MON Service

Deploying additional monitors

A typical Ceph cluster has three or five monitor daemons that are spread across different hosts. We recommend deploying five monitors if there are five or more nodes in your cluster.

Ceph deploys monitor daemons automatically as the cluster grows and Ceph scales back monitor daemons automatically as the cluster shrinks. The smooth execution of this automatic growing and shrinking depends upon proper subnet configuration.

The cephadm bootstrap procedure assigns the first monitor daemon in the cluster to a particular subnet. cephadm designates that subnet as the default subnet of the cluster. New monitor daemons will be assigned by default to that subnet unless cephadm is instructed to do otherwise.

If all of the ceph monitor daemons in your cluster are in the same subnet, manual administration of the ceph monitor daemons is not necessary. cephadm will automatically add up to five monitors to the subnet, as needed, as new hosts are added to the cluster.

By default, cephadm will deploy 5 daemons on arbitrary hosts. See Placement Specification for details of specifying the placement of daemons.

Designating a Particular Subnet for Monitors

To designate a particular IP subnet for use by ceph monitor daemons, use a command of the following form, including the subnet’s address in CIDR format (e.g., 10.1.2.0/24):

ceph config set mon public_network *<mon-cidr-network>*

For example:

ceph config set mon public_network 10.1.2.0/24

Cephadm deploys new monitor daemons only on hosts that have IP addresses in the designated subnet.

You can also specify two public networks by using a list of networks:

ceph config set mon public_network *<mon-cidr-network1>,<mon-cidr-network2>*

For example:

ceph config set mon public_network 10.1.2.0/24,192.168.0.1/24

Deploying Monitors on a Particular Network

You can explicitly specify the IP address or CIDR network for each monitor and control where each monitor is placed. To disable automated monitor deployment, run this command:

ceph orch apply mon --unmanaged

To deploy each additional monitor:

ceph orch daemon add mon *<host1:ip-or-network1>

For example, to deploy a second monitor on newhost1 using an IP address 10.1.2.123 and a third monitor on newhost2 in network 10.1.2.0/24, run the following commands:

ceph orch apply mon --unmanaged
ceph orch daemon add mon newhost1:10.1.2.123
ceph orch daemon add mon newhost2:10.1.2.0/24

Now, enable automatic placement of Daemons

ceph orch apply mon --placement="newhost1,newhost2,newhost3" --dry-run

See Placement Specification for details of specifying the placement of daemons.

Finally apply this new placement by dropping --dry-run

ceph orch apply mon --placement="newhost1,newhost2,newhost3"

Moving Monitors to a Different Network

To move Monitors to a new network, deploy new monitors on the new network and subsequently remove monitors from the old network. It is not advised to modify and inject the monmap manually.

First, disable the automated placement of daemons:

ceph orch apply mon --unmanaged

To deploy each additional monitor:

ceph orch daemon add mon *<newhost1:ip-or-network1>*

For example, to deploy a second monitor on newhost1 using an IP address 10.1.2.123 and a third monitor on newhost2 in network 10.1.2.0/24, run the following commands:

ceph orch apply mon --unmanaged
ceph orch daemon add mon newhost1:10.1.2.123
ceph orch daemon add mon newhost2:10.1.2.0/24

Subsequently remove monitors from the old network:

ceph orch daemon rm *mon.<oldhost1>*

Update the public_network:

ceph config set mon public_network *<mon-cidr-network>*

For example:

ceph config set mon public_network 10.1.2.0/24

Now, enable automatic placement of Daemons

ceph orch apply mon --placement="newhost1,newhost2,newhost3" --dry-run

See Placement Specification for details of specifying the placement of daemons.

Finally apply this new placement by dropping --dry-run

ceph orch apply mon --placement="newhost1,newhost2,newhost3"