We address the challenge of managing large amounts of numerical data within computing grids consisting of a federation of clusters. We claim that storing, accessing, updating and sharing such data should be considered by applications as an external service. We propose a hierarchical architecture for this service, based on a peer-to-peer approach. This architecture is illustrated through a software platform called JuxMem (for Juxtaposed Memory), which provides transparent access to mutable data, while enhancing data persistence in a dynamic environment. Managing the volatility of storage resources is specially emphasized. As a proof of concept, we describe a prototype implementation on top of the JXTA peer-to-peer framework, and we report on a preliminary experimental evaluation.