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===============
Block Devices
===============
A block is a sequence of bytes (for example, a 512-byte block of data).
Block-based storage interfaces are the most common way to store data with
rotating media such as hard disks, CDs, floppy disks, and even traditional
9-track tape. The ubiquity of block device interfaces makes a virtual block
device an ideal candidate to interact with a mass data storage system like Ceph.
Ceph block devices are thin-provisioned, resizable and store data striped over
multiple OSDs in a Ceph cluster. Ceph block devices leverage
:abbr:`RADOS (Reliable Autonomic Distributed Object Store)` capabilities
such as snapshotting, replication and consistency. Ceph's
:abbr:`RADOS (Reliable Autonomic Distributed Object Store)` Block Devices (RBD)
interact with OSDs using kernel modules or the ``librbd`` library.
.. ditaa:: +------------------------+ +------------------------+
| Kernel Module | | librbd |
+------------------------+-+------------------------+
| RADOS Protocol |
+------------------------+-+------------------------+
| OSDs | | Monitors |
+------------------------+ +------------------------+
.. note:: Kernel modules can use Linux page caching. For ``librbd``-based
applications, Ceph supports `RBD Caching`_.
Ceph's block devices deliver high performance with infinite scalability to
`kernel modules`_, or to :abbr:`KVMs (kernel virtual machines)` such as `Qemu`_, and
cloud-based computing systems like `OpenStack`_ and `CloudStack`_ that rely on
libvirt and Qemu to integrate with Ceph block devices. You can use the same cluster
to operate the `Ceph RADOS Gateway`_, the `Ceph FS filesystem`_, and Ceph block
devices simultaneously.
.. important:: To use RBD, you must have a running Ceph cluster.
.. toctree::
:maxdepth: 1
Commands <rados-rbd-cmds>
Kernel Modules <rbd-ko>
Snapshots<rbd-snapshot>
QEMU <qemu-rbd>
libvirt <libvirt>
Cache Settings <rbd-config-ref/>
OpenStack <rbd-openstack>
CloudStack <rbd-cloudstack>
Manpage rbd <../../man/8/rbd>
Manpage rbd-fuse <../../man/8/rbd-fuse>
Manpage ceph-rbdnamer <../../man/8/ceph-rbdnamer>
librbd <librbdpy>
.. _RBD Caching: ../rbd-config-ref/
.. _kernel modules: ../rbd-ko/
.. _Qemu: ../qemu-rbd/
.. _OpenStack: ../rbd-openstack
.. _CloudStack: ../rbd-cloudstack
.. _Ceph RADOS Gateway: ../../radosgw/
.. _Ceph FS filesystem: ../../cephfs/
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