Is it safe to run two Mimir instances writing to the same S3 bucket without multi-zone?

Grafana
1

Hi all,

I’m running Grafana Mimir in microservices mode, and I currently have:

  • Two completely separate Mimir instances (A and B) in 2 K8s Cluster, they are not streched or connected.
  • Each has its own ring, no shared coordination
  • Both are configured to write to the same S3 bucket
  • A load balancer sends remote_write traffic from Grafana Agent (Alloy) to either instance (round-robin)
  • Multi-zone mode is disabled

Now I’m wondering:

:red_question_mark: Is this safe? Will I get data loss or corruption?

Specifically:

  • What happens if Mimir-A crashes, Alloy switches to Mimir-B, and Mimir-A later replays its WAL?
  • Will this cause duplicate or overlapping blocks in S3?

Any help or clarification from the community or the Grafana team would be greatly appreciated!

Thanks :folded_hands: