Why is RDS burst balance seemingly random, never below 99%, and not correlated to system traffic?

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I'm trying make sense of the RDS "Burst Balance" metric that I'm seeing over the past couple months. According to the metrics reported in CloudWatch, my burst balance never seems to drop lower than 99%, but sometimes it stays near 99% for weeks and sometimes it recovers back to 100% rather quickly. The graph below is Burst Balance between 5/1/2020 and 7/23/2020.

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I'm confused because there seems to be no correlation between this metric and any other metric, such as system traffic reported by the load balancer. Also - why does it never drop below 99% and why does it sometimes take weeks to return to 100% even though there is very little traffic on my system past 5:00 PM every day, and virtually none on weekends? Here is my system traffic (load balancer request count) for that same time period:

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Why is the burst balance all over the place when the system traffic is very predictable and repetitive?

amazon-cloudwatch amazon-rds amazon-web-services database
1 Answers

The burst balance relates to disk usage on your RDS instance.

This assume you're using a GP2 volume which is bustable (it has credits that can be depleted too). Just like burstable instance classes (T2/T3) GP2 has a balance of available credits in which its IOPs can reach upto 3000 PIOPs.

GP2 provides dedicated IOPs of 3 times the number of GiB storage of the instance (For example if you have 100 GiB of storage then you will 300 PIOPs guaraneteed, anything over this will use your credits).

Once it reaches over this number you will start depleting this balance, but when under it will replenish. When its on the cusp of PIOPs usage then you're likely not to either replenish or gain balance (as you see).

To see more information over the usage check read and write Ops of your RDS instance from within CloudWatch metrics.You can combine this with performance insights to find any specific actions that might be leading to this.

More information about I/O Credits and Burst Performance is available from within the documentation.

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