Ceph Block Storage

Block storage will be familiar to those who have worked with traditional SAN (Storage Area Network) technologies. Allocations of desired capacity are provisioned on demand and presented as contiguous statically-sized volumes (sometimes referred to as images). Ceph RBD supports volumes up to 16 exabytes in size. These volumes are attached to the client operating system as virtualized disk drives that can be utilized much like local physical drives. In virtualized environments the attachment point is often at the hypervisor level (eg. QEMU / KVM). The hypervisor then presents volumes to the guest operating system via the virtio driver or as an emulated IDE or SCSI disk.Usually a filesystem is then created on the volume for traditional file storage. This strategy has the appeal that guest operating systems do not need to know about Ceph, which is especially useful for software delivered as an appliance image. Client operating systems running on bare metal can also directly map volumes using a Ceph kernel driver.

Ceph's block storage component is RBD, the RADOS Block Device. We will discuss RADOS in depth in the following chapters, but for now we'll note that RADOS is the underlying technology on which RBD is built. RBD provides reliable, distributed, and high performance block storage volumes to clients. RBD volumes are effectively striped over numerous objects scattered throughout the entire Ceph cluster, a strategy that is key for providing availability, durability, and performance to clients. The Linux kernel bundles a native RBD driver; thus clients need not install layered software to enjoy Ceph's block service. RBD also provides enterprise features including incremental (diff) and full-volume snapshots, thin provisioning, copy-on-write (COW) cloning, layering, and others. RBD clients also support in-memory caching, which can dramatically improve performance.

The Ceph RBD service is exploited by cloud platforms including OpenStack and CloudStack to provision both primary / boot devices and supplemental volumes. Within OpenStack, Ceph's RBD service is configured as a backend for the abstracted Cinder (block) and Glance (base image) components. RBD's copy-on-write functionality enables one to quickly spin up hundreds or even thousands of thin-provisioned instances (virtual machines).