Today, several data-storage systems allow applications to create and manage custom metadata to improve data search and navigability in large-scale storage systems. Our thesis is that, besides improving search and navigability, custom metadata can also serve as a two-way communication mechanism between applications and the storage layer to enable cross-layer optimizations in a uniform, application-independent and incremental fashion. Categories and Subject Descriptors D.4.3 [Operating Systems]: File Systems Management – distributed file systems. H.3.4 [Information Storage and Retrieval]: Systems and Software – distributed systems. General Terms Performance, Design, Standardization. Keywords Cross-layer optimization, distributed storage systems, custom metadata.