Add documentation on resizing prometheus' disk

docs/index.md

@@ -35,6 +35,7 @@ sre-guide/support/index.md
 sre-guide/manage-k8s/index.md
 sre-guide/node-scale-up/index.md
 sre-guide/common-problems-solutions.md
+sre-guide/prometheus-disk-resize.md
 ```
 
 (hub-deployment-guide)=

docs/sre-guide/prometheus-disk-resize.md

@@ -0,0 +1,103 @@
+# Resizing Prometheus' disk
+
+We may need to resize Prometheus' disk that collects metrics data as we store more and more data.
+
+On GCP clusters, the storage classes are set by default to permit auto-expansion.
+Therefore, simply defining a new persistent volume size in the support chart values and redeploying it, should suffice.
+However, this may not be the case on other cloud providers.
+The below steps will walk you through resizing the disk.
+
+## Resizing the disk
+
+```bash
+# Set the KUBE_EDITOR env var to point to a text editor you're comfortable with
+export KUBE_EDITOR="/usr/bin/nano"
+
+# Set the name of the cluster to work against
+export CLUSTER_NAME=...
+
+# Authenticate against the cluster
+deployer use-cluster-credentials $CLUSTER_NAME
+```
+
+1. Set the desired size of the Prometheus server persistent volume in the relevant `support.values.yaml` file.
+
+   ```yaml
+   prometheus:
+     server:
+       persistentVolume:
+         size: <desired-size>
+   ```
+
+1. Check the reclaim policy on the persistent volume.
+
+   ```bash
+   # List all the PVs. They are not namespaced.
+   kubectl get pv
+   ```
+
+1. Edit persistent volume's reclaim policy to be `Retain` if it is not already.
+   This will prevent us from losing the data Prometheus has already collected.
+
+   ```bash
+   kubectl edit pv <pv-name>
+   ```
+
+1. Check the value of `ALLOWVOLUMEEXPANSION` of the default storage class, identified by `(default)` next to it's name.
+
+   ```bash
+   kubectl get storageclass
+   ```
+
+1. Set `ALLOWVOLUMEEXPANSION` to `true` if it is not.
+   This will allow the persistent volumes to be dynamically resized.
+
+   ```bash
+   kubectl patch storageclass <storage-class-name> --patch '{\"allowVolumeExpansion\": true}'
+   ```
+
+```{note}
+At the point, you could try to redeploy the support chart and see if it succeeds.
+If it doesn't, continue with the steps.
+```
+
+1. Delete the persistent volume claim for the prometheus server.
+   Persistent volume claims cannot be patched so we will need to recreate it.
+
+   ```bash
+   # List all PVCs in the support namespace
+   kubectl -n support get pvc
+
+   # Delete the prometheus server PVC
+   kubectl -n support delete pvc support-prometheus-server
+   ```
+
+1. In another terminal with the `CLUSTER_NAME` variable set, redeploy the support chart.
+   It should fail with the PVC in a `Pending` state.
+
+   ```bash
+   deployer deploy-support $CLUSTER_NAME
+   ```
+
+1. Edit the persistent volume to have the same UID and resource version as the newly created PVC under`spec.claimRef`.
+
+   ```bash
+   # Get the UID and resource version of the PVC
+   kubectl -n support get pvc support-prometheus-server -o yaml
+
+   # Edit the PV to reference these values under `spec.claimRef`
+   kubectl edit pv <pv-name>
+   ```
+
+1. Delete the prometheus server pod and check that it comes back up.
+
+   ```bash
+   kubectl -n support delete pod support-prometheus-server-<hash>
+   kubectl -n support get pods --watch
+   ```
+
+1. Redeploy the support chart again and this time it should succeed.
+
+   ```bash
+   deployer deploy-support $CLUSTER_NAME
+   ```