Efficient storage and retrieval for wearable-device data

Technology described herein provides methods whereby a two-level indexing/hashing structure is used to efficiently coordinate storage of sensor measurements between local digital memory (e.g., at a mobile device) and remote digital memory (e.g., at a cloud storage system). The first level of the two-level indexing/hashing structure may be include an array of first-level nodes that are sorted according to priority values. The priority values may be determined based on user data-querying activity. The second level of the two-level indexing/hashing structure may include second-level hash tables wherein buckets are associated with memory blocks of a predefined size. Sensor measurements that were taken during a specific time period may be stored near each other in memory and may be downloaded for local storage if user activity suggests that the user frequently has interest in data from that time period.