Introduction

Proxmox VE 9 introduces tighter integration with Ceph, making it easier than ever to deploy hyper-converged clusters with shared, highly available storage. However, many small to medium-sized businesses jump into 3-node Proxmox VE 9 clusters with Ceph without fully understanding the risks and limitations that come with this minimal design.

While a 3-node Ceph cluster is technically supported, it sits at the bare minimum of fault tolerance — and even small misconfigurations or hardware failures can cause significant downtime.

In this post, we’ll explore the real-world risks, failure scenarios, and best practices to build a stable, resilient Proxmox VE 9 + Ceph cluster.

What Is a 3-Node Proxmox VE Cluster with Ceph?

A Proxmox VE 9 cluster allows multiple hypervisors (nodes) to share configuration, migrate VMs, and manage storage centrally. When integrated with Ceph, each node contributes local disks (OSDs) to form a distributed, redundant storage pool.

Typical setup:

• 3 Nodes: pve1, pve2, pve3

• Each node: runs MON, MGR, OSD

• Storage replication: 3 replicas across nodes

• Shared Ceph storage: used for VM disks, containers, and backups

This design is attractive because it’s simple, cost-effective, and uses internal disks — but it’s also where the risks begin.

Top Risks of a 3-Node Ceph Cluster on Proxmox VE 9

1. Quorum Fragility and Split-Brain Risk

Both Proxmox Cluster Manager (corosync) and Ceph MON daemons rely on quorum.
With only 3 nodes:

• Losing 1 node means you’re down to 2 votes, the absolute minimum.

• Losing or isolating 1 more node causes loss of quorum.

• Without quorum, both Proxmox and Ceph will freeze operations — no VM migration, no storage writes, no management access.

 Real-world example:

If a single node reboots for maintenance and another node temporarily loses network connectivity, the entire cluster may lose quorum and halt all VM activity.

Mitigation:

• Add a QDevice or 4th node for stable quorum.

• Separate cluster and storage networks physically or via VLANs.

2. OSD and Replication Overhead

In a 3-node Ceph cluster, each object is typically replicated 3 times — once per node.
That means:

• Effective usable capacity = total raw storage ÷ 3.

• Any single OSD or node failure triggers heavy rebalancing traffic across all nodes.

Example:

With 3 × 2 TB disks (6 TB raw), you get ~2 TB usable Ceph storage.
If one node goes down, Ceph starts copying data from remaining nodes to restore 3 replicas, potentially saturating the cluster network and impacting VM I/O performance.

Mitigation:

• Use 10GbE or faster network interconnects.

• Deploy NVMe-based OSDs for better recovery speed.

3. Network Instability Causes Major Disruptions

Ceph depends heavily on low-latency, high-bandwidth interconnects between nodes.
Even minor network jitter or packet loss can trigger slow requests, PG recovery loops, or temporary VM pauses.

Common mistakes:

• Using a single NIC for both cluster and Ceph traffic.

• Mixing management, replication, and public traffic on one network.

Mitigation:

• Use dual networks: one for Ceph cluster (replication) and one for Proxmox management.

• Avoid shared switches with other traffic like backup or iSCSI.

4. Maintenance Risks and Downtime

In a 3-node setup:

• Taking one node down for updates means 33% of your cluster is offline.

• Ceph automatically marks OSDs on that node as “down” and starts rebalancing.

• When you bring the node back, it triggers another rebalance — doubling the network load.

Result: Extended maintenance windows and reduced performance stability.

Mitigation:

• Temporarily pause Ceph backfilling before planned maintenance:

  ceph osd set noout


  (Don’t forget to unset noout after maintenance!)

5.  No True High Availability (HA) for Storage

Even though Ceph provides data redundancy, Ceph itself must be healthy for VM HA to work.

In a 3-node setup:

• Losing 1 node + 1 MON service can break quorum.

• If Ceph becomes read-only, VMs pause even though data is intact.

• Restoring from this state can require manual intervention or Ceph repair commands.

Mitigation:

• Run one MON and one MGR per node (total 3).

• Use Ceph dashboard or ceph -s to monitor health before migrations or updates.

6. Rebalance Storms After Failures

When one node fails, Ceph starts rebalancing all Placement Groups (PGs) to maintain 3 replicas. In small clusters, this causes:

• High I/O load

• Increased latency

• Possible VM freeze

During recovery, Ceph can consume all bandwidth and CPU resources — leaving little for VM operations.

Mitigation:

• Tune recovery speed with:

  ceph tell 'osd.*' injectargs '--osd-max-backfills 2'


• Upgrade to 4 or 5 nodes to spread recovery load.

7. Limited Flexibility for Scaling

In a 3-node cluster:

• You can’t easily expand storage capacity without disrupting data distribution.

• Adding new OSDs or nodes triggers a full cluster rebalance.

Larger clusters (5+ nodes) scale smoothly — Ceph balances data across more nodes, reducing impact per failure.

Mitigation:

• Start with 4–5 nodes if budget allows.

• Use consistent disk sizes across nodes to avoid skewed distribution.

8. Difficult Troubleshooting for New Administrators

A small Ceph cluster often enters degraded states after reboots or maintenance.
Admins unfamiliar with Ceph’s recovery logic may misinterpret normal behavior as failure and make things worse (e.g., marking OSDs out prematurely).

Mitigation:

• Train your team on Ceph commands:

  ceph -s
ceph health detail
ceph osd tree
ceph pg stat


• Test recovery in a lab environment before production.

Risk Summary Table

Best Practices for Safer 3-Node Ceph Deployment

• Use SSD or NVMe OSDs for fast recovery

• Isolate Ceph traffic from VM traffic

• Deploy 1 MON + 1 MGR per node

• Set public_network and cluster_network separately

• Keep Ceph and Proxmox versions aligned

• Always check:

  ceph -s
pvecm status


  before any upgrade or reboot.

Conclusion

A 3-node Proxmox VE 9 cluster with Ceph can work — but it’s a tightrope walk. It’s perfect for labs, testing, or small environments, but not ideal for critical production workloads where uptime and performance are non-negotiable.

For production-grade reliability:

• Add a 4th node or QDevice for quorum stability.

• Use dedicated networks for Ceph replication.

• Monitor Ceph health continuously.

Investing in proper design upfront prevents the nightmare scenarios that come with “minimum viable” Ceph clusters.

Contact us today for a free Ceph consultation — our experts can help you deploy Ceph clusters the right way.