Introduction

The VolumeReplicationClass is a cluster-scoped Kubernetes custom resource provided by the Volume Replication Operator (VRO). It defines the replication driver and scheduling parameters used by VolumeReplication objects. For Rook-based RBD mirroring, the scheduling interval in the VolumeReplicationClass controls how often snapshot-based replication syncs data to the peer cluster, directly affecting your Recovery Point Objective (RPO).

VolumeReplicationClass Architecture

flowchart TD
    A[VolumeReplicationClass\n schedulingInterval: 1h] --> B[VolumeReplication CR]
    C[PVC / RBD Image] --> B
    B --> D[Volume Replication Operator]
    D -->|snapshot every 1h| E[Ceph RBD Snapshot]
    E -->|mirror to peer| F[Site B Ceph Cluster]

Prerequisites

• Volume Replication Operator installed in the Kubernetes cluster
• Rook CephBlockPool with mirroring enabled in snapshot mode
• CSI addons deployed (required by VRO for snapshot scheduling)

Step 1: Install the Volume Replication Operator

# Install via operator hub or directly
kubectl apply -f https://raw.githubusercontent.com/csi-addons/volume-replication-operator/main/config/crd/bases/replication.storage.openshift.io_volumereplicationclasses.yaml
kubectl apply -f https://raw.githubusercontent.com/csi-addons/volume-replication-operator/main/config/crd/bases/replication.storage.openshift.io_volumereplications.yaml

# Install the operator itself
kubectl apply -f https://raw.githubusercontent.com/csi-addons/volume-replication-operator/main/config/manager/manager.yaml

# Verify the operator is running
kubectl get pods -n volume-replication-operator-system

Step 2: Create a VolumeReplicationClass with a Specific Interval

The schedulingInterval sets the replication frequency using a duration format (minutes, hours):

# vrc-hourly.yaml
apiVersion: replication.storage.openshift.io/v1alpha1
kind: VolumeReplicationClass
metadata:
  name: rook-volumereplicationclass-1h
spec:
  provisioner: rook-ceph.rbd.csi.ceph.com
  parameters:
    # Scheduling interval for snapshot-based mirroring
    schedulingInterval: "1h"
    # Optional: offset within the interval to stagger snapshots
    schedulingStartTime: "00:00:00"
    # Mirroring mode: snapshot or journal
    mirroringMode: snapshot
    # Replication secret reference
    replication.storage.openshift.io/replication-secret-name: rook-csi-rbd-provisioner
    replication.storage.openshift.io/replication-secret-namespace: rook-ceph

kubectl apply -f vrc-hourly.yaml

Step 3: Create VolumeReplicationClasses for Different RPO Tiers

Define multiple classes for different service tiers:

# vrc-tiers.yaml

# Tier 1: Critical workloads - replicate every 15 minutes
apiVersion: replication.storage.openshift.io/v1alpha1
kind: VolumeReplicationClass
metadata:
  name: rook-vrc-15min
spec:
  provisioner: rook-ceph.rbd.csi.ceph.com
  parameters:
    schedulingInterval: "15m"
    mirroringMode: snapshot
    replication.storage.openshift.io/replication-secret-name: rook-csi-rbd-provisioner
    replication.storage.openshift.io/replication-secret-namespace: rook-ceph
---
# Tier 2: Important workloads - replicate every hour
apiVersion: replication.storage.openshift.io/v1alpha1
kind: VolumeReplicationClass
metadata:
  name: rook-vrc-1hour
spec:
  provisioner: rook-ceph.rbd.csi.ceph.com
  parameters:
    schedulingInterval: "1h"
    mirroringMode: snapshot
    replication.storage.openshift.io/replication-secret-name: rook-csi-rbd-provisioner
    replication.storage.openshift.io/replication-secret-namespace: rook-ceph
---
# Tier 3: Standard workloads - replicate every 24 hours
apiVersion: replication.storage.openshift.io/v1alpha1
kind: VolumeReplicationClass
metadata:
  name: rook-vrc-daily
spec:
  provisioner: rook-ceph.rbd.csi.ceph.com
  parameters:
    schedulingInterval: "24h"
    schedulingStartTime: "02:00:00"
    mirroringMode: snapshot
    replication.storage.openshift.io/replication-secret-name: rook-csi-rbd-provisioner
    replication.storage.openshift.io/replication-secret-namespace: rook-ceph

kubectl apply -f vrc-tiers.yaml
kubectl get volumereplicationclass

Step 4: Enable Snapshot Scheduling on the CephBlockPool

The CephBlockPool must have snapshot scheduling enabled to match the VRC interval:

# pool-snapshot-mirror.yaml
apiVersion: ceph.rook.io/v1
kind: CephBlockPool
metadata:
  name: replicapool
  namespace: rook-ceph
spec:
  replicated:
    size: 3
  mirroring:
    enabled: true
    mode: snapshot
    snapshotSchedules:
      # Match the most frequent VRC interval
      - interval: 15m
      - interval: 1h
      - interval: 24h
        startTime: "02:00:00"

kubectl apply -f pool-snapshot-mirror.yaml

Step 5: Create a VolumeReplication Using the Class

# volume-replication-app.yaml
apiVersion: replication.storage.openshift.io/v1alpha1
kind: VolumeReplication
metadata:
  name: db-volume-replication
  namespace: production
spec:
  # Reference the VolumeReplicationClass
  volumeReplicationClass: rook-vrc-15min
  # Source volume to replicate
  dataSource:
    apiGroup: ""
    kind: PersistentVolumeClaim
    name: database-pvc
  # Initial replication state (primary site)
  replicationState: primary
  # Whether to automatically resync when replication state changes
  autoResync: false

kubectl apply -f volume-replication-app.yaml

# Check replication status
kubectl describe volumereplication db-volume-replication -n production

Step 6: Verify Snapshot Schedule on Ceph

kubectl -n rook-ceph exec -it deploy/rook-ceph-tools -- bash

# Check snapshot schedules for the mirrored image
rbd mirror image snapshot schedule list \
  replicapool/csi-vol-<image-id> \
  --format json

# Check scheduled snapshot status
rbd mirror image status replicapool/csi-vol-<image-id>

# List recent snapshots
rbd snap list replicapool/csi-vol-<image-id> | grep mirror

Step 7: Monitor Replication Lag

The time between snapshot creation and completion on the peer indicates actual RPO:

# Check last sync time and lag
kubectl get volumereplication db-volume-replication -n production \
  -o jsonpath='{.status.lastSyncTime}'

# Check replication state details
kubectl describe volumereplication db-volume-replication -n production | grep -A10 "Status:"

# Check via Prometheus
# Metric: rbd_mirror_image_replaying_lag_seconds
# Alert if lag > scheduled interval

Troubleshooting

# VolumeReplication stuck in unknown state
kubectl describe volumereplication db-volume-replication -n production

# Check VRO operator logs
kubectl logs -n volume-replication-operator-system deploy/volume-replication-operator-controller-manager | tail -30

# Verify CSI addon supports scheduling
kubectl get csiaddon -A

# Check if snapshot schedule was created on the image
kubectl -n rook-ceph exec -it deploy/rook-ceph-tools -- \
  rbd mirror image snapshot schedule ls replicapool

Summary

VolumeReplicationClass resources define the replication driver and scheduling interval for snapshot-based RBD mirroring. Multiple classes with different schedulingInterval values enable tiered RPO objectives (15 minutes for critical workloads, hourly for important, daily for standard). The interval in the VolumeReplicationClass must align with the snapshotSchedules configured on the CephBlockPool. VolumeReplication objects then reference the appropriate class to schedule replication for specific PVCs.