A keymap service architecture for a distributed storage system. A system may implement storage nodes configured to store replicas of data objects, where each of the replicas is accessible via a respective locator value, and keymap instances each configured to store keymap entries corresponding respectively to the data objects. A given keymap entry may indicate a mapping from a given key value corresponding to a given data object to each respective locator value of its replicas. Each of the keymap instances may store a replica of the given keymap entry and may index its respective stored keymap entries within a respective index data structure including hierarchically arranged index nodes corresponding to keymap entries. For a given keymap entry having a given corresponding index node, each tag value associated with each ancestor of the given corresponding index node may be a prefix of the given key value.