Configuration database

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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 priority order that Ceph uses to set options is:

  1. Compiled-in default values

  2. Ceph cluster configuration database

  3. Local ceph.conf file

  4. 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.

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.
Important: 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.
Note: 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

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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. These sections are described in Table 1.

Table 1. Configuration database sections
Section Description
global Affects all daemons and clients.
mon Affects all Ceph Monitors.
mgr Affects all Ceph Managers.
osd Affects all Ceph OSDs.
mds Affects all Ceph Metadata Servers.
client Affects all Ceph Clients, including mounted file systems, block devices, and RADOS Gateways.

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:
type:location

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 the foo host.

For example: 
ceph config set osd/host:magna045 debug_osd 20
class:device-class

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.

For example:
ceph config set osd/class:hdd osd_max_backfills 8

Administrative commands

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

ls

Lists the available configuration options.

dump

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.

set WHO OPTION VALUE

Sets a configuration option in the Ceph configuration database, where WHO is the target daemon, OPTION is the option to set, and VALUE is the desired value.

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.

For more information, see Setting a specific configuration at runtime.