PlanetScale has deployed millions of Amazon Elastic Block Store (EBS) volumes across the world. We create and destroy tens of thousands of them every day as we stand up databases for customers, take backups, and test our systems end-to-end. Through this experience, we have an unique viewpoint into the failure rate and mechanisms of EBS, and have spent a lot of time working on how to mitigate them.
In complex systems, failure isn’t a binary outcome. Cloud native systems are built without single paths of failure, but partial failure can still result in degraded performance, loss of user-facing availability, and undefined behavior. Often, minor failure in one part of the stack appears as a full failure in others.
For example, if a single instance inside of a multi-node distributed caching system runs out of networking resources, the downstream application will interpret error cases as cache misses to avoid failing the request. This will overwhelm the database when the application floods it with queries to fetch data as though it was missing. In this, a partial failure at one level results in a full failure of the database tier, causing downtime.
Defining Failure
While full failure and data loss is very rare with EBS, “slow” is often as bad as “failed”, and that happens much much more often.
Here’s what “slow” looks like, from the AWS Console:
This volume has been operating steadily for at least 10 hours. AWS has reported it at 67% idle, with write latency measuring at single-digit ms/operation. Well within expectations. Suddenly, at around 16:00, write latency spikes to 200ms-500ms/operation, idle time races to zero, and the volume is effectively blocked from reading and writing data.
To the application running on top of this database: this is a failure. To the user, this is a 500 response on a webpage after a 10 second wait. To you, this is an incident. At PlanetScale, we consider this full failure because our customers do.
The EBS documentation is useful in helping us understand what promises AWS’ gp3 is able to make:
When attached to an EBS–optimized instance, General Purpose SSD (gp2 and gp3) volumes are designed to deliver at least 90 percent of their provisioned IOPS performance 99 percent of the time in a given year
This means a volume is expected to experience under 90% of its provisioned performance 1% of the time. That’s 14 minutes of every day or 86 hours out of the year of potential impact. This rate of degradation far exceeds that of a single disk drive or SSD. This is the cost of separating storage and compute and the sheer complexity of the software and networking components between the client and the backing disks for the volume.
In our experience, the documentation is accurate: sometimes volumes pass in and out of their provisioned performance in small time windows:
However, these short windows are enough to have impact on real-time workloads:
Production systems are not built to handle this level of sudden variance. When there are no guarantees, even overprovisioning doesn’t solve the problem. If this were a once-in-a-million chance, it would be different, but as we’ll discuss below, that is far from the case.
The True Rate of Failure
At PlanetScale, we see failures like this on a daily basis - the rate of failure is frequent enough that we’ve built systems that monitor EBS volumes directly to minimize impact.
This is not a secret, it's from the documentation. AWS doesn’t describe how failure is distributed for gp3 volumes, but in our experience it tends to last 1-10 minutes at a time. This is likely the time needed for a failover in a network or compute component.
Let's assume the following: Each degradation event is random, meaning the level of reduced performance is somewhere between 1% and 89% of provisioned, and your application is designed to withstand losing 50% of its expected throughput before erroring. If each individual failure event lasts 10 minutes, every volume would experience about 43 events per month, with at least 21 of them causing downtime!
In a large database composed of many shards, this failure compounds. Assume a 256 shard database where each shard has one primary and two replicas: a total of 768 gp3 EBS volumes provisioned. If we take the 50% threshold from above, there is a 99.65% chance you have at least one node experiencing a production-impacting event at any given time.
Even if you use io2, which AWS sells at 4x to 10x the price, you’d still be expected to be in a failure condition roughly one third of the time in any given year on just that one database!
To make matters worse, we also see these frequently as correlated failure inside of a single zone, even using io2 volumes:
With enough volumes, the rate of experiencing EBS failure is 100%: our automated mitigations are consistently recycling underperforming EBS volumes to reduce customer-impact, and we expect to see multiple events on a daily basis.
That’s the true rate of failure of EBS: it’s constant, variable, and all by design. Because there are no performance guarantees when volumes are not operating to their specifications, it is extremely difficult to plan around for workloads that require consistent performance. You can pay for additional nines, but with enough drives over a long enough timeframe, failure is guaranteed.
Handling Failure
At PlanetScale, our mitigations have clamped down on the expected maximum time for an impact window. We monitor metrics such as read/write latency and idle % closely, and we've even developed basic tests like making sure we can write to a file. This allows us to respond quickly to performance issues, and ensures that an EBS volume isn’t ‘stuck’.
When we detect that an EBS volume is in a degraded state using these heuristics, we can perform a zero-downtime reparent in seconds to another node in the cluster, and automatically bring up a replacement volume. This doesn’t reduce the impact to zero, as it’s impossible to detect this failure before it happens, but it does ensure the majority of the cases don’t require a human to remediate and are over before users notice.
This is why we built PlanetScale Metal. With a shared-nothing architecture that uses local storage instead of network-attached storage like EBS, the rest of the shards and nodes in a database are able to continue to operate without problem.
