============================= Ceph RADOS Block Device (RBD) ============================= If you use KVM, QEMU or Hyper-V as your hypervisor, you can configure the Compute service to use `Ceph RADOS block devices (RBD) `__ for volumes. Ceph is a massively scalable, open source, distributed storage system. It is comprised of an object store, block store, and a POSIX-compliant distributed file system. The platform can auto-scale to the exabyte level and beyond. It runs on commodity hardware, is self-healing and self-managing, and has no single point of failure. Due to its open-source nature, you can install and use this portable storage platform in public or private clouds. .. figure:: ../../figures/ceph-architecture.png Ceph architecture .. note:: **Supported Ceph versions** The current `release cycle model for Ceph `_ targets a new release yearly on 1 March, with there being at most two active stable releases at any time. For a given OpenStack release, *Cinder supports the current Ceph active stable releases plus the two prior releases.* For example, at the time of the OpenStack Wallaby release in April 2021, the Ceph active supported releases are Pacific and Octopus. The Cinder Wallaby release therefore supports Ceph Pacific, Octopus, Nautilus, and Mimic. Additionally, it is expected that the version of the Ceph client available to Cinder or any of its associated libraries (os-brick, cinderlib) is aligned with the Ceph server version. Mixing server and client versions is *unsupported* and may lead to anomalous behavior. The minimum requirements for using Ceph with Hyper-V are Ceph Pacific and Windows Server 2016. RADOS ~~~~~ Ceph is based on Reliable Autonomic Distributed Object Store (RADOS). RADOS distributes objects across the storage cluster and replicates objects for fault tolerance. RADOS contains the following major components: *Object Storage Device (OSD) Daemon* The storage daemon for the RADOS service, which interacts with the OSD (physical or logical storage unit for your data). You must run this daemon on each server in your cluster. For each OSD, you can have an associated hard drive disk. For performance purposes, pool your hard drive disk with raid arrays, or logical volume management (LVM). By default, the following pools are created: data, metadata, and RBD. *Meta-Data Server (MDS)* Stores metadata. MDSs build a POSIX file system on top of objects for Ceph clients. However, if you do not use the Ceph file system, you do not need a metadata server. *Monitor (MON)* A lightweight daemon that handles all communications with external applications and clients. It also provides a consensus for distributed decision making in a Ceph/RADOS cluster. For instance, when you mount a Ceph shared on a client, you point to the address of a MON server. It checks the state and the consistency of the data. In an ideal setup, you must run at least three ``ceph-mon`` daemons on separate servers. Ways to store, use, and expose data ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ To store and access your data, you can use the following storage systems: *RADOS* Use as an object, default storage mechanism. *RBD* Use as a block device. The Linux kernel RBD (RADOS block device) driver allows striping a Linux block device over multiple distributed object store data objects. It is compatible with the KVM RBD image. *CephFS* Use as a file, POSIX-compliant file system. Ceph exposes RADOS; you can access it through the following interfaces: *RADOS Gateway* OpenStack Object Storage and Amazon-S3 compatible RESTful interface (see `RADOS_Gateway `__). *librados* and its related C/C++ bindings *RBD and QEMU-RBD* Linux kernel and QEMU block devices that stripe data across multiple objects. RBD pool ~~~~~~~~ The RBD pool used by the Cinder backend is configured with option ``rbd_pool``, and by default the driver expects exclusive management access to that pool, as in being the only system creating and deleting resources in it, since that's the recommended deployment choice. Pool sharing is strongly discouraged, and if we were to share the pool with other services, within OpenStack (Nova, Glance, another Cinder backend) or outside of OpenStack (oVirt), then the stats returned by the driver to the scheduler would not be entirely accurate. The inaccuracy would be that the actual size in use by the cinder volumes would be lower than the reported one, since it would be also including the used space by the other services. We can set the ``rbd_exclusive_cinder_pool`` configuration option to ``false`` to fix this inaccuracy, but this has a performance impact. .. warning:: Setting ``rbd_exclusive_cinder_pool`` to ``false`` will increase the burden on the Cinder driver and the Ceph cluster, since a request will be made for each existing image, to retrieve its size, during the stats gathering process. For deployments with large amount of volumes it is recommended to leave the default value of ``true``, and accept the inaccuracy, as it should not be particularly problematic. Driver options ~~~~~~~~~~~~~~ The following table contains the configuration options supported by the Ceph RADOS Block Device driver. .. config-table:: :config-target: Ceph storage cinder.volume.drivers.rbd RBD Mirroring ~~~~~~~~~~~~~ The cinder RBD driver supports mirroring between multiple clusters. You can configure it on the cinder side with the usual replication configuration. Refer to the :doc:`documentation ` for more information. You will also have to configure replication on the Ceph side. To do so you may refer to the `Ceph documentation `_. Note that with the RBD driver in cinder you need to configure the pool replication option in image mode. For instance, if your pool is named ``volumes``, the command would be: ``rbd mirror pool enable volumes image``. RBD QoS ~~~~~~~~~~~~~ Currently, the Cinder RBD driver supports the following QoS options compatible with Ceph Octopus release and above: .. list-table:: :header-rows: 1 * - Cinder Value - Ceph Mapping * - ``total_iops_sec`` - ``rbd_qos_iops_limit`` * - - * - ``read_iops_sec`` - ``rbd_qos_read_iops_limit`` * - - * - ``write_iops_sec`` - ``rbd_qos_write_iops_limit`` * - - * - ``total_bytes_sec`` - ``rbd_qos_bps_limit`` * - - * - ``read_bytes_sec`` - ``rbd_qos_read_bps_limit`` * - - * - ``write_bytes_sec`` - ``rbd_qos_write_bps_limit`` * - - * - ``total_iops_sec_max`` - ``rbd_qos_bps_burst`` * - - * - ``read_iops_sec_max`` - ``rbd_qos_read_iops_burst`` * - - * - ``write_iops_sec_max`` - ``rbd_qos_write_iops_burst`` * - - * - ``total_bytes_sec_max`` - ``rbd_qos_bps_burst`` * - - * - ``read_bytes_sec_max`` - ``rbd_qos_read_bps_burst`` * - - * - ``write_bytes_sec_max`` - ``rbd_qos_write_bps_burst`` * - - For more information on QoS settings you may refer to `Ceph QoS documentation `_.