1. A method comprising: storing, by a storage server comprising one or more processors, backup data for a plurality of client systems; wherein the plurality of client systems includes a first client system and one or more other client systems; wherein the one or more other client systems includes at least a second client system; wherein the first client system is associated with a first retention duration goal; wherein the first retention duration goal indicates that the first client system desires to be able to recover data at least as old as a first specified data age; wherein the storage server does not guarantee that the first retention duration goal will be met; wherein the second client system is associated with a second retention duration goal; wherein the second retention duration goal indicates that the second client system desires to be able to recover data at least as old as a second specified data age; wherein the storage server does not guarantee that the second retention duration goal will be met; receiving, at the storage server, a request from the first client system to store new backup data for the first client system; and in response to the request from the first client system, the storage server determining which backup data to delete to make space for the new backup data based, at least in part, on the second retention duration goal associated with the second client system; wherein determining which backup data to delete to make space for the new backup data of the first client system includes determining to delete an amount of backup data associated with the second client system that is newer than the second specified data age, while retaining at least a guaranteed amount of backup data for the second client system.