CephFS Async Mirroring

There is a way to backup CephFS file systems by running a mirror daemon (cephfs-mirror) which takes the subvolumes and performs an asynchronous replication of snapshots to a remote CephFS file system [1]. A share for Manila is a subvolume in CephFS.

This is analagous to but different from Manila's own share replication. Share replication is done on a per-share basis, and there is no straightforward way to plugin CephFS mirroring solution as there is no way to manage an automatic failover.

A possible way to implement this is by referencing Cinder's spec [2] where Cinder provides a common mechanism for exposing replication functionality on a per volume basis.

[1] https://docs.ceph.com/en/latest/dev/cephfs-mirroring/
[2] https://docs.openstack.org/cinder/latest/contributor/replication.html