Ceph monitors are responsible for monitoring the health of the entire cluster as well as maintaining the cluster membership state, state of peer nodes, and cluster configuration information. The Ceph monitor performs these tasks by maintaining a master copy of the cluster map. The cluster map includes Monitor maps, OSD maps, the PG map, the CRUSH map, and the MDS map. All these maps are collectively known as cluster maps. Let's take a quick look at the functionality of each map:
- Monitor map: It holds end-to-end information about the monitor node, which includes the Ceph cluster id, monitor hostname, and IP address with the port number. It also stores the current epoch for map creation and last changed time too. You can check your cluster's monitor map by executing the following:
# ceph mon dump - OSD map: It stores some common fields, such as cluster ID, epoch for OSD map creation and last changed, and information related to pools, such as pool names, pool ID, type, replication level, and PGs. It also stores OSD information such as count, state, weight, last clean interval, and OSD host information. You can check your cluster's OSD maps by executing the following:
# ceph osd dump - PG map: It holds the PG version, time stamp, last OSD map epoch, full ratio, and near full ratio information. It also keeps track of each PG ID, object count, state, state stamp, up and acting OSD sets, and finally, the scrub details. To check your cluster PG map, execute the following:
# ceph pg dump - CRUSH map: It holds information on your clusters devices, buckets, failure domain hierarchy, and the rules defined for the failure domain when storing data. To check your cluster CRUSH map, execute the following:
# ceph osd crush dump - MDS map: This stores information on the current MDS map epoch, map creation and modification time, data and metadata pool ID, cluster MDS count, and the MDS state. To check your cluster MDS map, execute the following:
# ceph mds dump
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