RBD 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.

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