Chapter 1. The basics of Ceph configuration

As a storage administrator, you need to have a basic understanding of how to view the Ceph configuration, and how to set the Ceph configuration options for the Red Hat Ceph Storage cluster. You can view and set the Ceph configuration options at runtime.

1.1. Prerequisites

  • Installation of the Red Hat Ceph Storage software.

1.2. Ceph configuration

All Red Hat Ceph Storage clusters have a configuration, which defines:

  • Cluster Identity
  • Authentication settings
  • Ceph daemons
  • Network configuration
  • Node names and addresses
  • Paths to keyrings
  • Paths to OSD log files
  • Other runtime options

A deployment tool, such as cephadm, will typically create an initial Ceph configuration file for you. However, you can create one yourself if you prefer to bootstrap a Red Hat Ceph Storage cluster without using a deployment tool.

Additional Resources

1.3. The Ceph configuration database

The Ceph Monitor manages a configuration database of Ceph options that centralize configuration management by storing configuration options for the entire storage cluster. By centralizing the Ceph configuration in a database, this simplifies storage cluster administration.

The overall priority order that Ceph uses to set options is:

  • Compiled-in default values
  • Ceph cluster configuration database
  • Local ceph.conf file
  • Runtime override, using the ceph daemon DAEMON-NAME config set or ceph tell DAEMON-NAME injectargs commands

There are still a few Ceph options that can be defined in the local Ceph configuration file, which is /etc/ceph/ceph.conf by default. However, ceph.conf has been deprecated for Red Hat Ceph Storage 5.

cephadm uses a basic ceph.conf file that only contains a minimal set of options for connecting to Ceph Monitors, authenticating, and fetching configuration information. In most cases, cephadm uses only the mon_host option. To avoid using ceph.conf only for the mon_host option, use DNS SRV records to perform operations with Monitors.


Red Hat recommends that you use the assimilate-conf administrative command to move valid options into the configuration database from the ceph.conf file. For more information about assimilate-conf, see Administrative Commands.

Ceph allows you to make changes to the configuration of a daemon at runtime. This capability can be useful for increasing or decreasing the logging output, by enabling or disabling debug settings, and can even be used for runtime optimization.


When the same option exists in the configuration database and the Ceph configuration file, the configuration database option has a lower priority than what is set in the Ceph configuration file.

Sections and Masks

Just as you can configure Ceph options globally, per daemon type, or by a specific daemon in the Ceph configuration file, you can also configure the Ceph options in the configuration database according to these sections. Ceph configuration options can have a mask associated with them. These masks can further restrict which daemons or clients the options apply to.

Masks have two forms:

The type is a CRUSH property, for example, rack or host. The location is a value for the property type. For example, host:foo limits the option only to daemons or clients running on a particular node, foo in this example.
The device-class is the name of the CRUSH device class, such as hdd or ssd. For example, class:ssd limits the option only to Ceph OSDs backed by solid state drives (SSD). This mask has no effect on non-OSD daemons of clients.

Administrative Commands

The Ceph configuration database can be administered with the sub-command ceph config ACTION. These are the actions you can do:

Dumps the entire configuration database of options for the storage cluster.
get WHO
Dumps the configuration for a specific daemon or client. For example, WHO can be a daemon, like mds.a.
Sets a configuration option in the Ceph configuration database.
show WHO
Shows the reported running configuration for a running daemon. These options might be different from those stored by the Ceph Monitors, if there is a local configuration file in use or options have been overridden on the command line or at run time. Also, the source of the option values is reported as part of the output.
assimilate-conf -i INPUT_FILE -o OUTPUT_FILE
Assimilate a configuration file from the INPUT_FILE and move any valid options into the Ceph Monitors’ configuration database. Any options that are unrecognized, invalid, or cannot be controlled by the Ceph Monitor return in an abbreviated configuration file stored in the OUTPUT_FILE. This command can be useful for transitioning from legacy configuration files to a centralized configuration database. Note that when you assimilate a configuration and the Monitors or other daemons have different configuration values set for the same set of options, the end result depends on the order in which the files are assimilated.
help OPTION -f json-pretty
Displays help for a particular OPTION using a JSON-formatted output.

Additional Resources

1.4. Using the Ceph metavariables

Metavariables simplify Ceph storage cluster configuration dramatically. When a metavariable is set in a configuration value, Ceph expands the metavariable into a concrete value.

Metavariables are very powerful when used within the [global], [osd], [mon], or [client] sections of the Ceph configuration file. However, you can also use them with the administration socket. Ceph metavariables are similar to Bash shell expansion.

Ceph supports the following metavariables:

Expands to the Ceph storage cluster name. Useful when running multiple Ceph storage clusters on the same hardware.
Expands to one of osd or mon, depending on the type of the instant daemon.
Expands to the daemon identifier. For osd.0, this would be 0.
Expands to the host name of the instant daemon.
Expands to $type.$id.

1.5. Viewing the Ceph configuration at runtime

The Ceph configuration files can be viewed at boot time and run time.


  • Root-level access to the Ceph node.
  • Access to admin keyring.


  1. To view a runtime configuration, log in to a Ceph node running the daemon and execute:


    ceph daemon DAEMON_TYPE.ID config show

    To see the configuration for osd.0, you can log into the node containing osd.0 and execute this command:


    [root@osd ~]# ceph daemon osd.0 config show

  2. For additional options, specify a daemon and help.


    [root@osd ~]# ceph daemon osd.0 help

1.6. Viewing a specific configuration at runtime

Configuration settings for Red Hat Ceph Storage can be viewed at runtime from the Ceph Monitor node.


  • A running Red Hat Ceph Storage cluster.
  • Root-level access to the Ceph Monitor node.


  1. Log into a Ceph node and execute:


    ceph daemon DAEMON_TYPE.ID config get PARAMETER


    [root@mon ~]# ceph daemon osd.0 config get public_addr

1.7. Setting a specific configuration at runtime

To set a specific Ceph configuration at runtime, use the ceph config set command.


  • A running Red Hat Ceph Storage cluster.
  • Root-level access to the Ceph Monitor or OSD nodes.


  1. Set the configuration on all Monitor or OSD daemons :


    ceph config set DAEMON CONFIG-OPTION VALUE


    [root@ceph5 ~]# ceph config set osd debug_osd 10

  2. Validate that the option and value are set:


    [root@ceph5 ~]# ceph config dump
    osd      advanced debug_osd  10/10

    • To remove the configuration option from all daemons:


      ceph config rm DAEMON CONFIG-OPTION VALUE


      [root@ceph5 ~]# ceph config rm osd debug_osd

    • To set the configuration for a specific daemon:




      [root@ceph5 ~]# ceph config set osd.0 debug_osd 10

    • To validate that the configuration is set for the specified daemon:


      [root@ceph5 ~]# ceph config dump
      osd.0      advanced debug_osd     10/10

    • To remove the configuration for a specific daemon:




      root@ceph5 ~]# ceph config rm osd.0 debug_osd


If you use a client that does not support reading options from the configuration database, or if you still need to use ceph.conf to change your cluster configuration for other reasons, run the following command:

ceph config set mgr mgr/cephadm/manage_etc_ceph_ceph_conf false

You must maintain and distribute the ceph.conf file across the storage cluster.

1.8. OSD Memory Target

BlueStore keeps OSD heap memory usage under a designated target size with the osd_memory_target configuration option.

The option osd_memory_target sets OSD memory based upon the available RAM in the system. Use this option when TCMalloc is configured as the memory allocator, and when the bluestore_cache_autotune option in BlueStore is set to true.

Ceph OSD memory caching is more important when the block device is slow; for example, traditional hard drives, because the benefit of a cache hit is much higher than it would be with a solid state drive. However, this must be weighed in to a decision to colocate OSDs with other services, such as in a hyper-converged infrastructure (HCI) or other applications.

1.8.1. Setting the OSD memory target

Use the osd_memory_target option to set the maximum memory threshold for all OSDs in the storage cluster, or for specific OSDs. An OSD with an osd_memory_target option set to 16 GB may use up to 16 GB of memory.


Configuration options for individual OSDs take precedence over the settings for all OSDs.


  • A running Red Hat Ceph Storage cluster.
  • Root-level access to all hosts in the storage cluster.


  • To set osd_memory_target for all OSDs in the storage cluster:


    ceph config set osd osd_memory_target VALUE

    VALUE is the number of GBytes of memory to be allocated to each OSD in the storage cluster.

  • To set osd_memory_target for a specific OSD in the storage cluster:


    ceph config set osd_memory_target VALUE

    .id is the ID of the OSD and VALUE is the number of GBytes of memory to be allocated to the specified OSD. For example, to configure the OSD with ID 8 to use up to 16 GBytes of memory:


    [ceph@cephadm /]# ceph config set osd.8 osd_memory_target _16G_

  • To set an individual OSD to use one maximum amount of memory and configure the rest of the OSDs to use another amount, specify the individual OSD first:


    [ceph@cephadm /]# ceph config set osd osd_memory_target 16G
    ceph config set osd.8 osd_memory_target 8G

1.9. MDS Memory Cache Limit

MDS servers keep their metadata in a separate storage pool, named cephfs_metadata, and are the users of Ceph OSDs. For Ceph File Systems, MDS servers have to support an entire Red Hat Ceph Storage cluster, not just a single storage device within the storage cluster, so their memory requirements can be significant, particularly if the workload consists of small-to-medium-size files, where the ratio of metadata to data is much higher.

Example: Set the mds_cache_memory_limit to 2000000000 bytes


For a large Red Hat Ceph Storage cluster with a metadata-intensive workload, do not put an MDS server on the same node as other memory-intensive services, doing so gives you the option to allocate more memory to MDS, for example, sizes greater than 100 GB.

Additional Resources

1.10. Additional Resources

  • See the general Ceph configuration options in Appendix A for specific option descriptions and usage.