Cortex is a horizontally scalable, highly available, multi-tenant, long term storage for Prometheus.
Grafana Mimir is an open source software project that provides a scalable long-term storage for Prometheus. Mimir allows you to scale to 1 billion metrics and beyond, with simplified deployment, high availability, multi-tenancy, durable storage, and blazing fast query performance that is up to 40x faster than Cortex