Document data retention configuration

Author: cevian

README.md

@@ -41,6 +41,23 @@ By default, the `timescale-observability` Helm chart sets up a single-instance o
 interested in a replicated setup for high-availabilty with automated backups, please see
 [this github repo](https://github.com/timescale/timescaledb-kubernetes/tree/master/charts/timescaledb-single) for additional instructions.
 
+# Configuring data retention
+
+By default, data is stored for 90 days and then deleted.
+This default can be changed in SQL by using the SQL function
+`set_default_retention_period(new interval)`.  For example,
+```
+SELECT set_default_retention_period(180 * INTERVAL '1 day')
+```
+
+You can also override this default on a per-metric basis using
+the SQL function `set_metric_retention_period(metric_name, interval)`
+and undo this override with `reset_metric_retention_period(metric_name)`.
+
+Note: The default applies to all metrics that do not have override,
+no matter whether they were created before or after the call to
+`set_default_retention_period`.
+
 # Working with SQL data
 
 We describe how to use our pre-defined views and functions to work with the prometheus data in [the SQL schema doc](docs/sql_schema.md).
@@ -57,6 +74,11 @@ This is used as a dependency from the `timescale-observability` Helm chart and c
 
 ## Non-Helm installation methods
 
+Any non-helm installations also need to make sure the `drop_chunks` procedure on a regular
+basis (e.g. via CRON). We recommend executing it every 30 minutes.
+This is necessary to execute data retention policies according to the configured policy.
+This is set up automatically in helm.
+
 ### Binaries
 
 You can download pre-built binaries for the Timescale-Prometheus Connector [on our release page](https://github.com/timescale/timescale-prometheus/releases).