1. A method, comprising: receiving data objects for storage in at least: one or more first storage devices characterized by a first write endurance; and one or more second storage devices characterized by a second write endurance that is lower than the first write endurance; estimating respective write-activity levels for the data objects; and for each data object, choosing whether to store the data object in the first storage devices or in the second storage devices based on an estimated write-activity level of the data object, and storing the data object in the chosen storage devices.