Synchronously replicating a dataset across cloud-based storage systems, including adding a cloud-based storage system to a set of storage systems that the dataset is synchronously replicated across, where access operations are applied to the dataset equivalently through all storage systems in the set, all storage systems in the set store a separate copy of the dataset, and operations to modify the dataset performed and completed through any of the storage systems in the set are reflected in access operations to read the dataset, the cloud-based storage system including one or more cloud computing instances executing a storage controller application, a virtual drive layer that includes one or more cloud computing instances with local storage for storing at least a portion of the dataset as block data, and an object storage layer for storing at least a portion of the dataset as object data.