Address review comments

Author: xing-yang

content/en/blog/_posts/2023-05-08-volume-group-snapshot-alpha.md

@@ -1,6 +1,6 @@
 ---
 layout: blog
-title: "Introducing Volume Group Snapshot"
+title: "Kubernetes 1.27: Introducing An API For Volume Group Snapshots"
 date: 2023-05-08T10:00:00-08:00
 slug: kubernetes-1-27-volume-group-snapshot-alpha
 ---
@@ -11,17 +11,17 @@ Volume group snapshot is introduced as an Alpha feature in Kubernetes v1.27.
 This feature introduces a Kubernetes API that allows users to take a crash consistent
 snapshot for multiple volumes together. It uses a label selector to group multiple
 PersistentVolumeClaims for snapshotting.
-This new feature is only supported for CSI volume drivers.
+This new feature is only supported for [CSI](https://kubernetes-csi.github.io/docs/) volume drivers.
 
-## What is Volume Group Snapshot
+## An overview of volume group snapshots
 
 Some storage systems provide the ability to create a crash consistent snapshot of
 multiple volumes. A group snapshot represents “copies” from multiple volumes that
 are taken at the same point-in-time. A group snapshot can be used either to rehydrate
 new volumes (pre-populated with the snapshot data) or to restore existing volumes to
 a previous state (represented by the snapshots).
 
-## Why add Volume Group Snapshots to Kubernetes?
+## Why add volume group snapshots to Kubernetes?
 
 The Kubernetes volume plugin system already provides a powerful abstraction that
 automates the provisioning, attaching, mounting, resizing, and snapshotting of block
@@ -30,9 +30,9 @@ and file storage.
 Underpinning all these features is the Kubernetes goal of workload portability:
 Kubernetes aims to create an abstraction layer between distributed applications and
 underlying clusters so that applications can be agnostic to the specifics of the
-cluster they run on and application deployment requires no “cluster specific” knowledge.
+cluster they run on and application deployment requires no cluster specific knowledge.
 
-There is already a [VolumeSnapshot API](https://github.com/kubernetes/enhancements/tree/master/keps/sig-storage/177-volume-snapshot)
+There is already a [VolumeSnapshot](/docs/concepts/storage/volume-snapshots/) API
 that provides the ability to take a snapshot of a persistent volume to protect against
 data loss or data corruption. However, there are other snapshotting functionalities
 not covered by the VolumeSnapshot API.
@@ -45,25 +45,26 @@ If snapshots for the data volume and the logs volume are taken at different time
 the application will not be consistent and will not function properly if it is restored
 from those snapshots when a disaster strikes.
 
-It is true that we can quiesce the application first, take an individual snapshot from
+It is true that you can quiesce the application first, take an individual snapshot from
 each volume that is part of the application one after the other, and then unquiesce the
-application after all the individual snapshots are taken. This way we will get application
-consistent snapshots.
-However, application quiesce is time consuming. Sometimes it may not be possible to
-quiesce an application. Taking individual snapshots one after another may also take
-longer time compared to taking a consistent group snapshot. Some users may not want
-to do application quiesce very frequently for these reasons. For example, a user may
-want to run weekly backups with application quiesce and nightly backups without
-application quiesce but with consistent group support which provides crash consistency
-across all volumes in the group.
+application after all the individual snapshots are taken. This way, you would get
+application consistent snapshots.
+
+However, sometimes it may not be possible to quiesce an application or the application
+quiesce can be too expensive so you want to do it less frequently. Taking individual
+snapshots one after another may also take longer time compared to taking a consistent
+group snapshot. Some users may not want to do application quiesce very often for these
+reasons. For example, a user may want to run weekly backups with application quiesce
+and nightly backups without application quiesce but with consistent group support which
+provides crash consistency across all volumes in the group.
 
 ## Kubernetes Volume Group Snapshots API
 
 Kubernetes Volume Group Snapshots introduce [three new API objects](https://github.com/kubernetes-csi/external-snapshotter/blob/master/client/apis/volumegroupsnapshot/v1alpha1/types.go) for managing snapshots:
 
 `VolumeGroupSnapshot`
 : Created by a Kubernetes user (or perhaps by your own automation) to request
-creation of a volume group snapshot for multiple volumes.
+creation of a volume group snapshot for multiple persistent volume claims.
 It contains information about the volume group snapshot operation such as the
 timestamp when the volume group snapshot was taken and whether it is ready to use.
 The creation and deletion of this object represents a desire to create or delete a
@@ -81,41 +82,49 @@ was created with a one-to-one mapping.
 : Created by cluster administrators to describe how volume group snapshots should be
 created. including the driver information, the deletion policy, etc.
 
-The Volume Group Snapshot objects are defined as CustomResourceDefinitions (CRDs).
+These three API kinds are defined as CustomResourceDefinitions (CRDs).
 These CRDs must be installed in a Kubernetes cluster for a CSI Driver to support
 volume group snapshots.
 
 ## How do I use Kubernetes Volume Group Snapshots
 
-Volume Group Snapshot feature is implemented in the
+Volume group snapshots are implemented in the
 [external-snapshotter](https://github.com/kubernetes-csi/external-snapshotter) repository. Implementing volume
 group snapshots meant adding or changing several components:
 
-* Kubernetes Volume Group Snapshot CRDs
+* Added new CustomResourceDefinitions for VolumeGroupSnapshot and two supporting APIs.
 * Volume group snapshot controller logic is added to the common snapshot controller.
 * Volume group snapshot validation webhook logic is added to the common snapshot validation webhook.
-* Logic to make CSI calls is added to CSI Snapshotter sidecar controller.
+* Adding logic to make CSI calls into the snapshotter sidecar controller.
 
 The volume snapshot controller, CRDs, and validation webhook are deployed once per
 cluster, while the sidecar is bundled with each CSI driver.
 
 Therefore, it makes sense to deploy the volume snapshot controller, CRDs, and validation
-webhook as a cluster addon. It is strongly recommended that Kubernetes distributors
+webhook as a cluster addon. I strongly recommend that Kubernetes distributors
 bundle and deploy the volume snapshot controller, CRDs, and validation webhook as part
 of their Kubernetes cluster management process (independent of any CSI Driver).
 
 ### Creating a new group snapshot with Kubernetes
 
 Once a VolumeGroupSnapshotClass object is defined and you have volumes you want to
-snapshot together, you may create a new group snapshot by creating a VolumeGroupSnapshot
+snapshot together, you may request a new group snapshot by creating a VolumeGroupSnapshot
 object.
 
 The source of the group snapshot specifies whether the underlying group snapshot
 should be dynamically created or if a pre-existing VolumeGroupSnapshotContent
-should be used. One of the following members in the source must be set.
+should be used.
+
+A pre-existing VolumeGroupSnapshotContent is created by a cluster administrator.
+It contains the details of the real volume group snapshot on the storage system which
+is available for use by cluster users.
+
+One of the following members in the source of the group snapshot must be set.
 
-* Selector - Selector is a label query over persistent volume claims that are to be grouped together for snapshotting. This labelSelector will be used to match the label added to a PVC.
-* VolumeGroupSnapshotContentName - specifies the name of a pre-existing VolumeGroupSnapshotContent object representing an existing volume group snapshot.
+* `selector` - a label query over PersistentVolumeClaims that are to be grouped
+together for snapshotting. This labelSelector will be used to match the label
+added to a PVC.
+* `volumeGroupSnapshotContentName` - specifies the name of a pre-existing VolumeGroupSnapshotContent object representing an existing volume group snapshot.
 
 For dynamic provisioning, a selector must be set so that the snapshot controller can
 find PVCs with the matching labels to be snapshotted together.
@@ -187,7 +196,7 @@ snapshots that are part of a group snapshot.
 
 ## As a storage vendor, how do I add support for group snapshots to my CSI driver?
 
-To implement the volume group snapshot feature, a CSI driver MUST:
+To implement the volume group snapshot feature, a CSI driver **must**:
 
 * Implement a new group controller service.
 * Implement group controller RPCs: `CreateVolumeGroupSnapshot`, `DeleteVolumeGroupSnapshot`, and `GetVolumeGroupSnapshot`.
@@ -216,7 +225,7 @@ The alpha implementation of volume group snapshots for Kubernetes has the follow
 limitations:
 
 * Does not support reverting an existing PVC to an earlier state represented by a snapshot that is part of a group snapshot (only supports provisioning a new volume from a snapshot).
-* No application consistency guarantees beyond any guarantees provided by the storage system (e.g. crash consistency).
+* No application consistency guarantees beyond any guarantees provided by the storage system (e.g. crash consistency). See this [doc](https://github.com/kubernetes/community/blob/master/wg-data-protection/data-protection-workflows-white-paper.md#quiesce-and-unquiesce-hooks) for more discussions on application consistency.
 
 ## What’s next?