Add documentation comments for volume expand controller

pkg/controller/volume/expand/cache/volume_resize_map.go

@@ -88,6 +88,11 @@ func NewVolumeResizeMap(kubeClient clientset.Interface) VolumeResizeMap {
 }
 
 // AddPVCUpdate adds pvc for resizing
+// This function intentionally allows addition of PVCs for which pv.Spec.Size >= pvc.Spec.Size,
+// the reason being - lack of transaction in k8s means after successful resize, we can't guarantee that when we update PV,
+// pvc update will be successful too and after resize we alyways update PV first.
+// If for some reason we weren't able to update PVC after successful resize, then we are going to reprocess
+// the PVC and hopefully after a no-op resize in volume plugin, PVC will be updated with right values as well.
 func (resizeMap *volumeResizeMap) AddPVCUpdate(pvc *v1.PersistentVolumeClaim, pv *v1.PersistentVolume) {
 	if pv.Spec.ClaimRef == nil || pvc.Namespace != pv.Spec.ClaimRef.Namespace || pvc.Name != pv.Spec.ClaimRef.Name {
 		glog.V(4).Infof("Persistent Volume is not bound to PVC being updated : %s", util.ClaimToClaimKey(pvc))

pkg/controller/volume/expand/pvc_populator.go

@@ -80,6 +80,11 @@ func (populator *pvcPopulator) Sync() {
 			glog.V(5).Infof("Error getting persistent volume for pvc %q : %v", pvc.UID, err)
 			continue
 		}
+		// We are only going to add PVCs which are:
+		//  - bound
+		//  - pvc.Spec.Size > pvc.Status.Size
+		// These 2 checks are already performed in AddPVCUpdate function before adding pvc for resize
+		// and hence we do not repeat those checks here.
 		populator.resizeMap.AddPVCUpdate(pvc, pv)
 	}
 }

pkg/volume/util/operationexecutor/operation_generator.go

@@ -756,6 +756,9 @@ func (og *operationGenerator) GenerateExpandVolumeFunc(
 				return expandErr
 			}
 			newSize = updatedSize
+			// k8s doesn't have transactions, we can't guarantee that after updating PV - updating PVC will be
+			// successful, that is why all PVCs for which pvc.Spec.Size > pvc.Status.Size must be reprocessed
+			// until they reflect user requested size in pvc.Status.Size
 			updateErr := resizeMap.UpdatePVSize(pvcWithResizeRequest, newSize)
 
 			if updateErr != nil {
@@ -766,6 +769,8 @@ func (og *operationGenerator) GenerateExpandVolumeFunc(
 		}
 
 		// No Cloudprovider resize needed, lets mark resizing as done
+		// Rest of the volume expand controller code will assume PVC as *not* resized until pvc.Status.Size
+		// reflects user requested size.
 		if !volumePlugin.RequiresFSResize() {
 			glog.V(4).Infof("Controller resizing done for PVC %s", pvcWithResizeRequest.QualifiedName())
 			err := resizeMap.MarkAsResized(pvcWithResizeRequest, newSize)