doc: Add doc for volume expansion feature

Signed-off-by: Shuo Wu <shuo@rancher.com>

README.md

@@ -212,6 +212,7 @@ More examples are available at `./examples/`
 ### [Storage Tags](./docs/storage-tags.md)
 ### [Customized default setting](./docs/customized-default-setting.md)
 ### [Taint Toleration](./docs/taint-toleration.md)
+### [Volume Expansion](./docs/expansion.md)
 
 ### [Restoring Stateful Set volumes](./docs/restore_statefulset.md)
 ### [Google Kubernetes Engine](./docs/gke.md)

docs/expansion.md

@@ -0,0 +1,80 @@
+# Volume Expansion
+
+## Overview
+- Longhorn supports both ONLINE and OFFLINE volume expansion. 
+- Longhorn will expand frontend (e.g. block device) then expand filesystem.
+
+## Prerequisite:
+Longhorn version v0.8.0 or higher.
+
+## Expand a Longhorn volume
+There are two ways to expand a Longhorn volume:
+
+#### Via PVC 
+- This method is applied only if:
+  1. Kubernetes version v1.16 or higher.
+  2. The PVC is dynamically provisioned by the Kubernetes with Longhorn StorageClass.
+  3. The field `allowVolumeExpansion` should be `true` in the related StorageClass.
+- This method is recommended if it's applicable. Since the PVC and PV will be updated automatically and everything keeps consistent after expansion.
+- Usage: Find the corresponding PVC for Longhorn volume then modify requested `storage` of the PVC spec. e.g.,
+```
+apiVersion: v1
+kind: PersistentVolumeClaim
+metadata:
+  annotations:
+    kubectl.kubernetes.io/last-applied-configuration: |
+      {"apiVersion":"v1","kind":"PersistentVolumeClaim","metadata":{"annotations":{},"name":"longhorn-simple-pvc","namespace":"default"},"spec":{"accessModes":["ReadWriteOnce"],"resources":{"requests":{"storage":"1Gi"}},"storageClassName":"longhorn"}}
+    pv.kubernetes.io/bind-completed: "yes"
+    pv.kubernetes.io/bound-by-controller: "yes"
+    volume.beta.kubernetes.io/storage-provisioner: driver.longhorn.io
+  creationTimestamp: "2019-12-21T01:36:16Z"
+  finalizers:
+  - kubernetes.io/pvc-protection
+  name: longhorn-simple-pvc
+  namespace: default
+  resourceVersion: "162431"
+  selfLink: /api/v1/namespaces/default/persistentvolumeclaims/longhorn-simple-pvc
+  uid: 0467ae73-22a5-4eba-803e-464cc0b9d975
+spec:
+  accessModes:
+  - ReadWriteOnce
+  resources:
+    requests:
+      storage: 1Gi
+  storageClassName: longhorn
+  volumeMode: Filesystem
+  volumeName: pvc-0467ae73-22a5-4eba-803e-464cc0b9d975
+status:
+  accessModes:
+  - ReadWriteOnce
+  capacity:
+    storage: 1Gi
+  phase: Bound
+```
+Modify `spec.resources.requests.storage` of this PVC.
+
+
+#### Via Longhorn UI
+- If your Kubernetes version is v1.14 or v1.15, this method is the only choice for Longhorn volume expansion. 
+- Notice that The volume size will be updated after the expansion but the capacity of corresponding PVC and PV won't change. Users need to take care of them.
+- Usage: On the volume page of Longhorn UI, click `Expand` for the volume.
+
+
+## Frontend expansion
+- Longhorn will expand a Longhorn volume's frontend even if the volume is in the maintenance mode.
+- For the OFFLINE expansion, Longhorn will automatically attach the `detached` volume to a random node then do expansion.
+ 
+  For the ONLINE expansion, users can read/write the volume while expansion. 
+  
+- Rebuilding/adding replicas is not allowed during the expansion and vice versa. 
+ 
+
+## Filesystem expansion
+Longhorn will try to expand the file system only if:
+1. The expanded size should be greater than the current size.
+2. There is a Linux filesystem in the Longhorn volume. 
+3. The filesystem used in the Longhorn volume is one of the followings:
+  3.1 ext4
+  3.2 XFS
+4. The Longhorn volume is not in maintanence mode
+5. The Longhorn volume is using block device frontend.