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Introduction:
Traditional disaster recovery (DR) solutions often treat a WAN link failure as a critical event—broken synchronous replication, I/O backpressure that throttles production VMs, or manual administrative heroics to re‑establish sync. Proxmox Virtual Environment paired with Ceph storage fundamentally changes this dynamic by leveraging asynchronous replication (RBD mirroring). When your WAN link drops or degrades, the local Ceph cluster simply pauses outbound replication without any impact on running virtual machines, turning a potential outage into a temporary, self‑healing expansion of your Recovery Point Objective (RPO).
Learning Objectives:
– Differentiate between snapshot‑based and journal‑based Ceph RBD mirroring for disaster recovery.
– Configure asynchronous replication between two Proxmox/Ceph clusters to eliminate I/O backpressure during WAN failures.
– Monitor and automatically recover from network degradation, including handling journal loops and forced re‑sync scenarios.
You Should Know:
1. Understanding Ceph’s Asynchronous Replication Architecture
Ceph’s RBD (RADOS Block Device) mirroring operates asynchronously by design. Unlike synchronous replication that waits for an acknowledgement from the remote site before confirming a write, asynchronous replication writes locally first then replicates changes in the background. This eliminates any WAN‑induced latency from your production I/O path.
Two mirroring modes:
– Snapshot‑based: Periodic snapshots are shipped to the remote cluster. Simpler but higher potential data loss.
– Journal‑based (rbd-mirror): Every write is recorded in a journal and replayed asynchronously. Provides near‑continuous replication with minimal RPO.
When the WAN drops, the local journal queues changes; when the link returns, Ceph automatically calculates the delta and resumes. A prolonged outage may cause the journal to wrap and force a full re‑sync, but short‑term degradation has zero effect on VM performance.
2. Setting Up Ceph RBD Mirroring Between Two Proxmox Clusters
Prerequisites: Two Proxmox clusters (primary and secondary), each with Ceph deployed, and network connectivity between them.
Step‑by‑step (Linux – primary cluster):
1. Enable mirroring on the primary pool
rbd mirror pool enable <pool_name> journal
2. Create a mirroring peer (remote cluster)
On primary, capture the cluster FSID and client keyring from secondary.
On secondary cluster ceph auth get client.bootstrap-mds -o /tmp/bootstrap-key Copy keyring to primary, then bootstrap peer rbd mirror pool peer add <pool_name> client.bootstrap-mds@secondary --remote-cluster secondary
3. Enable mirroring on a specific image
rbd mirror image enable <pool_name>/<image_name> journal
4. Verify mirroring status
rbd mirror image status <pool_name>/<image_name> Look for "state: replaying" or "up+stopped"
Windows admin note: For cross‑platform monitoring, use `rbd.exe` from the Ceph Windows package or SSH into the Proxmox node via PowerShell with `plink`.
3. Simulating WAN Link Degradation and Observing Resilience
Test how your DR reacts to real‑world network instability without breaking production.
Use `tc` (traffic control) on Linux to inject latency/packet loss on the replication interface:
Add a 200ms delay with 5% packet loss on eth0 (the replication link) tc qdisc add dev eth0 root netem delay 200ms loss 5% Monitor Ceph mirroring status during degradation watch -1 2 'rbd mirror pool status <pool_name>' Remove the simulation tc qdisc del dev eth0 root
What to observe:
– Local VM I/O remains unaffected (run `fio` or `dd` on a test VM).
– Mirroring states change to `up+stopped` or `up+error` but automatically resume when `tc` is removed.
– The remote cluster’s RPO temporarily expands; Ceph logs show “mirroring paused – remote unreachable.”
4. Handling Journal Loops and Forced Re‑Sync
If the WAN link stays down longer than the journal retention window, Ceph may wrap the journal, requiring a full re‑sync of the disk image. Recognise and recover from this condition.
Symptoms of journal loop:
– `rbd mirror image status` shows `state: failed` with error “journal replay: entry not found”.
– Remote image is stale; replication never catches up.
Step‑by‑step forced re‑sync:
1. Stop mirroring on the affected image
rbd mirror image disable <pool_name>/<image_name> --force
2. Demote the remote image (if it became primary erroneously)
rbd mirror image demote <pool_name>/<image_name>
3. Re‑enable mirroring and force a full resync
rbd mirror image enable <pool_name>/<image_name> journal rbd mirror image resync <pool_name>/<image_name>
4. Monitor re‑sync progress
rbd status <pool_name>/<image_name> Shows "mirroring resyncing"
Mitigation: Increase journal size and set appropriate `rbd_journal_max_payload_bytes` to survive longer outages without forced re‑sync.
5. Monitoring RPO Expansion and Setting Alerts
Asynchronous replication naturally allows RPO to grow during WAN degradation. Track this metric to meet your recovery objectives.
Command to compute approximate RPO lag (in seconds):
On primary cluster, get last journal entry timestamp rados -p <pool_name> getomapval journal.1.<image_id> last_entry_timestamp - | xxd -r -p | date -f -
For production monitoring, deploy Prometheus + Ceph exporter:
– Install `prometheus-ceph-exporter` on a Proxmox node.
– Configure alert rule:
- alert: CephMirroringLag
expr: ceph_rbd_mirroring_replay_delay_seconds > 300
annotations:
summary: "RPO exceeded 5 minutes for {{ $labels.image }}"
Cloud hardening: If using cloud‑native WAN links (AWS Direct Connect or Azure ExpressRoute), implement route‑based VPN failover (e.g., WireGuard) so Ceph can switch to a backup path automatically when primary latency exceeds a threshold.
6. Disaster Recovery Failover and Failback Procedures
When the primary site fails entirely, initiate planned failover to the secondary cluster.
Failover steps (secondary cluster):
1. Promote the mirrored images
rbd mirror image promote <pool_name>/<image_name> --force
2. Start all VMs on the secondary Proxmox cluster (either manually or via HA groups).
3. Update DNS or load balancer records to redirect traffic to the secondary site.
Failback after primary is restored:
1. Re‑establish mirroring from secondary (now primary) back to original primary.
rbd mirror image demote <pool_name>/<image_name> on current primary rbd mirror image promote <pool_name>/<image_name> on original primary
2. Reverse replication direction using the same peer configuration.
Automation: Script this with `pvesh` (Proxmox API) and `rbd` commands to reduce RTO from hours to minutes.
7. Hardening Your DR Network for Worst‑Case Scenarios
While Ceph tolerates degradation, intentional hardening prevents the “prolonged outage → forced re‑sync” scenario.
Linux commands to bond multiple WAN links for replication traffic:
Create a bond interface (mode 1 = active‑backup) nmcli connection add type bond ifname bond0 mode active-backup miimon 100 nmcli connection add type ethernet ifname eth0 master bond0 nmcli connection add type ethernet ifname eth1 master bond0 Assign the bond to Ceph’s replication subnet nmcli connection modify bond0 ipv4.addresses 10.10.10.10/24
Windows‑side: Use PowerShell to monitor replication link health and trigger email alerts if packet loss exceeds 10% for 5 minutes.
while ($true) {
$result = Test-Connection -ComputerName remote-ceph-mon -Count 10 -ErrorAction SilentlyContinue
$loss = ($result | Where-Object {$_.StatusCode -1e 0}).Count / 10 100
if ($loss -gt 10) { Send-MailAlert -Subject "Ceph WAN Degraded" }
Start-Sleep -Seconds 300
}
Security note: Encrypt replication traffic with Ceph’s `msgr2` protocol and enforce mutual TLS to prevent interception or tampering of journal data across the WAN.
What Undercode Say:
– Asynchronous replication is a business continuity feature, not a compromise. Many architects fear “lost writes” during a WAN outage, but Ceph’s design prioritises production availability over strict consistency. For most DR scenarios, a temporary RPO expansion is far cheaper than a production slowdown.
– Proactive monitoring of journal depth and RPO lag turns resilience into observability. Teams that set alerts on `rbd mirroring replay delay` can predict forced re‑sync conditions before they happen, then apply temporary QoS or route changes to keep the journal from wrapping.
Analysis: Charles Crampton’s insight highlights a silent killer of traditional DR: the assumption that WAN links are always reliable. By adopting Proxmox + Ceph, organisations eliminate the “split‑brain or slowdown” dilemma. The real innovation is that Ceph treats network degradation as a normal state, not an emergency. This shifts DR testing from periodic “fire drills” to continuous resilience. However, the warning about journal loops is critical—without proper sizing (e.g., `rbd_journal_size = 5GB` for high‑change workloads), a 24‑hour outage still forces a costly full re‑sync. The sweet spot is combining Ceph’s native async model with a second‑layer WAN aggregator (SD‑WAN or bonded links) to keep the journal window safe.
Expected Output:
Prediction:
– +1 By 2028, asynchronous storage replication will become the default for multi‑site DR in hyperconverged infrastructures, as enterprises realise that synchronous “zero RPO” is often a myth on real‑world WANs.
– +1 Proxmox + Ceph will see accelerated adoption in mid‑sized businesses, displacing legacy SAN‑based replication that requires expensive dedicated links.
– -1 Without better automated alerting for journal exhaustion, many first‑time Ceph DR deployments will still experience silent data loss after prolonged network partitions. Tooling like Ceph’s built‑in `rbd mirror pool status –verbose` must be integrated into mainstream observability stacks (Datadog, Grafana Cloud).
– +1 The rise of cheap, high‑latency satellite internet (Starlink for DR) will favour asynchronous systems like Ceph, creating new hybrid DR architectures where “degraded” becomes the new normal.
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