Configuration Guide

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

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.

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:

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.

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.

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:

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:

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

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

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

ceph config set mgr mgr/cephadm/manage_etc_ceph_ceph_conf false

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 into a decision to colocate OSDs with other services, such as in a hyper-converged infrastructure (HCI) or other applications.

The OSD daemons adjust the memory consumption based on the osd_memory_target configuration option. The option osd_memory_target sets OSD memory based upon the available RAM in the system.

If Red Hat Ceph Storage is deployed on dedicated nodes that do not share memory with other services, cephadm automatically adjusts the per-OSD consumption based on the total amount of RAM and the number of deployed OSDs.

ceph config set osd osd_memory_target_autotune true

Once the storage cluster is upgraded to Red Hat Ceph Storage 5.0, for cluster maintenance such as addition of OSDs or replacement of OSDs, Red Hat recommends setting osd_memory_target_autotune parameter to true to autotune osd memory as per system memory.

Cephadm starts with a fraction mgr/cephadm/autotune_memory_target_ratio, which defaults to 0.7 of the total RAM in the system, subtract off any memory consumed by non-autotuned daemons such as non-OSDS and for OSDs for which osd_memory_target_autotune is false, and then divide by the remaining OSDs.

By default, autotune_memory_target_ratio is 0.2 for hyper-converged infrastructure and 0.7 for other environments.

The osd_memory_target parameter is calculated as follows:

osd_memory_target = TOTAL_RAM_OF_THE_OSD_NODE (in Bytes) * (autotune_memory_target_ratio) / NUMBER_OF_OSDS_IN_THE_OSD_NODE - (SPACE_ALLOCATED_FOR_OTHER_DAEMONS (in Bytes))

SPACE_ALLOCATED_FOR_OTHER_DAEMONS may optionally include the following daemon space allocations:

For example, if a node has 24 OSDs and has 251 GB RAM space, then osd_memory_target is 7860684936.

The final targets are reflected in the configuration database with options. You can view the limits and the current memory consumed by each daemon from the ceph orch ps output under MEM LIMIT column.

[ceph: root@host01 /]# ceph config set mgr mgr/cephadm/autotune_memory_target_ratio 0.2

You can manually set a specific memory target for an OSD in the storage cluster.

[ceph: root@host01 /]# ceph config set osd.123 osd_memory_target 7860684936

You can manually set a specific memory target for an OSD host in the storage cluster.

ceph config set osd/host:HOSTNAME osd_memory_target TARGET_BYTES

[ceph: root@host01 /]# ceph config set osd/host:host01 osd_memory_target 1000000000

ceph orch host label add HOSTNAME _no_autotune_memory

You can exclude an OSD from memory autotuning by disabling the autotune option and setting a specific memory target.

[ceph: root@host01 /]# ceph config set osd.123 osd_memory_target_autotune false
[ceph: root@host01 /]# ceph config set osd.123 osd_memory_target 16G

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

ceph_conf_overrides:
  osd:
    mds_cache_memory_limit=2000000000

Network configuration is critical for building a high performance Red Hat Ceph Storage cluster. The Ceph storage cluster does not perform request routing or dispatching on behalf of the Ceph client. Instead, Ceph clients make requests directly to Ceph OSD daemons. Ceph OSDs perform data replication on behalf of Ceph clients, which means replication and other factors impose additional loads on the networks of Ceph storage clusters.

Ceph has one network configuration requirement that applies to all daemons. The Ceph configuration file must specify the host for each daemon.

Some deployment utilities, such as cephadm creates a configuration file for you. Do not set these values if the deployment utility does it for you.

All Ceph clusters must use a public network. However, unless you specify an internal cluster network, Ceph assumes a single public network. Ceph can function with a public network only, but for large storage clusters, you will see significant performance improvement with a second private network for carrying only cluster-related traffic.

To support two networks, each Ceph Node will need to have more than one network interface card (NIC).

There are several reasons to consider operating two separate networks:

Network configuration settings are not required. Ceph can function with a public network only, assuming a public network is configured on all hosts running a Ceph daemon. However, Ceph allows you to establish much more specific criteria, including multiple IP networks and subnet masks for your public network. You can also establish a separate cluster network to handle OSD heartbeat, object replication, and recovery traffic.

Do not confuse the IP addresses you set in the configuration with the public-facing IP addresses network clients might use to access your service. Typical internal IP networks are often 192.168.0.0 or 10.0.0.0.

When you configured the networks, you can restart the cluster or restart each daemon. Ceph daemons bind dynamically, so you do not have to restart the entire cluster at once if you change the network configuration.

Messenger is the Ceph network layer implementation. Red Hat supports two messenger types:

In Red Hat Ceph Storage 4 and higher, async is the default messenger type. To change the messenger type, specify the ms_type configuration setting in the [global] section of the Ceph configuration file.

To configure Ceph networks, use the config set command within the cephadm shell. Note that the IP addresses you set in your network configuration are different from the public-facing IP addresses that network clients might use to access your service.

Ceph functions perfectly well with only a public network. However, Ceph allows you to establish much more specific criteria, including multiple IP networks for your public network.

You can also establish a separate, private cluster network to handle OSD heartbeat, object replication, and recovery traffic. For more information about the private network, see Configuring a private network.

The public network configuration allows you specifically define IP addresses and subnets for the public network.

When the user wants to place the Ceph Monitor daemons on hosts belonging to multiple network subnets, configuring multiple public networks to the cluster is necessary. An example of usage is a stretch cluster mode used for Advanced Cluster Management (ACM) in Metro DR for OpenShift Data Foundation.

You can configure multiple public networks to the cluster during bootstrap and once bootstrap is complete.

Network configuration settings are not required. Ceph assumes a public network with all hosts operating on it, unless you specifically configure a cluster network, also known as a private network.

If you create a cluster network, OSDs routes heartbeat, object replication, and recovery traffic over the cluster network. This can improve performance, compared to using a single network.

To assign a cluster network, use the --cluster-network option with the cephadm bootstrap command. The cluster network that you specify must define a subnet in CIDR notation (for example, 10.90.90.0/24 or fe80::/64).

You can also configure the cluster_network after boostrap.

By default, Red Hat Ceph Storage daemons use TCP ports 6800—​7100 to communicate with other hosts in the cluster. You can verify that the host’s firewall allows connection on these ports.

See the firewall’s documentation for more information.

You can enable encryption for all Ceph traffic over the network with the introduction of the messenger version 2 protocol. The secure mode setting for messenger v2 encrypts communication between Ceph daemons and Ceph clients, giving you end-to-end encryption.

The Ceph daemons bind to multiple ports allowing both the legacy, v1-compatible, and the new, v2-compatible, Ceph clients to connect to the same storage cluster. Ceph clients or other Ceph daemons connecting to the Ceph Monitor daemon will try to use the v2 protocol first, if possible, but if not, then the legacy v1 protocol will be used. By default, both messenger protocols, v1 and v2, are enabled. The new v2 port is 3300, and the legacy v1 port is 6789, by default.

By default, Ceph OSDs bind to the first available ports on a Ceph node beginning at port 6800. Ensure to open at least four ports beginning at port 6800 for each OSD that runs on the node:

Ports are node-specific. However, you might need to open more ports than the number of ports needed by Ceph daemons running on that Ceph node in the event that processes get restarted and the bound ports do not get released. Consider opening a few additional ports in case a daemon fails and restarts without releasing the port such that the restarted daemon binds to a new port. Also, consider opening the port range of 6800—​7300 on each OSD node.

If you set separate public and cluster networks, you must add rules for both the public network and the cluster network, because clients will connect using the public network and other Ceph OSD Daemons will connect using the cluster network.

If you put the cluster network into another zone, open the ports within that zone as appropriate.

Understanding how to configure a Ceph Monitor is an important part of building a reliable Red Hat Ceph Storage cluster. All storage clusters have at least one monitor. A Ceph Monitor configuration usually remains fairly consistent, but you can add, remove or replace a Ceph Monitor in a storage cluster.

Ceph monitors maintain a "master copy" of the cluster map. That means a Ceph client can determine the location of all Ceph monitors and Ceph OSDs just by connecting to one Ceph monitor and retrieving a current cluster map.

Before Ceph clients can read from or write to Ceph OSDs, they must connect to a Ceph Monitor first. With a current copy of the cluster map and the CRUSH algorithm, a Ceph client can compute the location for any object. The abilityto compute object locations allows a Ceph client to talk directly to Ceph OSDs, which is a very important aspect of Ceph’s high scalability and performance.

The primary role of the Ceph Monitor is to maintain a master copy of the cluster map. Ceph Monitors also provide authentication and logging services. Ceph Monitors write all changes in the monitor services to a single Paxos instance, and Paxos writes the changes to a key-value store for strong consistency. Ceph Monitors can query the most recent version of the cluster map during synchronization operations. Ceph Monitors leverage the key-value store’s snapshots and iterators, using the rocksdb database, to perform store-wide synchronization.

The cluster map is a composite of maps, including the monitor map, the OSD map, and the placement group map. The cluster map tracks a number of important events:

When there is a significant change in the state of the cluster, for example, a Ceph OSD goes down, a placement group falls into a degraded state, and so on. The cluster map gets updated to reflect the current state of the cluster. Additionally, the Ceph monitor also maintains a history of the prior states of the cluster. The monitor map, OSD map, and placement group map each maintain a history of their map versions. Each version is called an epoch.

When operating the Red Hat Ceph Storage cluster, keeping track of these states is an important part of the cluster administration.

A cluster will run sufficiently with a single monitor. However, a single monitor is a single-point-of-failure. To ensure high availability in a production Ceph storage cluster, run Ceph with multiple monitors so that the failure of a single monitor will not cause a failure of the entire storage cluster.

When a Ceph storage cluster runs multiple Ceph Monitors for high availability, Ceph Monitors use the Paxos algorithm to establish consensus about the master cluster map. A consensus requires a majority of monitors running to establish a quorum for consensus about the cluster map. For example, 1; 2 out of 3; 3 out of 5; 4 out of 6; and so on.

Red Hat recommends running a production Red Hat Ceph Storage cluster with at least three Ceph Monitors to ensure high availability. When you run multiple monitors, you can specify the initial monitors that must be members of the storage cluster to establish a quorum. This may reduce the time it takes for the storage cluster to come online.

[mon]
mon_initial_members = a,b,c

When you add monitor settings to the Ceph configuration file, you need to be aware of some of the architectural aspects of Ceph Monitors. Ceph imposes strict consistency requirements for a Ceph Monitor when discovering another Ceph Monitor within the cluster. Whereas Ceph clients and other Ceph daemons use the Ceph configuration file to discover monitors, monitors discover each other using the monitor map (monmap), not the Ceph configuration file.

A Ceph Monitor always refers to the local copy of the monitor map when discovering other Ceph Monitors in the Red Hat Ceph Storage cluster. Using the monitor map instead of the Ceph configuration file avoids errors that could break the cluster. For example, typos in the Ceph configuration file when specifying a monitor address or port. Since monitors use monitor maps for discovery and they share monitor maps with clients and other Ceph daemons, the monitor map provides monitors with a strict guarantee that their consensus is valid.

If Ceph Monitors discovered each other through the Ceph configuration file instead of through the monitor map, it would introduce additional risks because the Ceph configuration files are not updated and distributed automatically. Ceph Monitors might inadvertently use an older Ceph configuration file, fail to recognize a Ceph Monitor, fall out of a quorum, or develop a situation where Paxos is not able to determine the current state of the system accurately.

In most configuration and deployment cases, tools that deploy Ceph, such as cephadm, might help bootstrap the Ceph monitors by generating a monitor map for you.

A Ceph monitor requires a few explicit settings:

The bare minimum monitor settings for a Ceph Monitor in the Ceph configuration file includes a host name for each monitor if it is not configured for DNS and the monitor address. The Ceph Monitors run on port 6789 and 3300 by default.

You can use the following commands to set or read the storage cluster configuration options.

Here, WHO parameter might be name of the section or a Ceph daemon, OPTION is a configuration file, and VALUE can be either true or false.

ceph cephadm set-extra-ceph-conf

Each Red Hat Ceph Storage cluster has a unique identifier (fsid). If specified, it usually appears under the [global] section of the configuration file. Deployment tools usually generate the fsid and store it in the monitor map, so the value may not appear in a configuration file. The fsid makes it possible to run daemons for multiple clusters on the same hardware.

Ceph provides a default path where Ceph monitors store data.

Ceph monitors call the fsync() function often, which can interfere with Ceph OSD workloads.

Ceph monitors store their data as key-value pairs. Using a data store prevents recovering Ceph monitors from running corrupted versions through Paxos, and it enables multiple modification operations in one single atomic batch, among other advantages.

When a Red Hat Ceph Storage cluster gets close to its maximum capacity (specifies by the mon_osd_full_ratio parameter), Ceph prevents you from writing to or reading from Ceph OSDs as a safety measure to prevent data loss. Therefore, letting a production Red Hat Ceph Storage cluster approach its full ratio is not a good practice, because it sacrifices high availability. The default full ratio is .95, or 95% of capacity. This a very aggressive setting for a test cluster with a small number of OSDs.

A common scenario for test clusters involves a system administrator removing a Ceph OSD from the Red Hat Ceph Storage cluster to watch the cluster re-balance. Then, removing another Ceph OSD, and so on until the Red Hat Ceph Storage cluster eventually reaches the full ratio and locks up.

Ideally, you want to plan for a series of Ceph OSD failures where the cluster can recover to an active + clean state without replacing those Ceph OSDs immediately. You can run a cluster in an active + degraded state, but this is not ideal for normal operating conditions.

The following diagram depicts a simplistic Red Hat Ceph Storage cluster containing 33 Ceph Nodes with one Ceph OSD per host, each Ceph OSD Daemon reading from and writing to a 3TB drive. So this exemplary Red Hat Ceph Storage cluster has a maximum actual capacity of 99TB. With a mon osd full ratio of 0.95, if the Red Hat Ceph Storage cluster falls to 5 TB of remaining capacity, the cluster will not allow Ceph clients to read and write data. So the Red Hat Ceph Storage cluster’s operating capacity is 95 TB, not 99 TB.

It is normal in such a cluster for one or two OSDs to fail. A less frequent but reasonable scenario involves a rack’s router or power supply failing, which brings down multiple OSDs simultaneously, for example, OSDs 7-12. In such a scenario, you should still strive for a cluster that can remain operational and achieve an active + clean state, even if that means adding a few hosts with additional OSDs in short order. If your capacity utilization is too high, you might not lose data, but you could still sacrifice data availability while resolving an outage within a failure domain if capacity utilization of the cluster exceeds the full ratio. For this reason, Red Hat recommends at least some rough capacity planning.

Identify two numbers for your cluster:

To determine the mean average capacity of an OSD within a cluster, divide the total capacity of the cluster by the number of OSDs in the cluster. Consider multiplying that number by the number of OSDs you expect to fail simultaneously during normal operations (a relatively small number). Finally, multiply the capacity of the cluster by the full ratio to arrive at a maximum operating capacity. Then, subtract the amount of data from the OSDs you expect to fail to arrive at a reasonable full ratio. Repeat the foregoing process with a higher number of OSD failures (for example, a rack of OSDs) to arrive at a reasonable number for a near full ratio.

Ceph monitors know about the cluster by requiring reports from each OSD, and by receiving reports from OSDs about the status of their neighboring OSDs. Ceph provides reasonable default settings for interaction between monitor and OSD, however, you can modify them as needed.

When you run a production cluster with multiple monitors which is recommended, each monitor checks to see if a neighboring monitor has a more recent version of the cluster map. For example, a map in a neighboring monitor with one or more epoch numbers higher than the most current epoch in the map of the instant monitor. Periodically, one monitor in the cluster might fall behind the other monitors to the point where it must leave the quorum, synchronize to retrieve the most current information about the cluster, and then rejoin the quorum.

These roles enable a leader to delegate synchronization duties to a provider, which prevents synchronization requests from overloading the leader and improving performance. In the following diagram, the requester has learned that it has fallen behind the other monitors. The requester asks the leader to synchronize, and the leader tells the requester to synchronize with a provider.

Once synchronization is complete, Ceph requires trimming across the cluster. Trimming requires that the placement groups are active + clean.

Ceph daemons pass critical messages to each other, which must be processed before daemons reach a timeout threshold. If the clocks in Ceph monitors are not synchronized, it can lead to a number of anomalies.

For example:

Clock drift may still be noticeable with NTP even though the discrepancy is not yet harmful. Ceph clock drift and clock skew warnings can get triggered even though NTP maintains a reasonable level of synchronization. Increasing your clock drift may be tolerable under such circumstances. However, a number of factors such as workload, network latency, configuring overrides to default timeouts, and other synchronization options can influence the level of acceptable clock drift without compromising Paxos guarantees.

The cephx protocol is enabled by default. Cryptographic authentication has some computational costs, though they are generally quite low. If the network environment connecting clients and hosts is considered safe and you cannot afford authentication computational costs, you can disable it. When deploying a Ceph storage cluster, the deployment tool will create the client.admin user and keyring.

When cephx is enabled, Ceph will look for the keyring in the default search path, which includes /etc/ceph/$cluster.$name.keyring. You can override this location by adding a keyring option in the [global] section of the Ceph configuration file, but this is not recommended.

Execute the following procedures to enable cephx on a cluster with authentication disabled. If you or your deployment utility have already generated the keys, you may skip the steps related to generating keys.

The following procedure describes how to disable Cephx. If your cluster environment is relatively safe, you can offset the computation expense of running authentication.

However, it may be easier during setup or troubleshooting to temporarily disable authentication.

When you run Ceph with authentication enabled, the ceph administrative commands and Ceph clients require authentication keys to access the Ceph storage cluster.

The most common way to provide these keys to the ceph administrative commands and clients is to include a Ceph keyring under the /etc/ceph/ directory. The file name is usually ceph.client.admin.keyring or $cluster.client.admin.keyring. If you include the keyring under the /etc/ceph/ directory, you do not need to specify a keyring entry in the Ceph configuration file.

To do so, execute the following command:

# scp USER@HOSTNAME:/etc/ceph/ceph.client.admin.keyring /etc/ceph/ceph.client.admin.keyring

Replace USER with the user name used on the host with the client.admin key and HOSTNAME with the host name of that host.

You can specify the key itself in the Ceph configuration file using the key setting, which is not recommended, or a path to a key file using the keyfile setting.

Administrative users or deployment tools might generate daemon keyrings in the same way as generating user keyrings. By default, Ceph stores daemons keyrings inside their data directory. The default keyring locations, and the capabilities necessary for the daemon to function.

The daemon data directory locations default to directories of the form:

/var/lib/ceph/$type/CLUSTER-ID

/var/lib/ceph/osd/ceph-12

You can override these locations, but it is not recommended.

Ceph provides fine-grained control so you can enable or disable signatures for service messages between the client and Ceph. You can enable or disable signatures for messages between Ceph daemons.

When you create pools and set the number of placement groups for the pool, Ceph uses default values when you do not specifically override the defaults.

You can set these values when running pool commands.

By default, Ceph makes 3 replicas of objects. If you want to set 4 copies of an object as the default value, a primary copy and three replica copies, reset the default values as shown in osd_pool_default_size. If you want to allow Ceph to write a lesser number of copies in a degraded state, set osd_pool_default_min_size to a number less than the osd_pool_default_size value.

[ceph: root@host01 /]# ceph config set global osd_pool_default_size 4  # Write an object 4 times.
[ceph: root@host01 /]# ceph config set global osd_pool_default_min_size 1  # Allow writing one copy in a degraded state.

Ensure you have a realistic number of placement groups. Red Hat recommends approximately 100 per OSD. For example, total number of OSDs multiplied by 100 divided by the number of replicas, that is, osd_pool_default_size. For 10 OSDs and osd_pool_default_size = 4, we would recommend approximately (100 * 10) / 4 = 250.

[ceph: root@host01 /]# ceph config set global osd_pool_default_pg_num 250
[ceph: root@host01 /]# ceph config set global osd_pool_default_pgp_num 250

All Ceph clusters have a configuration, which defines:

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 cluster without using a deployment tool.

For your convenience, each daemon has a series of default values. Many are set by the ceph/src/common/config_opts.h script. You can override these settings with a Ceph configuration file or at runtime by using the monitor tell command or connecting directly to a daemon socket on a Ceph node.

In addition to making multiple copies of objects, Ceph ensures data integrity by scrubbing placement groups. Ceph scrubbing is analogous to the fsck command on the object storage layer.

For each placement group, Ceph generates a catalog of all objects and compares each primary object and its replicas to ensure that no objects are missing or mismatched.

Light scrubbing (daily) checks the object size and attributes. Deep scrubbing (weekly) reads the data and uses checksums to ensure data integrity.

Scrubbing is important for maintaining data integrity, but it can reduce performance. Adjust the following settings to increase or decrease scrubbing operations.

When you add Ceph OSDs to a cluster or remove them from the cluster, the CRUSH algorithm rebalances the cluster by moving placement groups to or from Ceph OSDs to restore the balance. The process of migrating placement groups and the objects they contain can reduce the cluster operational performance considerably. To maintain operational performance, Ceph performs this migration with the 'backfill' process, which allows Ceph to set backfill operations to a lower priority than requests to read or write data.

When the cluster starts or when a Ceph OSD terminates unexpectedly and restarts, the OSD begins peering with other Ceph OSDs before a write operation can occur.

If a Ceph OSD crashes and comes back online, usually it will be out of sync with other Ceph OSDs containing more recent versions of objects in the placement groups. When this happens, the Ceph OSD goes into recovery mode and seeks to get the latest copy of the data and bring its map back up to date. Depending upon how long the Ceph OSD was down, the OSD’s objects and placement groups may be significantly out of date. Also, if a failure domain went down, for example, a rack, more than one Ceph OSD might come back online at the same time. This can make the recovery process time consuming and resource intensive.

To maintain operational performance, Ceph performs recovery with limitations on the number of recovery requests, threads, and object chunk sizes which allows Ceph to perform well in a degraded state.

After you have completed your initial Ceph configuration, you can deploy and run Ceph. When you execute a command such as ceph health or ceph -s, the Ceph Monitor reports on the current state of the Ceph storage cluster. The Ceph Monitor knows about the Ceph storage cluster by requiring reports from each Ceph OSD daemon, and by receiving reports from Ceph OSD daemons about the status of their neighboring Ceph OSD daemons. If the Ceph Monitor does not receive reports, or if it receives reports of changes in the Ceph storage cluster, the Ceph Monitor updates the status of the Ceph cluster map.

Ceph provides reasonable default settings for Ceph Monitor and OSD interaction. However, you can override the defaults. The following sections describe how Ceph Monitors and Ceph OSD daemons interact for the purposes of monitoring the Ceph storage cluster.

Each Ceph OSD daemon checks the heartbeat of other Ceph OSD daemons every 6 seconds. To change the heartbeat interval, change the value at runtime:

ceph config set osd osd_heartbeat_interval TIME_IN_SECONDS

[ceph: root@host01 /]# ceph config set osd osd_heartbeat_interval 60

If a neighboring Ceph OSD daemon does not send heartbeat packets within a 20 second grace period, the Ceph OSD daemon might consider the neighboring Ceph OSD daemon down. It can report it back to a Ceph Monitor, which updates the Ceph cluster map. To change the grace period, set the value at runtime:

ceph config set osd osd_heartbeat_grace TIME_IN_SECONDS

[ceph: root@host01 /]# ceph config set osd osd_heartbeat_grace 30

By default, two Ceph OSD Daemons from different hosts must report to the Ceph Monitors that another Ceph OSD Daemon is down before the Ceph Monitors acknowledge that the reported Ceph OSD Daemon is down.

However, there is the chance that all the OSDs reporting the failure are in different hosts in a rack with a bad switch that causes connection problems between OSDs.

To avoid a "false alarm," Ceph considers the peers reporting the failure as a proxy for a "subcluster" that is similarly laggy. While this is not always the case, it may help administrators localize the grace correction to a subset of the system that is performing poorly.

Ceph uses the mon_osd_reporter_subtree_level setting to group the peers into the "subcluster" by their common ancestor type in the CRUSH map.

By default, only two reports from a different subtree are required to report another Ceph OSD Daemon down. Administrators can change the number of reporters from unique subtrees and the common ancestor type required to report a Ceph OSD Daemon down to a Ceph Monitor by setting the mon_osd_min_down_reporters and mon_osd_reporter_subtree_level values at runtime:

ceph config set mon mon_osd_min_down_reporters NUMBER

[ceph: root@host01 /]# ceph config set mon mon_osd_min_down_reporters 4

ceph config set mon mon_osd_reporter_subtree_level CRUSH_ITEM

[ceph: root@host01 /]# ceph config set mon mon_osd_reporter_subtree_level host
[ceph: root@host01 /]# ceph config set mon mon_osd_reporter_subtree_level rack
[ceph: root@host01 /]# ceph config set mon mon_osd_reporter_subtree_level osd

If a Ceph OSD daemon cannot peer with any of the Ceph OSD daemons defined in its Ceph configuration file or the cluster map, it pings a Ceph Monitor for the most recent copy of the cluster map every 30 seconds. You can change the Ceph Monitor heartbeat interval by setting the value at runtime:

ceph config set osd osd_mon_heartbeat_interval TIME_IN_SECONDS

[ceph: root@host01 /]# ceph config set osd osd_mon_heartbeat_interval 60

If a Ceph OSD Daemon does not report to a Ceph Monitor, the Ceph Monitor marks the Ceph OSD Daemon down after the mon_osd_report_timeout, which is 900 seconds, elapses. A Ceph OSD Daemon sends a report to a Ceph Monitor when a reportable event such as a failure, a change in placement group stats, a change in up_thru or when it boots within 5 seconds.

You can change the Ceph OSD Daemon minimum report interval by setting the osd_mon_report_interval value at runtime:

ceph config set osd osd_mon_report_interval TIME_IN_SECONDS

To get, set, and verify the config you can use the following example:

[ceph: root@host01 /]# ceph config get osd osd_mon_report_interval
5
[ceph: root@host01 /]# ceph config set osd osd_mon_report_interval 20
[ceph: root@host01 /]# ceph config dump | grep osd

global                  advanced  osd_pool_default_crush_rule                  -1
  osd                   basic     osd_memory_target                            4294967296
  osd                   advanced  osd_mon_report_interval                      20