Read IOPS limits for RDS read-replica?

4
votes

I've noticed strange thing happened on my PostgreSQL Amazon RDS Read replica.

We've done "stress-test" of dozens parallel high-load read requests. Performance was really good in the beginning of the test, but then rapidly decreased while PostgresSQL itself kept holding dozens of select queries which were performed fast before it stacked.

I've opened Monitor statistics tab in RDS console and have seen that along with visible performance reducing Read IOPS number also decreased from 3000/sec to 300/sec and didn't go upper then 300/sec iops for long time. At the same time CPU usage was really low ~3%, there weren't any problems with RAM or storage space.

So my question: are any documented limitations of Read IOPS for read replica? It looks like Amazon RDS automatically reduced high limit of IOPS after really high load (3000/sec).

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Read-replica server runs on db.t2.large instance with 100 GB General Purpose (SSD) storage type with disabled fixed IOPS feature.

1
performancepostgresqlamazon-rds

1 Answers

3
votes

The behavior you describe is exactly as documented for the underlying storage class GP2.

GP2 is designed to [...] deliver a consistent baseline performance of 3 IOPS/GB

GP2 volumes smaller than 1 TB can also burst up to 3,000 IOPS.

https://aws.amazon.com/ebs/details/

3 IOPS/GB on a 100GB volume is 300 IOPS.

See also http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/EBSVolumeTypes.html for a description of how IOPS credits work. While your system isn't busy, it will build up credits that can be used for the next burst.