feat: longhorn guide (#36)

* feat: longhorn guide

* fix: remove line

Author: lorenzocorallo

docs/infrastructure/03-Guides/add-storage.md

@@ -0,0 +1,80 @@
+---
+title: Add Storage
+---
+
+## Core Concepts
+
+Many applications and services need a physical volume or storage to store their data or configuration.  
+As you should know, we use AKS (Kubernetes) to host everything from databases to telegram bots.  
+In our AKS cluster we have (ATTOW) a single node (VM) `Standard_B2ms` which allows to attach only 4 disks,
+one of which is the VM's disk itself.
+This means that you can only attach 3 managed disks to the Cluster, resulting in limited deployable services.
+
+To solve this problem, we added [Longhorn](https://longhorn.io/) so that we can create multiple
+[PVC](https://kubernetes.io/docs/concepts/storage/persistent-volumes) from the VM's disk 
+(ATTOW it's a 128GB Standard SSD).
+
+:::important
+Longhorn is a handy tool to create multiple PVC/volumes without creating new disks, but this comes with degraded performance and stability.  
+If you want to deploy something critical like a production database, it's recommended to create a dedicated
+[Managed Disk](https://learn.microsoft.com/en-us/azure/virtual-machines/managed-disks-overview).
+:::
+
+## Add a Longhorn volume
+
+First of all, you must login in [our Longhorn dashboard](https://longhorn.polinetwork.org) and check
+how much space is available for scheduling a new volume.
+:::tip 
+- `Schedulable` is space you can use to create new volumes
+- `Reserved` is space reserved to the VM, not available to create new volumes
+
+How much `Reserved` space is configured in the `Node` page, but at least 20-25 GB is recommended.
+:::
+
+Once you verified you have sufficient `Schedulable` space available, you can create the PVC in the k8s manifest:
+
+```yaml title="my-longhorn-example.yaml"
+# add-highlight-start
+apiVersion: v1
+kind: PersistentVolumeClaim
+metadata:
+  name: <name>-pvc
+spec:
+  accessModes:
+    - ReadWriteOnce
+  storageClassName: longhorn-static
+  resources:
+    requests:
+      storage: <size>
+# add-highlight-end
+---
+kind: Pod
+apiVersion: v1
+metadata:
+  name: my-longhorn-example-pod
+  namespace: <namespace>
+spec:
+  containers:
+    - name: busybox
+      image: registry.k8s.io/e2e-test-images/busybox:1.29-4
+      command:
+        - '/bin/sleep'
+        - '10000'
+      # add-highlight-start
+      volumeMounts:
+        - name: my-volume # Name of the volume defined below
+          mountPath: /app/data # Where you need to mount the volume
+      # add-highlight-end
+# add-highlight-start
+  volumes:
+    - name: my-volume # it can be whatever you want
+      persistentVolumeClaim:
+        claimName: <name>-pvc # the name of the PVC defined above
+# add-highlight-end
+```
+
+Parameters:
+- `namespace` k8s namespace
+- `name` should identify the app where the volume will be used
+- `size` size of the volume. Check [resource units](https://kubernetes.io/docs/concepts/configuration/manage-resources-containers/#meaning-of-memory).
+    e.g. `300M`, `1Gi`, `200Mi`