Grafana Mimir is an open-source, horizontally scalable, and highly performant time series database created by Grafana Labs, designed to handle massive scale in collecting and querying metrics, surpassing limitations of existing systems like Cortex and Thanos. The database features a split-and-merge compactor to overcome storage limits and employs query sharding for faster query performance, achieving up to 40 times speed improvements in some cases. The project, named after the Norse figure Mimir, aims to maintain Grafana Labs' open-source philosophy while providing commercial viability and agility, and plans include expanding support beyond Prometheus metrics to others like OpenTelemetry and Datadog. The development of Mimir involved learning from Grafana's existing enterprise products and collaboration across teams, emphasizing continual improvement and open-source contributions.