document how to enable prometheus alerts

docs/howto/features/storage-quota.md

@@ -138,6 +138,59 @@ deployer deploy <cluster_name> <hub_name>
 
 Once this is deployed, the hub will automatically enforce the storage quota for each user. If a user's home directory exceeds the quota, the user's pod may not be able to start successfully.
 
+## Enabling alerting through Prometheus Alertmanager
+
+Once we have enabled storage quotas, we want to be alerted when the disk usage of the NFS server exceeds a certain threshold so that we can take appropriate action.
+
+To do this, we need to create a Prometheus rule that will alert us when the disk usage of the NFS server exceeds a certain threshold using Alertmanager. 
+
+First, we need to enable Alertmanager in the hub's support values file (for example, [here's the one for the `nasa-veda` cluster](https://github.com/2i2c-org/infrastructure/blob/main/config/clusters/nasa-veda/support.values.yaml)).
+
+```yaml
+prometheus:
+  alertmanager:
+    enabled: true
+```
+
+Then, we need to create a Prometheus rule that will alert us when the disk usage of the NFS server exceeds a certain threshold. For example, to alert us when the disk usage of the NFS server exceeds 90% of the total disk size, we would add the following to the hub's support values file:
+
+```yaml
+prometheus:
+  serverFiles:
+    alerting_rules.yml:
+      groups:
+        - name: <cluster_name> jupyterhub-home-nfs EBS volume full
+          rules:
+            - alert: jupyterhub-home-nfs-ebs-full
+              expr: node_filesystem_avail_bytes{mountpoint="/shared-volume", component="shared-volume-metrics"} / node_filesystem_size_bytes{mountpoint="/shared-volume", component="shared-volume-metrics"} < 0.1
+              for: 15m
+              labels:
+                severity: critical
+                channel: pagerduty
+                cluster: <cluster_name>
+              annotations:
+                summary: "jupyterhub-home-nfs EBS volume full in namespace {{ $labels.namespace }}"
+```
+
+And finally, we need to configure Alertmanager to send alerts to PagerDuty.
+
+```yaml
+prometheus:
+  alertmanager:
+    enabled: true
+    config:
+      route:
+        group_wait: 10s
+        group_interval: 5m
+        receiver: pagerduty
+        repeat_interval: 3h
+        routes:
+          - receiver: pagerduty
+            match:
+              channel: pagerduty
+```
+
+
 ## Troubleshooting
 
 ### Checking the NFS server is running properly