Host Management

Listing Hosts

Run a command of this form to list hosts associated with the cluster:

ceph orch host ls [--format yaml] [--host-pattern <name>] [--label <label>] [--host-status <status>]

In commands of this form, the arguments “host-pattern”, “label” and “host-status” are optional and are used for filtering.

  • “host-pattern” is a regex that matches against hostnames and returns only matching hosts.

  • “label” returns only hosts with the specified label.

  • “host-status” returns only hosts with the specified status (currently “offline” or “maintenance”).

  • Any combination of these filtering flags is valid. It is possible to filter against name, label and status simultaneously, or to filter against any proper subset of name, label and status.

Adding Hosts

Hosts must have these Requirements installed. Hosts without all the necessary requirements will fail to be added to the cluster.

To add each new host to the cluster, perform two steps:

  1. Install the cluster’s public SSH key in the new host’s root user’s authorized_keys file:

    ssh-copy-id -f -i /etc/ceph/ root@*<new-host>*

    For example:

    ssh-copy-id -f -i /etc/ceph/ root@host2
    ssh-copy-id -f -i /etc/ceph/ root@host3
  2. Tell Ceph that the new node is part of the cluster:

    ceph orch host add *<newhost>* [*<ip>*] [*<label1> ...*]

    For example:

    ceph orch host add host2
    ceph orch host add host3

    It is best to explicitly provide the host IP address. If an IP is not provided, then the host name will be immediately resolved via DNS and that IP will be used.

    One or more labels can also be included to immediately label the new host. For example, by default the _admin label will make cephadm maintain a copy of the ceph.conf file and a client.admin keyring file in /etc/ceph:

    ceph orch host add host4 --labels _admin

Removing Hosts

A host can safely be removed from a the cluster after all daemons are removed from it.

To drain all daemons from a host, run a command of the following form:

ceph orch host drain *<host>*

The ‘_no_schedule’ label will be applied to the host. See Special host labels

All osds on the host will be scheduled to be removed. You can check osd removal progress with the following:

ceph orch osd rm status

see Remove an OSD for more details about osd removal

You can check if there are no deamons left on the host with the following:

ceph orch ps <host>

Once all daemons are removed you can remove the host with the following:

ceph orch host rm <host>

Offline host removal

If a host is offline and can not be recovered it can still be removed from the cluster with the following:

ceph orch host rm <host> --offline --force

This can potentially cause data loss as osds will be forcefully purged from the cluster by calling osd purge-actual for each osd. Service specs that still contain this host should be manually updated.

Host labels

The orchestrator supports assigning labels to hosts. Labels are free form and have no particular meaning by itself and each host can have multiple labels. They can be used to specify placement of daemons. See Placement by labels

Labels can be added when adding a host with the --labels flag:

ceph orch host add my_hostname --labels=my_label1
ceph orch host add my_hostname --labels=my_label1,my_label2

To add a label a existing host, run:

ceph orch host label add my_hostname my_label

To remove a label, run:

ceph orch host label rm my_hostname my_label

Special host labels

The following host labels have a special meaning to cephadm. All start with _.

  • _no_schedule: Do not schedule or deploy daemons on this host.

    This label prevents cephadm from deploying daemons on this host. If it is added to an existing host that already contains Ceph daemons, it will cause cephadm to move those daemons elsewhere (except OSDs, which are not removed automatically).

  • _no_autotune_memory: Do not autotune memory on this host.

    This label will prevent daemon memory from being tuned even when the osd_memory_target_autotune or similar option is enabled for one or more daemons on that host.

  • _admin: Distribute client.admin and ceph.conf to this host.

    By default, an _admin label is applied to the first host in the cluster (where bootstrap was originally run), and the client.admin key is set to be distributed to that host via the ceph orch client-keyring ... function. Adding this label to additional hosts will normally cause cephadm to deploy config and keyring files in /etc/ceph.

Maintenance Mode

Place a host in and out of maintenance mode (stops all Ceph daemons on host):

ceph orch host maintenance enter <hostname> [--force]
ceph orch host maintenance exit <hostname>

Where the force flag when entering maintenance allows the user to bypass warnings (but not alerts)

See also Fully qualified domain names vs bare host names

Rescanning Host Devices

Some servers and external enclosures may not register device removal or insertion with the kernel. In these scenarios, you’ll need to perform a host rescan. A rescan is typically non-disruptive, and can be performed with the following CLI command:

ceph orch host rescan <hostname> [--with-summary]

The with-summary flag provides a breakdown of the number of HBAs found and scanned, together with any that failed:

ceph orch host rescan rh9-ceph1 --with-summary
Ok. 2 adapters detected: 2 rescanned, 0 skipped, 0 failed (0.32s)

Creating many hosts at once

Many hosts can be added at once using ceph orch apply -i by submitting a multi-document YAML file:

service_type: host
hostname: node-00
- example1
- example2
service_type: host
hostname: node-01
- grafana
service_type: host
hostname: node-02

This can be combined with service specifications (below) to create a cluster spec file to deploy a whole cluster in one command. see cephadm bootstrap --apply-spec also to do this during bootstrap. Cluster SSH Keys must be copied to hosts prior to adding them.

Setting the initial CRUSH location of host

Hosts can contain a location identifier which will instruct cephadm to create a new CRUSH host located in the specified hierachy.

service_type: host
hostname: node-00
  rack: rack1


The location attribute will be only affect the initial CRUSH location. Subsequent changes of the location property will be ignored. Also, removing a host will no remove any CRUSH buckets.

See also Types and Buckets.

SSH Configuration

Cephadm uses SSH to connect to remote hosts. SSH uses a key to authenticate with those hosts in a secure way.

Default behavior

Cephadm stores an SSH key in the monitor that is used to connect to remote hosts. When the cluster is bootstrapped, this SSH key is generated automatically and no additional configuration is necessary.

A new SSH key can be generated with:

ceph cephadm generate-key

The public portion of the SSH key can be retrieved with:

ceph cephadm get-pub-key

The currently stored SSH key can be deleted with:

ceph cephadm clear-key

You can make use of an existing key by directly importing it with:

ceph config-key set mgr/cephadm/ssh_identity_key -i <key>
ceph config-key set mgr/cephadm/ssh_identity_pub -i <pub>

You will then need to restart the mgr daemon to reload the configuration with:

ceph mgr fail

Configuring a different SSH user

Cephadm must be able to log into all the Ceph cluster nodes as an user that has enough privileges to download container images, start containers and execute commands without prompting for a password. If you do not want to use the “root” user (default option in cephadm), you must provide cephadm the name of the user that is going to be used to perform all the cephadm operations. Use the command:

ceph cephadm set-user <user>

Prior to running this the cluster SSH key needs to be added to this users authorized_keys file and non-root users must have passwordless sudo access.

Customizing the SSH configuration

Cephadm generates an appropriate ssh_config file that is used for connecting to remote hosts. This configuration looks something like this:

Host *
User root
StrictHostKeyChecking no
UserKnownHostsFile /dev/null

There are two ways to customize this configuration for your environment:

  1. Import a customized configuration file that will be stored by the monitor with:

    ceph cephadm set-ssh-config -i <ssh_config_file>

    To remove a customized SSH config and revert back to the default behavior:

    ceph cephadm clear-ssh-config
  2. You can configure a file location for the SSH configuration file with:

    ceph config set mgr mgr/cephadm/ssh_config_file <path>

    We do not recommend this approach. The path name must be visible to any mgr daemon, and cephadm runs all daemons as containers. That means that the file either need to be placed inside a customized container image for your deployment, or manually distributed to the mgr data directory (/var/lib/ceph/<cluster-fsid>/mgr.<id> on the host, visible at /var/lib/ceph/mgr/ceph-<id> from inside the container).

Fully qualified domain names vs bare host names


cephadm demands that the name of the host given via ceph orch host add equals the output of hostname on remote hosts.

Otherwise cephadm can’t be sure that names returned by ceph * metadata match the hosts known to cephadm. This might result in a CEPHADM_STRAY_HOST warning.

When configuring new hosts, there are two valid ways to set the hostname of a host:

  1. Using the bare host name. In this case:

  • hostname returns the bare host name.

  • hostname -f returns the FQDN.

  1. Using the fully qualified domain name as the host name. In this case:

  • hostname returns the FQDN

  • hostname -s return the bare host name

Note that man hostname recommends hostname to return the bare host name:

The FQDN (Fully Qualified Domain Name) of the system is the name that the resolver(3) returns for the host name, such as, It is usually the hostname followed by the DNS domain name (the part after the first dot). You can check the FQDN using hostname --fqdn or the domain name using dnsdomainname.

You cannot change the FQDN with hostname or dnsdomainname.

The recommended method of setting the FQDN is to make the hostname
be an alias for the fully qualified name using /etc/hosts, DNS, or
NIS. For example, if the hostname was "ursula", one might have
a line in /etc/hosts which reads ursula

Which means, man hostname recommends hostname to return the bare host name. This in turn means that Ceph will return the bare host names when executing ceph * metadata. This in turn means cephadm also requires the bare host name when adding a host to the cluster: ceph orch host add <bare-name>.