Ceres is a component of Graphite as one of the time-series storage options available for use. Ceres provides a file format for incoming metrics to be persisted when received from the network. See also Whisper
Ceres is a time-series database format intended to replace Whisper as the default storage format for Graphite. In contrast with Whisper, Ceres is not a fixed-size database and is designed to better support sparse data of arbitrary fixed-size resolutions. This allows Graphite to distribute individual time-series across multiple servers or mounts.
Expected features such as roll-up aggregation and data expiration are not provided by Ceres itself, but instead are implemented as maintenance plugins in Carbon.