A block is a sequence of bytes (for example, a 512-byte block of data). Block-based storage interfaces are common way of storing data on a physcial storage device (e.g Hard Disk Drive, Solid State Drive). Ceph can emulate a block-based storage device that can be used as a virtual block device, a RBD image.
RBDs are thin-provisioned, resizable and store data striped over multiple OSDs in a Ceph cluster. Ceph block devices leverage RADOS capabilities such as snapshotting, replication and consistency. Ceph’s RADOS Block Devices (RBD) interact with OSDs using kernel modules or the
Using RBD devices
See this knowledge base article: KB045234 Mapping Ceph RBD Images On A Client
LVM + RBD
By Default LVM will filter out rbd devices and not allow PV to be created using a rbd device. To fix add the types option to /etc/lvm/lvm.conf in the devices section.
types = [ "rbd", 16 ]