Single-node Prometheus limits
Memory: ~10M active series per 64GB node. Retention: practically 2-4 weeks before disk and queries get slow. HA: scrape duplication, not data replication. Past these, you're scaling out.
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Remote write — the shared on-ramp
Prometheus pushes samples via remote_write to a long-term-storage backend. All three big systems consume this protocol. The push side is solved; the storage side is where they differ.
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Mimir vs Cortex vs Thanos
Mimir (Grafana Labs): multi-tenant from the start, S3-backed blocks, horizontally scalable ingesters. The most operationally polished option in 2026. Cortex: older sibling, similar architecture, less active. Thanos: sidecar pattern, uploads Prometheus blocks to S3 directly. Simpler to bolt on; harder to scale to many tenants.