This document is for a development version of Ceph.

Configuring Ceph

When Ceph services start, the initialization process activates a series of daemons that run in the background. A Ceph Storage Cluster runs at a minimum three types of daemons:

Ceph Storage Clusters that support the Ceph File System also run at least one Ceph Metadata Server (ceph-mds). Clusters that support Ceph Object Storage run Ceph RADOS Gateway daemons (radosgw) as well.

Each daemon has a number of configuration options, each of which has a default value. You may adjust the behavior of the system by changing these configuration options. Be careful to understand the consequences before overriding default values, as it is possible to significantly degrade the performance and stability of your cluster. Also note that default values sometimes change between releases, so it is best to review the version of this documentation that aligns with your Ceph release.

Option names

All Ceph configuration options have a unique name consisting of words formed with lower-case characters and connected with underscore (_) characters.

When option names are specified on the command line, either underscore (_) or dash (-) characters can be used interchangeable (e.g., --mon-host is equivalent to --mon_host).

When option names appear in configuration files, spaces can also be used in place of underscore or dash. We suggest, though, that for clarity and convenience you consistently use underscores, as we do throughout this documentation.

Config sources

Each Ceph daemon, process, and library will pull its configuration from several sources, listed below. Sources later in the list will override those earlier in the list when both are present.

  • the compiled-in default value

  • the monitor cluster’s centralized configuration database

  • a configuration file stored on the local host

  • environment variables

  • command line arguments

  • runtime overrides set by an administrator

One of the first things a Ceph process does on startup is parse the configuration options provided via the command line, environment, and local configuration file. The process will then contact the monitor cluster to retrieve configuration stored centrally for the entire cluster. Once a complete view of the configuration is available, the daemon or process startup will proceed.

Bootstrap options

Some configuration options affect the process’s ability to contact the monitors, to authenticate, and to retrieve the cluster-stored configuration. For this reason, these options might need to be stored locally on the node, and set by means of a local configuration file. These options include the following:


This is a list of IP addresses or hostnames that are separated by commas, whitespace, or semicolons. Hostnames are resolved via DNS. All A and AAAA records are included in the search list.




This is the list of monitors that the Ceph process initially contacts when first establishing communication with the Ceph cluster. This overrides the known monitor list that is derived from MonMap updates sent to older Ceph instances (like librados cluster handles). This option is expected to be useful primarily for debugging.



  • mon_dns_srv_name

  • mon_data, osd_data, mds_data, mgr_data, and similar options that define which local directory the daemon stores its data in.

  • keyring, keyfile, and/or key, which can be used to specify the authentication credential to use to authenticate with the monitor. Note that in most cases the default keyring location is in the data directory specified above.

In most cases, the default values of these options are suitable. There is one exception to this: the mon_host option that identifies the addresses of the cluster’s monitors. When DNS is used to identify monitors, a local Ceph configuration file can be avoided entirely.

Skipping monitor config

Pass the option --no-mon-config to any process to skip the step that retrieves configuration information from the cluster monitors. This is useful in cases where configuration is managed entirely via configuration files, or when the monitor cluster is down and some maintenance activity needs to be done.

Configuration sections

Any given process or daemon has a single value for each configuration option. However, values for an option may vary across different daemon types even daemons of the same type. Ceph options that are stored in the monitor configuration database or in local configuration files are grouped into sections to indicate which daemons or clients they apply to.

These sections include:


Settings under global affect all daemons and clients in a Ceph Storage Cluster.


log_file = /var/log/ceph/$cluster-$type.$id.log


Settings under mon affect all ceph-mon daemons in the Ceph Storage Cluster, and override the same setting in global.


mon_cluster_log_to_syslog = true


Settings in the mgr section affect all ceph-mgr daemons in the Ceph Storage Cluster, and override the same setting in global.


mgr_stats_period = 10


Settings under osd affect all ceph-osd daemons in the Ceph Storage Cluster, and override the same setting in global.


osd_op_queue = wpq


Settings in the mds section affect all ceph-mds daemons in the Ceph Storage Cluster, and override the same setting in global.


mds_cache_memory_limit = 10G


Settings under client affect all Ceph Clients (e.g., mounted Ceph File Systems, mounted Ceph Block Devices, etc.) as well as Rados Gateway (RGW) daemons.


objecter_inflight_ops = 512

Sections may also specify an individual daemon or client name. For example,, osd.123, and client.smith are all valid section names.

Any given daemon will draw its settings from the global section, the daemon or client type section, and the section sharing its name. Settings in the most-specific section take precedence, so for example if the same option is specified in both global, mon, and on the same source (i.e., in the same configurationfile), the value will be used.

If multiple values of the same configuration option are specified in the same section, the last value wins.

Note that values from the local configuration file always take precedence over values from the monitor configuration database, regardless of which section they appear in.


Metavariables simplify Ceph Storage Cluster configuration dramatically. When a metavariable is set in a configuration value, Ceph expands the metavariable into a concrete value at the time the configuration value is used. Ceph metavariables are similar to variable expansion in the Bash shell.

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 a daemon or process type (e.g., mds, osd, or mon)




Expands to the daemon or client identifier. For osd.0, this would be 0; for mds.a, it would be a.




Expands to the host name where the process is running.


Expands to $type.$id.




Expands to daemon pid.



The Configuration File

On startup, Ceph processes search for a configuration file in the following locations:

  1. $CEPH_CONF (i.e., the path following the $CEPH_CONF environment variable)

  2. -c path/path (i.e., the -c command line argument)

  3. /etc/ceph/$cluster.conf

  4. ~/.ceph/$cluster.conf

  5. ./$cluster.conf (i.e., in the current working directory)

  6. On FreeBSD systems only, /usr/local/etc/ceph/$cluster.conf

where $cluster is the cluster’s name (default ceph).

The Ceph configuration file uses an ini style syntax. You can add comment text after a pound sign (#) or a semi-colon (;). For example:

# <--A number (#) sign precedes a comment.
; A comment may be anything.
# Comments always follow a semi-colon (;) or a pound (#) on each line.
# The end of the line terminates a comment.
# We recommend that you provide comments in your configuration file(s).

Config file section names

The configuration file is divided into sections. Each section must begin with a valid configuration section name (see Configuration sections, above) surrounded by square brackets. For example,

debug_ms = 0

debug_ms = 1

debug_ms = 10

debug_ms = 10

Config file option values

The value of a configuration option is a string. If it is too long to fit in a single line, you can put a backslash (\) at the end of line as the line continuation marker, so the value of the option will be the string after = in current line combined with the string in the next line:

foo = long long ago\
long ago

In the example above, the value of “foo” would be “long long ago long ago”.

Normally, the option value ends with a new line, or a comment, like

obscure_one = difficult to explain # I will try harder in next release
simpler_one = nothing to explain

In the example above, the value of “obscure one” would be “difficult to explain”; and the value of “simpler one would be “nothing to explain”.

If an option value contains spaces, and we want to make it explicit, we could quote the value using single or double quotes, like

line = "to be, or not to be"

Certain characters are not allowed to be present in the option values directly. They are =, #, ; and [. If we have to, we need to escape them, like

secret = "i love \# and \["

Every configuration option is typed with one of the types below:


64-bit signed integer, Some SI prefixes are supported, like “K”, “M”, “G”, “T”, “P”, “E”, meaning, respectively, 103, 106, 109, etc. And “B” is the only supported unit. So, “1K”, “1M”, “128B” and “-1” are all valid option values. Some times, a negative value implies “unlimited” when it comes to an option for threshold or limit.


42, -1


It is almost identical to integer. But a negative value will be rejected.


256, 0


Free style strings encoded in UTF-8, but some characters are not allowed. Please reference the above notes for the details.


"hello world", "i love \#", yet-another-name


one of the two values true or false. But an integer is also accepted, where “0” implies false, and any non-zero values imply true.


true, false, 1, 0


a single address optionally prefixed with v1, v2 or any for the messenger protocol. If the prefix is not specified, v2 protocol is used. Please see Address formats for more details.


v1:, v2:,, 2409:8a1e:8fb6:aa20:1260:4bff:fe92:18f5::567, [::1]:6789


a set of addresses separated by “,”. The addresses can be optionally quoted with [ and ].


[v1:,v2:], v1:,v1: [2409:8a1e:8fb6:aa20:1260:4bff:fe92:18f5::567], [2409:8a1e:8fb6:aa20:1260:4bff:fe92:18f5::568]


the string format of a uuid defined by RFC4122. And some variants are also supported, for more details, see Boost document.




denotes a 64-bit unsigned integer. Both SI prefixes and IEC prefixes are supported. And “B” is the only supported unit. A negative value will be rejected.


1Ki, 1K, 1KiB and 1B.


denotes a duration of time. By default the unit is second if not specified. Following units of time are supported:

  • second: “s”, “sec”, “second”, “seconds”

  • minute: “m”, “min”, “minute”, “minutes”

  • hour: “hs”, “hr”, “hour”, “hours”

  • day: “d”, “day”, “days”

  • week: “w”, “wk”, “week”, “weeks”

  • month: “mo”, “month”, “months”

  • year: “y”, “yr”, “year”, “years”


1 m, 1m and 1 week

Monitor configuration database

The monitor cluster manages a database of configuration options that can be consumed by the entire cluster, enabling streamlined central configuration management for the entire system. The vast majority of configuration options can and should be stored here for ease of administration and transparency.

A handful of settings may still need to be stored in local configuration files because they affect the ability to connect to the monitors, authenticate, and fetch configuration information. In most cases this is limited to the mon_host option, although this can also be avoided through the use of DNS SRV records.

Sections and masks

Configuration options stored by the monitor can live in a global section, daemon type section, or specific daemon section, just like options in a configuration file can.

In addition, options may also have a mask associated with them to further restrict which daemons or clients the option applies to. Masks take two forms:

  1. type:location where type is a CRUSH property like rack or host, and location is a value for that property. For example, host:foo would limit the option only to daemons or clients running on a particular host.

  2. class:device-class where device-class is the name of a CRUSH device class (e.g., hdd or ssd). For example, class:ssd would limit the option only to OSDs backed by SSDs. (This mask has no effect for non-OSD daemons or clients.)

When setting a configuration option, the who may be a section name, a mask, or a combination of both separated by a slash (/) character. For example, osd/rack:foo would mean all OSD daemons in the foo rack.

When viewing configuration options, the section name and mask are generally separated out into separate fields or columns to ease readability.


The following CLI commands are used to configure the cluster:

  • ceph config dump will dump the entire monitors’ configuration database for the cluster.

  • ceph config get <who> will dump configuration options stored in the monitors’ configuration database for a specific daemon or client (e.g., mds.a).

  • ceph config get <who> <option> will show a configuration value stored in the monitors’ configuration database for a specific daemon or client (e.g., mds.a), or, if not present in the monitors’ configuration database, the compiled-in default value.

  • ceph config set <who> <option> <value> will set a configuration option in the monitors’ configuration database.

  • ceph config show <who> will show the reported running configuration for a running daemon. These settings may differ from those stored by the monitors if there are also local configuration files in use or options have been overridden on the command line or at run time. The source of the option values is reported as part of the output.

  • ceph config assimilate-conf -i <input file> -o <output file> will ingest a configuration file from input file and move any valid options into the monitors’ configuration database. Any settings that are unrecognized, invalid, or cannot be controlled by the monitor will be returned in an abbreviated config file stored in output file. This command is useful for transitioning from legacy configuration files to centralized monitor-based configuration.

Note that ceph config set <who> <option> <value> and ceph config get <who> <option> aren’t symmetric because the latter also shows compiled-in default values. In order to determine whether a configuration option is present in the monitors’ configuration database, use ceph config dump.


You can get help for a particular option with:

ceph config help <option>

Note that this will use the configuration schema that is compiled into the running monitors. If you have a mixed-version cluster (e.g., during an upgrade), you might also want to query the option schema from a specific running daemon:

ceph daemon <name> config help [option]

For example:

ceph config help log_file
log_file - path to log file
  (std::string, basic)
  Default (non-daemon):
  Default (daemon): /var/log/ceph/$cluster-$name.log
  Can update at runtime: false
  See also: [log_to_stderr,err_to_stderr,log_to_syslog,err_to_syslog]


ceph config help log_file -f json-pretty
    "name": "log_file",
    "type": "std::string",
    "level": "basic",
    "desc": "path to log file",
    "long_desc": "",
    "default": "",
    "daemon_default": "/var/log/ceph/$cluster-$name.log",
    "tags": [],
    "services": [],
    "see_also": [
    "enum_values": [],
    "min": "",
    "max": "",
    "can_update_at_runtime": false

The level property can be any of basic, advanced, or dev. The dev options are intended for use by developers, generally for testing purposes, and are not recommended for use by operators.

Runtime Changes

In most cases, Ceph permits changes to the configuration of a daemon at runtime. This can be used for increasing or decreasing the amount of logging output, for enabling or disabling debug settings, and for runtime optimization.

Configuration options can be updated via the ceph config set command. For example, to enable the debug log level on a specific OSD, run a command of this form:

ceph config set osd.123 debug_ms 20


If an option has been customized in a local configuration file, the central config setting will be ignored (it has a lower priority than the local configuration file).

Override values

Options can be set temporarily by using the tell or daemon interfaces on the Ceph CLI. These override values are ephemeral, which means that they affect only the current instance of the daemon and revert to persistently configured values when the daemon restarts.

Override values can be set in two ways:

  1. From any host, send a message to a daemon with a command of the following form:

    ceph tell <name> config set <option> <value>

    For example:

    ceph tell osd.123 config set debug_osd 20

    The tell command can also accept a wildcard as the daemon identifier. For example, to adjust the debug level on all OSD daemons, run a command of this form:

    ceph tell osd.* config set debug_osd 20
  2. On the host where the daemon is running, connect to the daemon via a socket in /var/run/ceph by running a command of this form:

    ceph daemon <name> config set <option> <value>

    For example:

    ceph daemon osd.4 config set debug_osd 20


In the output of the ceph config show command, these temporary values are shown with a source of override.

Viewing runtime settings

You can see the current options set for a running daemon with the ceph config show command. For example:

ceph config show osd.0

will show you the (non-default) options for that daemon. You can also look at a specific option with:

ceph config show osd.0 debug_osd

or view all options (even those with default values) with:

ceph config show-with-defaults osd.0

You can also observe settings for a running daemon by connecting to it from the local host via the admin socket. For example:

ceph daemon osd.0 config show

will dump all current settings:

ceph daemon osd.0 config diff

will show only non-default settings (as well as where the value came from: a config file, the monitor, an override, etc.), and:

ceph daemon osd.0 config get debug_osd

will report the value of a single option.

Changes since Nautilus

The Octopus release changed the way the configuration file is parsed. These changes are as follows:

  • Repeated configuration options are allowed, and no warnings will be printed. The value of the last one is used, which means that the setting last in the file is the one that takes effect. Before this change, we would print warning messages when lines with duplicated options were encountered, like:

    warning line 42: 'foo' in section 'bar' redefined
  • Invalid UTF-8 options were ignored with warning messages. But since Octopus, they are treated as fatal errors.

  • Backslash \ is used as the line continuation marker to combine the next line with current one. Before Octopus, it was required to follow a backslash with a non-empty line. But in Octopus, an empty line following a backslash is now allowed.

  • In the configuration file, each line specifies an individual configuration option. The option’s name and its value are separated with =, and the value may be quoted using single or double quotes. If an invalid configuration is specified, we will treat it as an invalid configuration file

    bad option ==== bad value
  • Before Octopus, if no section name was specified in the configuration file, all options would be set as though they were within the global section. This is now discouraged. Since Octopus, only a single option is allowed for configuration files without a section name.