As mentioned previously, working backups are a key strategy in ensuring that a failure does not result in the loss of data. Starting with the Jewel release, Ceph introduced RBD mirroring, which allows you to asynchronously mirror an RBD from one cluster to another. Note the difference between Cephs native replication, which is synchronous, and RBD mirroring. With synchronous replication, low latency between peers is essential, and asynchronous replication allows the two Ceph clusters to be geographically remote, as latency is no longer a factor.
By having a replicated copy of your RBD images on a separate cluster, you can dramatically reduce both your Recovery Time Objective (RTO) and Recovery Point Objective (RPO). The RTO is a measure of how long it takes from initiating recovery to when the data is usable. It is the worst case measurement of time between each data point and describes the expected data loss. A daily backup would have an RPO of 24 hours; for example, potentially, any data written up to 24 hours since the last backup would be lost if you had to restore from a backup.
With RBD mirroring, data is asynchronously replicated to the target RBD, and so, in most cases, the RPO should be under a minute. As the target RBD is also a replica and not a backup that would require to be first restored, the RTO is also likely going to be extremely low. Additionally, as the target RBD is stored on a separate Ceph cluster, it offers additional protection over snapshots, which could also be impacted if the Ceph cluster itself experiences issues. At first glance, this makes RBD mirroring seem like the perfect tool to protect against data loss, and in most cases, it is a very useful tool. RBD mirroring is not a replacement for a proper backup routine though. In the cases where data loss is caused by actions internal to the RBD, such as file system corruption or user error, these changes will be replicated to the target RBD. A separate isolated copy of your data is vital.
With that said, let's take a closer look into how RBD mirroring works.