One of the key components in RBD mirroring is the journal. The RBD mirroring journal stores all writes to the RBD and acknowledges to the client once they have been written. These writes are then written to the primary RBD image. The journal itself is stored as RADOS objects, prefixed similarly to how RBD images are. Separately, the remote rbd-mirror daemon polls the configured RBD mirrors and pulls the newly written journal objects across to the target cluster and replays them into the target RBD.