Block-layer data deduplication allows file systems and applications to reap the benefits of deduplication without requiring per-system or per-application modifications. However, important information about data context (e.g., data vs. metadata writes) is lost at the block layer. Passing such context to the block layer can help improve deduplication performance and reliability. We implemented a hinting interface in an open-source block-layer deduplication system, dmdedup, that passes relevant context to the block layer, and evaluated two hints, NODEDUP and PREFETCH. To allow upper storage layers to pass hints based on the available context, we modified the VFS and file system layers to expose a hinting interface to user applications. We show that passing the NODEDUP hint speeds up applications by up to 5.3× on modern machines because the overhead of deduplication is avoided when it is unlikely to be beneficial. We also show that the PREFETCH hint accelerates applications up to 1.8× by caching hashes for data that is likely to be accessed soon.