=============================== RBD Persistent Read-only Cache =============================== .. index:: Ceph Block Device; Persistent Read-only Cache Shared, Read-only Parent Image Cache ==================================== `Cloned RBD images`_ usually modify only a small fraction of the parent image. For example, in a VDI use-case, VMs are cloned from the same base image and initially differ only by hostname and IP address. During booting, all of these VMs read portions of the same parent image data. If we have a local cache of the parent image, this speeds up reads on the caching host. We also achieve reduction of client-to-cluster network traffic. RBD cache must be explicitly enabled in ``ceph.conf``. The ``ceph-immutable-object-cache`` daemon is responsible for caching the parent content on the local disk, and future reads on that data will be serviced from the local cache. .. note:: RBD shared read-only parent image cache requires the Ceph Nautilus release or later. .. ditaa:: +--------------------------------------------------------+ | QEMU | +--------------------------------------------------------+ | librbd (cloned images) | +-------------------+-+----------------------------------+ | librados | | ceph--immutable--object--cache | +-------------------+ +----------------------------------+ | OSDs/Mons | | local cached parent image | +-------------------+ +----------------------------------+ Enable RBD Shared Read-only Parent Image Cache ---------------------------------------------- To enable RBD shared read-only parent image cache, the following Ceph settings need to added in the ``[client]`` `section`_ of your ``ceph.conf`` file:: rbd parent cache enabled = true rbd plugins = parent_cache Immutable Object Cache Daemon ============================= Introduction and Generic Settings --------------------------------- The ``ceph-immutable-object-cache`` daemon is responsible for caching parent image content within its local caching directory. Using SSDs as the underlying storage is recommended because doing so provides better performance. The key components of the daemon are: #. **Domain socket based IPC:** The daemon listens on a local domain socket at startup and waits for connections from librbd clients. #. **LRU based promotion/demotion policy:** The daemon maintains in-memory statistics of cache hits for each cache file. It demotes the cold cache if capacity reaches the configured threshold. #. **File-based caching store:** The daemon maintains a simple file-based cache store. On promotion, the RADOS objects are fetched from RADOS cluster and stored in the local caching directory. When each cloned RBD image is opened, ``librbd`` tries to connect to the cache daemon through its Unix domain socket. After ``librbd`` is successfully connected, it coordinates with the daemon upon every subsequent read. In the case of an uncached read, the daemon promotes the RADOS object to the local caching directory and the next read of the object is serviced from the cache. The daemon maintains simple LRU statistics, which are used to evict cold cache files when required (for example, when the cache is at capacity and under pressure). Here are some important cache configuration settings: ``immutable_object_cache_sock`` :Description: The path to the domain socket used for communication between librbd clients and the ceph-immutable-object-cache daemon. :Type: String :Required: No :Default: ``/var/run/ceph/immutable_object_cache_sock`` ``immutable_object_cache_path`` :Description: The immutable object cache data directory. :Type: String :Required: No :Default: ``/tmp/ceph_immutable_object_cache`` ``immutable_object_cache_max_size`` :Description: The max size for immutable cache. :Type: Size :Required: No :Default: ``1G`` ``immutable_object_cache_watermark`` :Description: The high-water mark for the cache. The value is between (0, 1). If the cache size reaches this threshold the daemon will start to delete cold cache based on LRU statistics. :Type: Float :Required: No :Default: ``0.9`` The ``ceph-immutable-object-cache`` daemon is available within the optional ``ceph-immutable-object-cache`` distribution package. .. important:: ``ceph-immutable-object-cache`` daemon requires the ability to connect RADOS clusters. Running the Immutable Object Cache Daemon ----------------------------------------- ``ceph-immutable-object-cache`` daemon should use a unique Ceph user ID. To `create a Ceph user`_, with ``ceph`` specify the ``auth get-or-create`` command, user name, monitor caps, and OSD caps:: ceph auth get-or-create client.ceph-immutable-object-cache.{unique id} mon 'allow r' osd 'profile rbd-read-only' The ``ceph-immutable-object-cache`` daemon can be managed by ``systemd`` by specifying the user ID as the daemon instance:: systemctl enable ceph-immutable-object-cache@ceph-immutable-object-cache.{unique id} The ``ceph-immutable-object-cache`` can also be run in foreground by ``ceph-immutable-object-cache`` command:: ceph-immutable-object-cache -f --log-file={log_path} QOS Settings ------------ The immutable object cache supports throttling, controlled by the following settings: ``immutable_object_cache_qos_schedule_tick_min`` :Description: Minimum schedule tick for immutable object cache. :Type: Milliseconds :Required: No :Default: ``50`` ``immutable_object_cache_qos_iops_limit`` :Description: The desired immutable object cache IO operations limit per second. :Type: Unsigned Integer :Required: No :Default: ``0`` ``immutable_object_cache_qos_iops_burst`` :Description: The desired burst limit of immutable object cache IO operations. :Type: Unsigned Integer :Required: No :Default: ``0`` ``immutable_object_cache_qos_iops_burst_seconds`` :Description: The desired burst duration in seconds of immutable object cache IO operations. :Type: Seconds :Required: No :Default: ``1`` ``immutable_object_cache_qos_bps_limit`` :Description: The desired immutable object cache IO bytes limit per second. :Type: Unsigned Integer :Required: No :Default: ``0`` ``immutable_object_cache_qos_bps_burst`` :Description: The desired burst limit of immutable object cache IO bytes. :Type: Unsigned Integer :Required: No :Default: ``0`` ``immutable_object_cache_qos_bps_burst_seconds`` :Description: The desired burst duration in seconds of immutable object cache IO bytes. :Type: Seconds :Required: No :Default: ``1`` .. _Cloned RBD Images: ../rbd-snapshot/#layering .. _section: ../../rados/configuration/ceph-conf/#configuration-sections .. _create a Ceph user: ../../rados/operations/user-management#add-a-user