Grafana Labs undertook a significant project to transition their Cortex clusters from chunks to blocks storage, ensuring zero impact on Grafana Cloud customers. This complex migration was driven by the need for a more efficient and scalable storage solution, culminating in the launch of Grafana Mimir, an open-source storage engine for Prometheus. The migration strategy involved using shadow clusters to test the new blocks storage against the existing chunks storage, ensuring performance gains and data accuracy. The process required meticulous planning, including developing the blocksconvert tool to rewrite historical data from chunks to blocks, an operation that took months due to the vast amounts of data involved. The transition was executed through a phased approach, with features like shuffle sharding and a bucket index introduced to enhance scalability and performance. The successful migration marks a new era for Grafana Labs, promising further innovations in storing and querying Prometheus metrics at scale.