Add proposal for mount options

contributors/design-proposals/mount-options.md

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+# Mount options for mountable volume types
+
+## Goal
+
+Enable Kubernetes admins to specify mount options with mountable volumes
+such as  - `nfs`, `glusterfs` or `aws-ebs` etc.
+
+## Motivation
+
+We currently support network filesystems: NFS, Glusterfs, Ceph FS, SMB (Azure file), Quobytes, and local filesystems such as ext[3|4] and XFS.
+
+Mount time options that are operationally important and have no security implications should be suppported. Examples are NFS's TCP mode, versions, lock mode, caching mode; Glusterfs's caching mode; SMB's version, locking, id mapping; and more.
+
+## Design
+
+We are going to add support for mount options in PVs as a beta feature to begin with.
+
+Mount options can be specified as `mountOptions` annotations in PV. For example:
+
+``` yaml
+ apiVersion: v1
+  kind: PersistentVolume
+  metadata:
+    name: pv0003
+  annotations:
+    volume.beta.kubernetes.io/mountOptions: "hard,nolock,nfsvers=3"
+  spec:
+    capacity:
+      storage: 5Gi
+    accessModes:
+      - ReadWriteOnce
+    persistentVolumeReclaimPolicy: Recycle
+    nfs:
+      path: /tmp
+      server: 172.17.0.2
+```
+
+
+## Preventing users from specifying mount options in inline volume specs of Pod
+
+While mount options enable more flexibility in how volumes are mounted, it can result
+in user specifying options that are not supported or are known to be problematic when
+using inline volume specs.
+
+After much deliberation it was decided that - `mountOptions` as an API parameter will not be supported
+for inline volume specs.
+
+### Error handling and plugins that don't support mount option
+
+Kubernetes ships with volume plugins that don't support any kind of mount options. Such as `configmaps` or `secrets`,
+in those cases to prevent user from submitting volume definitions with bogus mount options - plugins can define a interface function
+such as:
+
+```go
+func SupportsMountOption() {
+   return false
+}
+```
+
+which will be used to validate the PV definition and API object will be *only* created if it passes the validation. Additionally
+support for user specified mount options will be also checked when volumes are being mounted.
+
+In other cases where plugin supports mount options (such as - `NFS` or `GlusterFS`) but mounting fails because of invalid mount
+option or otherwise - an Event API object will be created and attached to the appropriate object.