How RWO Backups are implemented
K8s does not prevent mounting a RWO PVC to multiple pods, if they are scheduled on the same host. K8up uses this fact to provide the ability to back up RWO PVCs.
For a given backup in a namespace K8up will list all the PVCs. The PVCs are then grouped depending on their type:
all RWX PVCs are grouped together
RWO PVCs are grouped by k8s node where they are currently mounted
K8up will then deploy backup jobs according to the grouping, a single job for all RWX PVCs and a job for each K8s node. The jobs themselves work as before, they loop over the mounted PVCs and do a file backup via restic.