1. A method, comprising: storing a data image comprising a plurality of data items, wherein the data image comprises a plurality of sparse data images stored as a plurality of data image layers, wherein at least some of the plurality of data image layers include a version of one or more of the data items which has changed relative to a prior version of the one or more data items included in another one of the plurality of data image layers, and wherein at least some of the plurality of data image layers do not include one or more data items that are included in one or more others of the plurality of data image layers; maintaining, for each of the plurality of data items, one or more probabilities indicating the likelihood that a particular data image layer includes a most recent version of the data item; and acquiring the most recent version of one of the plurality of data items, wherein said acquiring comprises: requesting the one of the plurality of data items from one or more primary data image layers according to the one or more probabilities for the one of the plurality of data items; and updating the one or more probabilities for the one of the plurality of data items based on results of said requesting.