1. A solid-state storage subsystem comprising: a non-volatile memory array comprising a user data quota portion and an over-provisioning quota portion, each quota portion comprising a plurality of solid-state storage elements; and a controller configured to: implement a data redundancy configuration with a plurality of data stripes, each data stripe comprising a respective subset of the plurality of storage elements of the user data quota portion; and be responsive to a failure of a storage element of the user data quota portion by at least: replacing the failed storage element of the user data quota portion with one of the storage elements of the over-provisioning quota portion; and rebuilding one or more of the data stripes that include the failed storage element using the storage element from the over-provisioning quota portion.