doc/cephadm: documentation for setting prometheus retention time

Signed-off-by: Adam King <adking@redhat.com>
(cherry picked from commit 11fc0ef194dc347de075cde1274933ec83164404)

diff --git a/doc/cephadm/services/monitoring.rst b/doc/cephadm/services/monitoring.rst
index a17beba6d1e18f40285edbbd734dd4b490414bc5..a36f62edeec79f875d662760a6319585ac0ffc4e 100644 (file)
--- a/doc/cephadm/services/monitoring.rst
+++ b/doc/cephadm/services/monitoring.rst
@@ -282,6 +282,32 @@ 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
 ------------------