A Ceph Block Device stripes a block device image over multiple objects in the Ceph Storage Cluster, where each object gets mapped to a placement group and distributed, and the placement groups are spread across separate ceph-osd daemons throughout the cluster.

Thin-provisioned snapshottable Ceph Block Devices are an attractive option for virtualization and cloud computing. In virtual machine scenarios, people typically deploy a Ceph Block Device with the rbd network storage driver in QEMU/KVM, where the host machine uses librbd to provide a block device service to the guest. Many cloud computing stacks use libvirt to integrate with hypervisors. You can use thin-provisioned Ceph Block Devices with QEMU and libvirt to support OpenStack and CloudStack among other solutions.

While we do not provide librbd support with other hypervisors at this time, you may also use Ceph Block Device kernel objects to provide a block device to a client. Other virtualization technologies such as Xen can access the Ceph Block Device kernel object(s). This is done with the command-line tool rbd.



A block is a sequence of bytes (often 512). Block-based storage interfaces are a mature and common way to store data on media including HDDs, SSDs, CDs, floppy disks, and even tape. The ubiquity of block device interfaces is a perfect fit for interacting with mass data storage including Ceph.

Ceph block devices are thin-provisioned, resizable, and store data striped over multiple OSDs. Ceph block devices leverage RADOS capabilities including snapshotting, replication and strong consistency. Ceph block storage clients communicate with Ceph clusters through kernel modules or the librbd library.


Kernel modules can use Linux page caching. For librbd-based applications, Ceph supports RBD Caching.

2019-JUN-27 __ Ceph Tech Talk - Intro to Ceph - YouTube - 45_58

Snapshot and Clone#

2019-JUN-27 __ Ceph Tech Talk - Intro to Ceph - YouTube - 48_53

RBD data layout#

2019-JUN-27 __ Ceph Tech Talk - Intro to Ceph - YouTube - 50_23

RBD Journaling mode#

2019-JUN-27 __ Ceph Tech Talk - Intro to Ceph - YouTube - 51_40

How to#


RBD stats#

Good article#

https://ceph.io/en/news/blog/search/?q=rbd https://ceph.io/en/news/blog/2015/ceph-validate-that-the-rbd-cache-is-active/ https://ceph.io/en/news/blog/2015/ceph-activate-rbd-readhead/