Metadata computation apparatus includes a host interface, a storage interface and one or more processors. The host interface is configured to communicate over a computer network with one or more remote hosts. The storage interface is configured to communicate with one or more non-volatile memories of one or more storage devices. The processors are configured to manage local storage or retrieval of media objects in the non-volatile memories, to compute metadata for a plurality of media objects that are stored, or are en-route for storage, on the storage devices, wherein the media objects are of multiple media types, wherein the computed metadata tags a target feature in the media objects of at least two different media types among the multiple media types, and to store, in the non-volatile memories, the metadata tagging the target feature found in the at least two different media types, for use by the hosts.