Abstract: Systems and methods are described for use of reserved storage capacity at a cloud provider network extension, sometimes referred to as a provider substrate extension (“PSE”) or “outpost”, as a data cache, thereby improving performance of the PSE. The reserved storage capacity may be reserved for a primary purpose, such as data replication in the event of failure of an in-use portion of the PSE's storage capacity. During non-failover operation, the reserved storage capacity may be otherwise unused. Embodiments of the present disclosure enable the reserved storage capacity, when not used for its primary purpose, to be used as a data cache to store information potentially used at the PSE.