doc/cephadm: documentation for setting prometheus retention time

Signed-off-by: Adam King <adking@redhat.com>

diff --git a/doc/cephadm/services/monitoring.rst b/doc/cephadm/services/monitoring.rst
index e64778ecceb39444dc25dd4757f12a1d7a779b4b..157332564e5d14004aa5865d5dacc3e303a11a4f 100644 (file)
--- a/doc/cephadm/services/monitoring.rst
+++ b/doc/cephadm/services/monitoring.rst
@@ -338,6 +338,32 @@ Due to performance reasons, monitoring of RBD images is disabled by default. For
 :ref:`prometheus-rbd-io-statistics`. If disabled, the overview and details dashboards will stay empty in Grafana
 and the metrics will not be visible in Prometheus.
 
+Setting up Prometheus
+-----------------------
+
+Setting Prometheus Retention Time
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Cephadm provides the option to set the Prometheus TDSB retention time using
+a ``retention_time`` field in the Prometheus service spec. The value defaults
+to 15 days (15d). If you would like a different value, such as 1 year (1y) you
+can apply a service spec similar to:
+
+.. code-block:: yaml
+
+    service_type: prometheus
+    placement:
+      count: 1
+    spec:
+      retention_time: "1y"
+
+.. note::
+
+  If you already had Prometheus daemon(s) deployed before and are updating an
+  existent spec as opposed to doing a fresh Prometheus deployment, you must also
+  tell cephadm to redeploy the Prometheus daemon(s) to put this change into effect.
+  This can be done with a ``ceph orch redeploy prometheus`` command.
+
 Setting up Grafana
 ------------------