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.
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
Brought to you by the Ceph Foundation
The Ceph Documentation is a community resource funded and hosted by the non-profit Ceph Foundation. If you would like to support this and our other efforts, please consider joining now.