Add clone from VolumeSnapshot docs

Signed-off-by: Alex Kalenyuk <akalenyu@redhat.com>

doc/clone-from-volumesnapshot-source.md

@@ -0,0 +1,82 @@
+# Data Volume cloning from a VolumeSnapshot source
+
+## Introduction
+Current CDI PVC cloning methods offer [smart cloning](./smart-clone.md) which uses a 1:1 cloning method that creates a temporary snapshot of the source PVC for each intended clone.  
+Some storage systems are designed to scale better with a 1:N method where a single snapshot of the source PVC is used to create multiple clones.  
+One examples is ceph, where this would be about utilizing RBD layering.  
+A much simpler example would be storage that can take around 5-10 minutes to snapshot a 200GB volume.  
+
+*Note: Data Volume cloning from snapshot source can work together with namespace transfer and size expansion*  
+
+### Prerequisites
+  1) The source snapshot and target PVC share the same provisioner
+  2) The user creating the DataVolume has permission to create the `datavolumes/source` resource in the source namespace
+  3) Storage supports expansion (if the user attempts clone to larger target)
+
+### Flow Description
+- DataVolume is created with a Snapshot source
+- Check if restore from said snapshot to desired target is possible (as described in Prerequisites):
+- If possible:
+    * Create the restore PVC
+    * Set the claim reference of the PV to point to the new target PVC ([namespace-transfer](./namespace-transfer.md))
+- If not possible:
+    * Attempt [host-assisted cloning](./clone-datavolume.md) between 2 PVCs where CDI creates a temporary restore PVC (which will be cleaned up) from the snapshot to act as the source.
+
+## Example
+To kick off the process, we need a source volume snapshot.  
+We create a simple Data Volume that will act as a source for the snapshot.
+```yaml
+apiVersion: cdi.kubevirt.io/v1beta1
+kind: DataVolume
+metadata:
+  name: golden-snap-source
+  namespace: golden-ns
+spec:
+  source:
+      http:
+         url: http://mirrors.nav.ro/fedora/linux/releases/33/Cloud/x86_64/images/Fedora-Cloud-Base-33-1.2.x86_64.qcow2
+  storage:
+    accessModes:
+    - ReadWriteOnce
+    resources:
+      requests:
+        storage: 8Gi
+```
+Next, we will create a VolumeSnapshot of the resulting PVC
+```yaml
+apiVersion: snapshot.storage.k8s.io/v1
+kind: VolumeSnapshot
+metadata:
+  name: golden-volumesnapshot
+  namespace: golden-ns
+spec:
+  volumeSnapshotClassName: csi-rbdplugin-snapclass
+  source:
+    persistentVolumeClaimName: golden-snap-source
+```
+Double check snapshot is ready to use (not a must, but nice to know)
+```bash
+$ kubectl get volumesnapshot golden-volumesnapshot 
+NAME                    READYTOUSE   SOURCEPVC            SOURCESNAPSHOTCONTENT   RESTORESIZE   SNAPSHOTCLASS                            SNAPSHOTCONTENT                                    CREATIONTIME   AGE
+golden-volumesnapshot   true ...
+```
+Now we will go ahead and create the target DataVolume that clones the above VolumeSnapshot
+```yaml
+apiVersion: cdi.kubevirt.io/v1beta1
+kind: DataVolume
+metadata:
+  name: cloned-datavolume
+  namespace: default
+spec:
+  source:
+    snapshot:
+      namespace: golden-ns
+      name: golden-volumesnapshot
+  storage:
+    storageClassName: rook-ceph-block
+    accessModes:
+      - ReadWriteOnce
+    resources:
+      requests:
+        storage: 9Gi
+```

doc/datavolumes.md

@@ -30,6 +30,7 @@ The following statuses are possible.
 * Import/Clone/UploadInProgress: The operation (import/clone/upload) is in progress.
 * SnapshotForSmartClone/SmartClonePVCInProgress: The Smart-Cloning operation is in progress.
 * CSICloneInProgress: The CSI Volume Clone operation is in progress
+* CloneFromSnapshotSourceInProgress: Clone from VolumeSnapshot source is in progress
 * Paused: A [multi-stage](#multi-stage-import) import is waiting to transfer a new checkpoint.
 * Succeeded: The operation has succeeded.
 * Failed: The operation has failed.

doc/efficient-cloning.md

@@ -25,12 +25,13 @@ _The required prerequisites in order to trigger efficient cloning methods_
     4) The user creating the DataVolume has permission to create the `datavolumes/source` resource in the source namespace
     5) The source volume is not in use
 
-### Note: Data Volume Cloning can work together with namespace transfer and size expansion  
+*Note: Data Volume Cloning can work together with namespace transfer and size expansion*  
 
 ### Additional Documentation
 * DataVolumes: [datavolumes](./datavolumes.md)
 * DataVolume Cloning: [clone-datavolumes](./clone-datavolume.md)
 * CSI Volume Cloning: [csi-cloning](./csi-cloning.md)
+* Cloning from VolumeSnapshot source: [clone-from-volumesnapshot-source](./clone-from-volumesnapshot-source.md)
 * Smart Cloning: [smart-clone](./smart-clone.md)
 * Namespace Transfer: [namespace-transfer](./namespace-transfer.md)
 * StorageProfile (description of cloneStrategy): [storageprofile](./storageprofile.md)