Simple definition
RBD (RADOS Block Device) provides distributed block volumes from a Ceph cluster.
Technical definition
An RBD image is split into objects stored in RADOS. Clients access the volume through librbd or a compatible driver, while Ceph distributes and protects the objects according to the pool configuration.
What is it used for?
Present resilient virtual disks to VMs, containers, or servers without relying on a traditional block-storage array.
Practical example
A Proxmox cluster can store VM disks as RBD images in a Ceph pool.
Common issues
- High network latency or a degraded Ceph cluster
- Poorly sized pool, replication, or CRUSH rules
- CephX permissions, mapping, or incompatible RBD features
How it works / role
When a client reads or writes a block, RBD maps that operation to the relevant RADOS objects. The appropriate OSDs serve or store the data, while the pool policy controls placement and data protection according to the cluster design.
Key takeaway: Exact behavior can vary by version, product, and architecture; always validate on the environment actually deployed.