Grafana Mimir for Long-Term Prometheus Storage#

Prometheus stores metrics on local disk with a practical retention limit of weeks to a few months. Beyond that, you need a long-term storage solution. Grafana Mimir is a horizontally scalable, multi-tenant time series database designed for exactly this purpose. It is API-compatible with Prometheus – Grafana queries Mimir using the same PromQL, and Prometheus pushes data to Mimir via remote_write.

Mimir is the successor to Cortex. Grafana Labs forked Cortex, rewrote significant portions for performance, and released Mimir under the AGPLv3 license. If you see references to Cortex architecture, the concepts map directly to Mimir with improvements.

Pull-Based Scraping Model#

Prometheus pulls metrics from targets rather than having targets push metrics to it. Every scrape interval (default 15s in the global config), Prometheus sends an HTTP GET to each target’s metrics endpoint. The target responds with all its current metric values in Prometheus exposition format.

This pull model has concrete advantages. Prometheus controls the scrape rate, so a misbehaving target cannot flood the system. You can scrape a target from your laptop with curl http://target:8080/metrics to see exactly what Prometheus sees. Targets that go down are immediately detectable because the scrape fails.

Why Monitor Your Monitoring#

If Prometheus runs out of memory and crashes, you lose all alerting. If its disk fills up, it stops ingesting and you have a blind spot that may last hours before anyone notices. If scrapes start timing out, metrics go stale and alerts based on rate() produce no data (which means they silently stop firing rather than triggering). Prometheus must be the most reliably monitored component in your stack.

What Cardinality Means#

In Prometheus, cardinality is the number of unique time series. Every unique combination of metric name and label key-value pairs constitutes one series. The metric http_requests_total{method="GET", path="/api/users", status="200"} is one series. Change any label value and you get a different series. http_requests_total{method="POST", path="/api/users", status="201"} is a second series.

A single metric name can produce thousands or millions of series depending on its labels. A metric with no labels is exactly one series. A metric with one label that has 10 possible values is 10 series. A metric with three labels, each having 100 possible values, is up to 1,000,000 series (100 x 100 x 100), though in practice not every combination occurs.