doc: added docs for snapshot-backed CephFS volumes

Signed-off-by: Robert Vasek <robert.vasek@cern.ch>

docs/cephfs-snapshot-backed-volumes.md

@@ -0,0 +1,61 @@
+# Provisioning and mounting CephFS snapshot-backed volumes
+
+Snapshot-backed volumes allow CephFS subvolume snapshots to be exposed as
+regular read-only PVCs. No data cloning is performed and provisioning such
+volumes is done in constant time.
+
+For more details please refer to [Snapshots as shallow read-only volumes](./design/proposals/cephfs-snapshot-shallow-ro-vol.md)
+design document.
+
+## Prerequisites
+
+Prerequisites for this feature are the same as for creating PVCs with snapshot
+volume source. See [Create snapshot and Clone Volume](./snap-clone.md) for more
+information.
+
+## Usage
+
+### Provisioning a snapshot-backed volume from a volume snapshot
+
+For provisioning new snapshot-backed volumes, following configuration must be
+set for storage class(es) and their PVCs respectively:
+
+* StorageClass:
+  * Specify `backingSnapshot: "true"` parameter.
+* PersistentVolumeClaim:
+  * Set `storageClassName` to point to your storage class with backing
+    snapshots enabled.
+  * Define `spec.dataSource` for your desired source volume snapshot.
+  * Set `spec.accessModes` to `ReadOnlyMany`. This is the only access mode that
+    is supported by this feature.
+
+### Mounting snapshots from pre-provisioned volumes
+
+Steps for defining a PersistentVolume and PersistentVolumeClaim for
+pre-provisioned CephFS subvolumes are identical to those described in
+[Static PVC with ceph-csi](./static-pvc.md), except one additional parameter
+must be specified: `backingSnapshotID`. CephFS-CSI driver will retrieve the
+snapshot identified by the given ID from within the specified subvolume, and
+expose it to workloads in read-only mode. Volume access mode must be set to
+`ReadOnlyMany`.
+
+Note that the snapshot retrieval is done by traversing `<rootPath>/.snap` and
+searching for a directory that contains `backingSnapshotID` value in its name.
+The specified snapshot ID does not necessarily need to be the complete directory
+name inside `<rootPath>/.snap`, however it must be complete enough to uniquely
+identify that directory.
+
+Example:
+
+```
+$ ls .snap
+_f279df14-6729-4342-b82f-166f45204233_1099511628283
+_a364870e-6729-4342-b82f-166f45204233_1099635085072
+```
+
+`f279df14-6729-4342-b82f-166f45204233` would be considered a valid value for
+`backingSnapshotID` volume parameter, whereas `6729-4342-b82f-166f45204233`
+would not, as it would be ambiguous.
+
+If the given snapshot ID is ambiguous, or no such snapshot is found, mounting
+the PVC will fail with INVALID_ARGUMENT error code.

docs/deploy-cephfs.md

@@ -81,6 +81,7 @@ you're running it inside a k8s cluster and find the config itself).
 | `pool`                                                                                              | no             | Ceph pool into which volume data shall be stored                                                                                                                                                                        |
 | `volumeNamePrefix`                                                                                  | no             | Prefix to use for naming subvolumes (defaults to `csi-vol-`).                                                                                                                                                           |
 | `snapshotNamePrefix`                                                                                | no             | Prefix to use for naming snapshots (defaults to `csi-snap-`)                                                                                                                                                            |
+| `backingSnapshot`                                                                                   | no             | Boolean value. The PVC shall be backed by the CephFS snapshot specified in its data source. `pool` parameter must not be specified. (defaults to `false`)                                                               |
 | `kernelMountOptions`                                                                                | no             | Comma separated string of mount options accepted by cephfs kernel mounter, by default no options are passed. Check man mount.ceph for options.                                                                          |
 | `fuseMountOptions`                                                                                  | no             | Comma separated string of mount options accepted by ceph-fuse mounter, by default no options are passed.                                                                                                                |
 | `csi.storage.k8s.io/provisioner-secret-name`, `csi.storage.k8s.io/node-stage-secret-name`           | for Kubernetes | Name of the Kubernetes Secret object containing Ceph client credentials. Both parameters should have the same value                                                                                                     |

examples/cephfs/storageclass.yaml

@@ -47,6 +47,11 @@ parameters:
   # If omitted, defaults to "csi-vol-".
   # volumeNamePrefix: "foo-bar-"
 
+  # (optional) Boolean value. The PVC shall be backed by the CephFS snapshot
+  # specified in its data source. `pool` parameter must not be specified.
+  # (defaults to `false`)
+  # backingSnapshot: "true"
+
 reclaimPolicy: Delete
 allowVolumeExpansion: true
 mountOptions: