We at MayaData have been using Cortex in production for more than 2 years now. Cortex is one of the core components of MayaData's Kubera SaaS platform (https://mayadata.io/product) and also packaged into its on-premise version. Cortex is used within Kubera for aggregating the Storage (node, disks, pools, volumes) metrics from over 1000s of Kubernetes clusters - and setup Alerts and notifications for managing the storage for stateful workloads. We use Cortex with Cassandra as a backend on GKE. For on-premise installations, the Cassandra backend is deployed on OpenEBS.