Prometheus Module

Provides a Prometheus exporter to pass on Ceph performance counters from the collection point in ceph-mgr. Ceph-mgr receives MMgrReport messages from all MgrClient processes (mons and OSDs, for instance) with performance counter schema data and actual counter data, and keeps a circular buffer of the last N samples. This module creates an HTTP endpoint (like all Prometheus exporters) and retrieves the latest sample of every counter when polled (or “scraped” in Prometheus terminology). The HTTP path and query parameters are ignored; all extant counters for all reporting entities are returned in text exposition format. (See the Prometheus documentation.)

Enabling prometheus output

The prometheus module is enabled with:

  1. ceph mgr module enable prometheus

Configuration

Note

The Prometheus manager module needs to be restarted for configuration changes to be applied.

By default the module will accept HTTP requests on port 9283 on all IPv4 and IPv6 addresses on the host. The port and listen address are both configurable with ceph config set, with keys mgr/prometheus/server_addr and mgr/prometheus/server_port. This port is registered with Prometheus’s registry.

  1. ceph config set mgr mgr/prometheus/server_addr 0.0.0.0
  2. ceph config set mgr mgr/prometheus/server_port 9283

Warning

The scrape_interval of this module should always be set to match Prometheus’ scrape interval to work properly and not cause any issues.

The Prometheus manager module is, by default, configured with a scrape interval of 15 seconds. The scrape interval in the module is used for caching purposes and to determine when a cache is stale.

It is not recommended to use a scrape interval below 10 seconds. It is recommended to use 15 seconds as scrape interval, though, in some cases it might be useful to increase the scrape interval.

To set a different scrape interval in the Prometheus module, set scrape_interval to the desired value:

  1. ceph config set mgr mgr/prometheus/scrape_interval 20

On large clusters (>1000 OSDs), the time to fetch the metrics may become significant. Without the cache, the Prometheus manager module could, especially in conjunction with multiple Prometheus instances, overload the manager and lead to unresponsive or crashing Ceph manager instances. Hence, the cache is enabled by default and cannot be disabled. This means that there is a possibility that the cache becomes stale. The cache is considered stale when the time to fetch the metrics from Ceph exceeds the configured scrape_interval.

If that is the case, a warning will be logged and the module will either

  • respond with a 503 HTTP status code (service unavailable) or,

  • it will return the content of the cache, even though it might be stale.

This behavior can be configured. By default, it will return a 503 HTTP status code (service unavailable). You can set other options using the ceph config set commands.

To tell the module to respond with possibly stale data, set it to return:

  1. ceph config set mgr mgr/prometheus/stale_cache_strategy return

To tell the module to respond with “service unavailable”, set it to fail:

  1. ceph config set mgr mgr/prometheus/stale_cache_strategy fail

RBD IO statistics

The module can optionally collect RBD per-image IO statistics by enabling dynamic OSD performance counters. The statistics are gathered for all images in the pools that are specified in the mgr/prometheus/rbd_stats_pools configuration parameter. The parameter is a comma or space separated list of pool[/namespace] entries. If the namespace is not specified the statistics are collected for all namespaces in the pool.

Example to activate the RBD-enabled pools pool1, pool2 and poolN:

  1. ceph config set mgr mgr/prometheus/rbd_stats_pools "pool1,pool2,poolN"

The module makes the list of all available images scanning the specified pools and namespaces and refreshes it periodically. The period is configurable via the mgr/prometheus/rbd_stats_pools_refresh_interval parameter (in sec) and is 300 sec (5 minutes) by default. The module will force refresh earlier if it detects statistics from a previously unknown RBD image.

Example to turn up the sync interval to 10 minutes:

  1. ceph config set mgr mgr/prometheus/rbd_stats_pools_refresh_interval 600

Statistic names and labels

The names of the stats are exactly as Ceph names them, with illegal characters ., - and :: translated to _, and ceph_ prefixed to all names.

All daemon statistics have a ceph_daemon label such as “osd.123” that identifies the type and ID of the daemon they come from. Some statistics can come from different types of daemon, so when querying e.g. an OSD’s RocksDB stats, you would probably want to filter on ceph_daemon starting with “osd” to avoid mixing in the monitor rocksdb stats.

The cluster statistics (i.e. those global to the Ceph cluster) have labels appropriate to what they report on. For example, metrics relating to pools have a pool_id label.

The long running averages that represent the histograms from core Ceph are represented by a pair of <name>_sum and <name>_count metrics. This is similar to how histograms are represented in Prometheus and they can also be treated similarly.

Pool and OSD metadata series

Special series are output to enable displaying and querying on certain metadata fields.

Pools have a ceph_pool_metadata field like this:

  1. ceph_pool_metadata{pool_id="2",name="cephfs_metadata_a"} 1.0

OSDs have a ceph_osd_metadata field like this:

  1. ceph_osd_metadata{cluster_addr="172.21.9.34:6802/19096",device_class="ssd",ceph_daemon="osd.0",public_addr="172.21.9.34:6801/19096",weight="1.0"} 1.0

Correlating drive statistics with node_exporter

The prometheus output from Ceph is designed to be used in conjunction with the generic host monitoring from the Prometheus node_exporter.

To enable correlation of Ceph OSD statistics with node_exporter’s drive statistics, special series are output like this:

  1. ceph_disk_occupation{ceph_daemon="osd.0",device="sdd", exported_instance="myhost"}

To use this to get disk statistics by OSD ID, use either the and operator or the * operator in your prometheus query. All metadata metrics (like `` ceph_disk_occupation`` have the value 1 so they act neutral with *. Using * allows to use group_left and group_right grouping modifiers, so that the resulting metric has additional labels from one side of the query.

See the prometheus documentation for more information about constructing queries.

The goal is to run a query like

  1. rate(node_disk_bytes_written[30s]) and on (device,instance) ceph_disk_occupation{ceph_daemon="osd.0"}

Out of the box the above query will not return any metrics since the instance labels of both metrics don’t match. The instance label of ceph_disk_occupation will be the currently active MGR node.

The following two section outline two approaches to remedy this.

Use label_replace

The label_replace function (cp. label_replace documentation) can add a label to, or alter a label of, a metric within a query.

To correlate an OSD and its disks write rate, the following query can be used:

  1. label_replace(rate(node_disk_bytes_written[30s]), "exported_instance", "$1", "instance", "(.*):.*") and on (device,exported_instance) ceph_disk_occupation{ceph_daemon="osd.0"}

Configuring Prometheus server

honor_labels

To enable Ceph to output properly-labeled data relating to any host, use the honor_labels setting when adding the ceph-mgr endpoints to your prometheus configuration.

This allows Ceph to export the proper instance label without prometheus overwriting it. Without this setting, Prometheus applies an instance label that includes the hostname and port of the endpoint that the series came from. Because Ceph clusters have multiple manager daemons, this results in an instance label that changes spuriously when the active manager daemon changes.

If this is undesirable a custom instance label can be set in the Prometheus target configuration: you might wish to set it to the hostname of your first mgr daemon, or something completely arbitrary like “ceph_cluster”.

node_exporter hostname labels

Set your instance labels to match what appears in Ceph’s OSD metadata in the instance field. This is generally the short hostname of the node.

This is only necessary if you want to correlate Ceph stats with host stats, but you may find it useful to do it in all cases in case you want to do the correlation in the future.

Example configuration

This example shows a single node configuration running ceph-mgr and node_exporter on a server called senta04. Note that this requires one to add an appropriate and unique instance label to each node_exporter target.

This is just an example: there are other ways to configure prometheus scrape targets and label rewrite rules.

prometheus.yml

  1. global:
  2.   scrape_interval:     15s
  3.   evaluation_interval: 15s
  5. scrape_configs:
  6.   - job_name: 'node'
  7.     file_sd_configs:
  8.       - files:
  9.         - node_targets.yml
  10.   - job_name: 'ceph'
  11.     honor_labels: true
  12.     file_sd_configs:
  13.       - files:
  14.         - ceph_targets.yml

ceph_targets.yml

  1. [
  2.     {
  3.         "targets": [ "senta04.mydomain.com:9283" ],
  4.         "labels": {}
  5.     }
  6. ]

node_targets.yml

  1. [
  2.     {
  3.         "targets": [ "senta04.mydomain.com:9100" ],
  4.         "labels": {
  5.             "instance": "senta04"
  6.         }
  7.     }
  8. ]

Notes

Counters and gauges are exported; currently histograms and long-running averages are not. It’s possible that Ceph’s 2-D histograms could be reduced to two separate 1-D histograms, and that long-running averages could be exported as Prometheus’ Summary type.

Timestamps, as with many Prometheus exporters, are established by the server’s scrape time (Prometheus expects that it is polling the actual counter process synchronously). It is possible to supply a timestamp along with the stat report, but the Prometheus team strongly advises against this. This means that timestamps will be delayed by an unpredictable amount; it’s not clear if this will be problematic, but it’s worth knowing about.