Optimus, our spare capacity leasing system, coordinates capacity allocations on millions of machines to improve global capacity utilization and meet fast growing business needs. Within Facebook’s infrastructure, spares are maintained to ensure capacity sufficiency and resiliency for daily peak traffic — spares remain idle for hours or days until mobilized. We will discuss the characteristics of spare capacity and the opportunities for dynamically leasing them to flexible workloads. We will share how elasticity manifests in our policies for capacity leases, how our models of fleet-wide capacity usage allow Optimus to make probabilistic guarantees on capacity availability, and showcase three workloads that effectively use leased capacity. We will also share some of the lessons we have learned in growing Optimus to its current state.