EBS baseline performance too high?

2
votes

I am trying to benchmark an RDS instance (postgres) on AWS.

I created the instance with a 30 GB "general purpose" SSD volume ("gp2"). according to the AWS docs, this should provide a baseline performance of 100 IOPS:

Between a minimum of 100 IOPS (at 33.33 GiB and below) and a maximum of 10,000 IOPS (at 3,334 GiB and above), baseline performance scales linearly at 3 IOPS per GiB of volume size.

but in addition to that, there is burst performance:

When using General Purpose (SSD) storage, your DB instance receives an initial I/O credit balance of 5.4 million I/O credits, which is enough to sustain a burst performance of 3,000 IOPS for 30 minutes.

As I'm interested in sustained database performance (= the baseline case), I have to get rid of all I/O credits before starting my tests. I did this by running pgbench.

In the following screenshot, you can see that I start pgbench at 11:00, and around 3 hours later the burst balance is finally used up, and write IOPS drops off:

write IOPS chart

So far, so good. the timing makes sense -- 3 * 60 * 60 * 600 = 6.48 million (I/O credits are also refilled during the burst).

What I don't understand: why doesn't IOPS drop down to the baseline rate (100), but stay at 380 instead? Is the documented formula for baseline performance not valid any more?

UPDATE: i've shut down this test instance now, but here are the details:

configuration details

1
performancepostgresqlamazon-web-servicesamazon-ec2amazon-rds
Hello, can you please provide us all of the RDS instance type details? This will be important in helping you solve your concern. Thanks. - Taterhead
@Taterhead i've added a screenshot of the configuration details. see anything interesting? - tlossen

1 Answers

0
votes

sorry for the delay in my response

Why the extra performance?

With the db.m3.xlarge (which falls under Standard - Previous Edition header) - you have an extra 500 Mbps of additional, dedicated capacity for Amazon Elastic Block Store. This is per the chart and details at this link.

In the first section of Amazon EBS Performance Tips, it says to use EBS optimized instances for increased performance. So, I'd say this was the main reason you were getting the extra IOPS over the 100, after you exhausted your burst credits.

Cost Considerations:

According to the end of the paragraph, having your M3, you will incur extra cost for the extra performance. However, if you were to select the M4, the extra performance incurs no extra cost.

So in sustained database performance cost analysis, I would consider just the base price of the M4 vs. base price of M3 + incurred performance cost the M3 will bring you.

Good luck.