage later on, so you need to consider if you really need that storage for long term or it’s better the flexibility of PIOPS.

Unfortunately both these options have a major impact on available IOPS during the change of storage with a likely very long “modifying status” for the RDS with no chance to apply further changes. While you experience a peak of usage and you are running out of IOPS, you will actually reduce even further your available IOPS as the RDS instance will allocate an entire new volume and fight with your application for the currently available IOPS.

Any other option?

Sharding. If your application supports sharding, you can create a second RDS instance doubling the available IOPS and changing the configuration of your application. You will control the downtime to create the new instance but you will have no easy way to go back in the future as you will need to merge manually the content of the two RDS instances.

Do nothing?

It does not matter too much if you are running General Purpose SSD (gp2) or a Provisioned IOPS (PIOPS) volume. Unfortunately there is no quick way to recover from a scenario where you are consuming the credit balance of the IOPS and almost all if there is any way to monitor that consumption of the burst capacity in a reliable way. If you can afford it, do nothing (immediately). If you have a predictable pattern in traffic – for example lower traffic during the night – it’s actually better not to act immediately, accept a temporary degradation of the RDS instance and plan the change in PIOPS or storage size when the load is lower and there are more available IOPS for the instance modification. The copy of the volume will be significantly faster and you will have a better control of the RDS instance.