๐ฐ Introduction
Within the Ceph ecosystem, users can choose between multiple storage interfaces based on their workload requirements:
- RBD (RADOS Block Device) โ a block-level storage system
- CephFS (Ceph File System) โ a distributed file-level storage system
Both are built upon Cephโs common foundation โ the RADOS distributed object store,
but they differ significantly in terms of architecture, performance characteristics, use cases, and management approach.
This article provides a detailed comparison of CephFS and RBD,
explaining their technical differences, real-world performance, and best-fit scenarios within environments such as Proxmox VE, PBS, and hybrid cloud deployments.
๐งฉ 1. Architectural Differences
| Aspect | RBD (Block Storage) | CephFS (File System) |
|---|---|---|
| Type | Block device (virtual disk) | Distributed file system |
| I/O Level | Block-level | File-level |
| Access Method | Mounted as virtual disk via librbd | Mounted as file system (Kernel / FUSE) |
| Metadata Handling | Direct OSD access (no MDS) | Requires MDS (Metadata Server) |
| Snapshot / Clone Support | โ Full support | โ Partial (per directory) |
| Deployment Complexity | Simple | Moderate (requires MDS) |
| Best Use Case | VM, container, database storage | File sharing, backup, development storage |
| Integration Interfaces | Proxmox, KVM, QEMU, OpenStack | Linux mount, NFS, Kubernetes PVs |
โ๏ธ 2. How They Work
1๏ธโฃ RBD โ Block-Level Storage
RBD maps Cephโs distributed objects directly into a virtual block device.
Each RBD volume behaves like a physical disk to the host or hypervisor.
VM / Host
โ
โโ> librbd (Client)
โ
โโ> RADOS Cluster โ OSDs
Advantages of RBD:
- No metadata overhead (no MDS required)
- Low latency, high IOPS
- Native snapshot and clone support
- Perfect fit for virtualization or database storage
๐ฆ In Proxmox VE, RBD is the default backend for VM and container disk storage.
2๏ธโฃ CephFS โ File-Level Storage
CephFS provides a POSIX-compliant distributed file system interface.
It requires MDS (Metadata Servers) to manage directory hierarchy, file locks, and user permissions.
Application
โ
โโ> CephFS Client (Kernel / FUSE)
โ
โโ> MDS (Metadata)
โ
โโ> RADOS Cluster โ OSDs
Advantages of CephFS:
- Multi-user concurrent file access
- Shared access across nodes and applications
- Ideal for backup, data lakes, and AI workloads
- Supports snapshots and quotas at the file-system level
๐ 3. Performance Comparison
| Metric | RBD | CephFS |
|---|---|---|
| Read/Write Latency | Low (direct block access) | Higher (metadata overhead via MDS) |
| IOPS | High | Moderate |
| Throughput | Excellent | Good (depends on MDS scaling) |
| Consistency | Strong | File-level consistency |
| Snapshot Efficiency | High | Moderate (metadata overhead) |
| Scalability | High | High (MDS scaling required) |
| Best Workload Type | Databases, VMs, OLTP | File sharing, backup, AI data sets |
In benchmark tests, RBD consistently outperforms CephFS in small random I/O workloads,
while CephFS excels in large sequential reads/writes and multi-user shared file environments.
๐ง 4. When to Use Each
โ Use RBD for:
- VM or container disk storage (Proxmox, KVM, OpenStack)
- Databases (MySQL, PostgreSQL, MongoDB)
- Transaction-heavy (OLTP) workloads
- High IOPS / low-latency environments
โ Use CephFS for:
- Multi-user shared file storage
- Backup and archive systems
- DevOps, AI, and machine learning datasets
- Kubernetes shared volumes (RWX mode)
โก 5. Integration in Proxmox VE
| Module | Integration Method | Advantage |
|---|---|---|
| RBD | Datacenter โ Storage โ Add โ RBD | High-performance VM/CT storage |
| CephFS | Datacenter โ Storage โ Add โ CephFS | Simple shared storage for backups and ISO images |
| PBS + CephFS | Mount CephFS for backup datastore | Enables snapshot-based, incremental backups |
| Hybrid Model | RBD for production, CephFS for backups | Balances performance and flexibility |
๐งฉ 6. Hybrid Architecture Example
โโโโโโโโโโโโโโโโโโโโโโโโโโโโ
โ Proxmox Cluster โ
โ โโโโโโโโโโโโโโโโโโโโโโโ โ
โ VM Storage โ RBD Pool โ
โ Backup โ CephFS Mount โ
โ PBS โ CephFS Datastore โ
โโโโโโโโโโโโโโโโโโโโโโโโโโโโ
โ
โโโโโโโโโโโโโโโโโโโโโโโโ
โ Ceph Cluster (RADOS) โ
โ OSDs + MON + MDS + MGR โ
โโโโโโโโโโโโโโโโโโโโโโโโ
Advantages:
- RBD provides high-performance VM storage
- CephFS handles shared and backup data
- Both share the same Ceph backend, simplifying management and maintenance
๐ 7. Security and Governance
- Use CephX or token-based access control for RBD volumes
- Configure ACLs and user isolation in CephFS
- Use Ceph Dashboard or Prometheus for ongoing I/O health monitoring
- Protect backup data with CephFS snapshots + PBS Verify Jobs for integrity validation
โ Conclusion
RBD and CephFS serve distinct yet complementary roles in enterprise environments:
| Storage Type | Strength | Recommended Use |
|---|---|---|
| RBD | High IOPS, low latency, strong consistency | VM storage, databases, performance-critical apps |
| CephFS | Shared file access, flexibility, snapshot support | Backup, AI data lake, file servers |
For enterprise Proxmox environments:
- Use RBD for production and compute workloads.
- Use CephFS for shared storage, backup, and analytics.
This hybrid approach delivers the best of both worlds โ
performance, reliability, and scalability โ while leveraging Cephโs unified storage foundation.
๐ฌ Coming next:
โCeph High Availability and Multi-Site Replication Strategiesโ โ
exploring Ceph mirroring, multi-site replication, and cross-datacenter disaster recovery design.