Merge pull request #7 from arangodb/storage-docs

Extended storage docs wrt local storage

docs/user/storage.md

@@ -16,3 +16,60 @@ The amount of storage needed is configured using the
 Note that configuring storage is done per group of servers.
 It is not possible to configure storage per individual
 server.
+
+## Local storage
+
+For optimal performance, ArangoDB should be configured with locally attached
+SSD storage.
+
+To accomplish this, one must create `PersistentVolumes` for all servers that
+need persistent storage (single, agents & dbservers).
+E.g. for a `cluster` with 3 agents and 5 dbservers, you must create 8 volumes.
+
+Note that each volume must have a capacity that is equal to or higher than the
+capacity needed for each server.
+
+To select the correct node, add a required node-affinity annotation as shown
+in the example below.
+
+```yaml
+apiVersion: v1
+kind: PersistentVolume
+metadata:
+  name: volume-agent-1
+  annotations:
+        "volume.alpha.kubernetes.io/node-affinity": '{
+            "requiredDuringSchedulingIgnoredDuringExecution": {
+                "nodeSelectorTerms": [
+                    { "matchExpressions": [
+                        { "key": "kubernetes.io/hostname",
+                          "operator": "In",
+                          "values": ["node-1"]
+                        }
+                    ]}
+                 ]}
+              }'
+spec:
+    capacity:
+      storage: 100Gi
+    accessModes:
+    - ReadWriteOnce
+    persistentVolumeReclaimPolicy: Delete
+    storageClassName: local-ssd
+    local:
+      path: /mnt/disks/ssd1
+```
+
+For Kubernetes 1.9 and up, you should create a `StorageClass` which is configured
+to bind volumes on their first use as shown in the example below.
+This ensures that the Kubernetes scheduler takes all constraints on a `Pod`
+that into consideration before binding the volume to a claim.
+
+```yaml
+kind: StorageClass
+apiVersion: storage.k8s.io/v1
+metadata:
+  name: local-ssd
+provisioner: kubernetes.io/no-provisioner
+volumeBindingMode: WaitForFirstConsumer
+```