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Four solutions for Prometheus long-term storage

Titans

Article from ADMIN 85/2025

By Martin Loschwitz

If you use Prometheus as a time series database, you will know that the more data it stores, the slower it becomes. Thanos, Cortex, Mimir, and M3DB set out to solve this problem in totally different ways. We reveal the candidates' strengths and weaknesses.

One key difference between current IT setups and those of the past is that current setups are typically designed for scalability. Computing power, RAM, disk space – all can ideally be added or removed dynamically in a state-of-the-art setup. For ongoing operations, this results in a few challenges that administrators were unlikely to confront until the advent of OpenStack, Ceph, Kubernetes, and the like. They include, for example, the need to know when it's the right time to add additional resources. "When they are needed" is the easy answer, because that directly prompts the next questions: When exactly are new resources needed, and how do admins work around bottlenecks and delays in delivery of the new equipment?"

Trending software helps answer these questions early enough that you can plan in peace and sleep well at night. Under the hood, you will often find a time series database that collects and saves metrics data from all instances of the entire setup at regular intervals before correlating the acquired data and alerting if defined threshold values are exceeded. Right now, Prometheus is probably the most widely used of the candidates in this field.

If you want to scale or store data in the long term for analysis, four open source approaches stand out from the crowd:

Thanos is a cluster solution with built-in long-term storage.
Cortex makes it easy to add a genuine long-term storage engine to Prometheus.
Mimir, on the other hand, is a Cortex fork that is now very different from its ancestor and has its own functions.
M3DB is a converter between the worlds, combining central design elements of both Thanos and Cortex.

You therefore need to make a decision. It makes sense to answer three questions first: