Where Has All The Read IOPS Gone
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Welp, holidays finished, so back into software delivery I go.
We just finished a 3 day Hackathon, which was pretty fun. The team I was in worked on a proof of concept for providing business intelligence on our customers to internal users by connecting and correlating our many disparate data sources. It was a fairly simple React.js website backed by a C# API, which connected to a number of different systems including Microsoft Dynamics CRM, Elasticsearch (specifically, our log stack), some crazy financial database called Helpy (which I don’t even seem to be able to Google) and finally a PostgreSQL database containing customer information.
It was cool to see it all come together over just a few days, but I doubt I’ll give it its own blog post like I did for the faster S3 clone that I did for the last Hackathon. Funnily enough, the hardest part was getting the damn thing deployed to an instance of IIS on a manually provisioned server inside our office. I’ve spent so much time using automated scripts that I wrote months ago in AWS to setup websites and API’s that I’ve completely forgotten how to just hack a webserver together.
Why didn’t I use the scripts you might ask? Well, I tried, and they really only work inside AWS on top of the AMI’s we’ve built up specifically for that purpose. A bit disappointing, but I got over it.
Hackathon fun aside, I had to deal with something else when I got back from my holidays, identifying and resolving a massive unexpected increase in Read IOPS usage for one of our production databases.
And for once, It had nothing to do with RavenDB.
While I was on holidays, one of our production API’s started exhibiting some pretty degraded performance. It happened suddenly too, it was running fine one minute and then all of a sudden its latency spiked hard. Unfortunately I was not there for the initial investigation, so the first half of this post is going to be a second-hand account, with all the inherent inaccuracy that entails.
Looking into the issue, all queries to the underlying database (a postgreSQL database hosted in RDS) were taking much longer than expected, but it wasn’t immediately obvious as to why. There had been no new deployments (so nothing had changed), and it didn’t look like the amount of traffic being received had drastically increased.
The RDS monitoring statistics showed the root cause of the performance problems. There had been a massive increase in the amount of Read IOPS being consumed by the database instance, from 300ish to 700ish. This had been happening for at least a few days before the performance went downhill, so the operative question was why it had become an issue all of a sudden.
Apparently, IO credits are a thing, like CPU credits.
With the increase in Read IOPS being sustained over many hours, the IO credits available to the instance were slowly being consumed, until they were all gone and performance tanked.
The way IO credits work, to my current understanding, is that when you provision an EBS volume, the size of the volume determines the minimum guaranteed IO throughput, at a rate of 3 IOPS per GB provisioned (unless you’ve specified provisioned IOPS, which is a whole different ballgame). The volume is capable of bursting to 3000 IOPS, but operating above the minimum will consume IO credits. Running out of IO credits means no more bursting, so if the performance of your service was reliant on pulling more than the minimum IO available, it will tank. Like ours did.
Apart from the unfortunate fact that unlike CPU credits, there is no way to monitor IO credits, the problem was relatively easy to solve. Either increase the volume size of the RDS instance (to increase the minimum IO performance) or leave the size the same, but switch to using provisioned IOPS. We opted for the cheaper option (bigger disk = more performance) and the performance stabilised.
This left us in a precarious position though, as we didn’t know what had cause the dramatic increase in Read IOPS.
PostgreSQL has some pretty good logs, which is a relief, considering no equivalent tool to SQL Server Profiler exists to my knowledge.
Getting the logs for a PostgreSQL instance hosted in RDS isn’t particularly complicated. Either edit the existing parameter group associated with the instance, or create a new one with logging enabled and then associate it with the instance. The logging options we were interested in were log_statement and log_min_duration_statement, which represent what activities to log and how long a query needs to be running before it should be logged, respectively. More information about working with PostgreSQL logs in RDS can be found at the appropriate AWS webpage. Getting to the logs manually is pretty easy. They are available for download when looking at the RDS instance in the AWS console/dashboard.
The logs showed that there was a class of query that was constantly being executed to determine the most recent piece of information in the database for a subset of the data (i.e.a select top x from y where a, b). The only piece of software that can access the database is a C# API, and it uses Entity Framework 6 for all database communication, so the queries were monstrous, but once our attention had been drawn to the appropriate place, it was pretty easy to see some issues:
- The data type on the first segmentation column was a string instead of an integer (only integer values were ever present in the column, and conceptually only integer values would ever be present)
- The data type on the second segmentation column was a string instead of a guid (similar to the integer comment above)
- The primary key on the table was a simple auto ID, whereas the actual primary key should have been a compound key with column a and b above, along with a third column
- There did not appear to be any appropriate indexes on the segmentation columns
What all of this meant was that the most frequent query being executed on the database was incredibly sub-optimal. It was likely that because the most frequent query needed to do a table scan in order to get an answer, and because the RDS instance was only specified as a t2.micro, postgreSQL was constantly churning memory, which would account for the high Read IOPS. The issue likely only occurred recently because we finally hit some threshold in traffic or database size where the churning manifested.
Fixing the problem was simple. Edit the EF models to change the types and specify appropriate indexes and add a new EF migration to be executed on the next deployment. The migration was a little more complex than usual because it had to upgrade data already in the database (for the wrong column types), but all in all, the fix didn’t take a huge amount of effort.
Like good software developers though, we had to make sure that it actually did fix the problem without actually deploying to production.
Thanks to previous efforts in traffic replication, I made the assumption that it would be relatively easy to replicate our production traffic into an updated version of the service.
I was wrong.
Our previous traffic replication had been accomplished using Gor replicating traffic from a single EC2 instance (a machine hosting RavenDB) to another similar instance. Pure HTTP traffic, no URLS, just IP addresses and Gor had been manually installed and configured. I had started work on replicating traffic from our API instances to a URL of my choosing, but I hadn’t quite finished it before I went on holidays.
I needed to replicate traffic from a variable number of instances to a publicly accessible URL, and I really didn’t want to have to manually install it on each machine (and then have to do it again next time I wanted to replicate traffic).
Creating a deployable Nuget package containing Gor and some helper Powershell scripts wasn’t overly difficult, I just used a similar pattern to the one we have for our Logstash deployment package. After deploying it though, it wouldn’t work. When I say it wouldn’t work, I mean I didn’t get any feedback at all when running it, no matter how I ran it. The issue was that Gor running on windows is dependent on WinPCap being present in order to function at all. This is easy enough to accomplish when you’re doing a manual install (just go grab WinPCap and install it), but unfortunately much harder when you want to be able to deploy it to any old machine in a completely unattended fashion. WinPCap doesn’t supply a silent installer (why god why), so I was forced to install NMap silently on the machines as part of the Gor deployment. Why NMap? It also installs WinPCap silently, because they’ve customised it.
Not my greatest victory, but I’ll take it.
With Gor working, I still had an issue where even though Gor seemed to be aware of the traffic occurring on the machine (I could log it to stdout for example), no matter what I did I could not get the traffic to replicate to another service via a standard URL available to the greater internet. Long story short, the issue was that Gor (as of version 0.15.1) does not and cannot use a proxy. Being that these instances did not have public IP addresses (no need, they were hidden behind a load balancer), they had to go through a proxy to get anything meaningful done on the internet.
We’re switching to using NAT Gateways for all internet traffic from private instances, so in the long term, this won’t be an issue. For now, I have to live with the fact that proxies are a thing, so I was forced to create an internal load balancer pointing at the same EC2 instances as the public URL I was going to use.
With that in place, I finally managed to replicate the production traffic to a test service with the changes applied.
And it worked amazingly. With the changes in place, our Read IOPS dropped from 700ish to like 10. Much more efficient. The only cost was that the service had to be made unavailable to apply the database updates (because they involved the transformation of the column contents). That only took like 5 minutes though, so I just did it at night when nobody noticed or cared.
I suppose the conclusion of this entire journey is to never underestimate the impact that early design decisions can have on the long term health of a service, and that the value of detailed monitoring should never be undervalued.
Honestly, we should have picked up this sort of thing much earlier than we did (probably through load testing), but it was good that we had the systems in place to react quickly when it did happen unexpectedly.
Another benefit of the whole thing is that it proved the value of our ability to spin up an entirely self contained environment as a mechanism for testing changes under relatively real circumstances. With the ability to replicate traffic at will now (thanks to the working Gor deployment), we’re in a much better position to try things out before we go live with them.