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content/en/blog/_posts/2023-04-11-volume-group-snapshot-alpha.md
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| --- | ||
| layout: blog | ||
| title: "Introducing Volume Group Snapshot" | ||
| date: 2023-04-11T10:00:00-08:00 | ||
| slug: kubernetes-1-27-volume-group-snapshot-alpha | ||
| --- | ||
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| **Author:** Xing Yang (VMware) | ||
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| 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. | ||
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| ## What is Volume Group Snapshot | ||
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| 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). | ||
|
|
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| ## Why add Volume Group Snapshots to Kubernetes? | ||
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| The Kubernetes volume plugin system already provides a powerful abstraction that | ||
| automates the provisioning, attaching, mounting, resizing, and snapshotting of block | ||
| and file storage. | ||
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| 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. | ||
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| There is already a [VolumeSnapshot API](https://github.com/kubernetes/enhancements/tree/master/keps/sig-storage/177-volume-snapshot) | ||
| 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. | ||
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| Some storage systems support consistent group snapshots that allow a snapshot to be | ||
| taken from multiple volumes at the same point-in-time to achieve write order consistency. | ||
| This can be useful for applications that contain multiple volumes. For example, | ||
| an application may have data stored in one volume and logs stored in another volume. | ||
| If snapshots for the data volume and the logs volume are taken at different times, | ||
| the application will not be consistent and will not function properly if it is restored | ||
| from those snapshots when a disaster strikes. | ||
|
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| It is true that we 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. | ||
|
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| ## Kubernetes Volume Group Snapshots API | ||
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| 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: | ||
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| `VolumeGroupSnapshot` | ||
| : Created by a Kubernetes user (or perhaps by your own automation) to request | ||
| creation of a volume group snapshot for multiple volumes. | ||
| 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 | ||
| cluster resource (a group snapshot). | ||
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| `VolumeGroupSnapshotContent` | ||
| : Created by the snapshot controller for a dynamically created VolumeGroupSnapshot. | ||
| It contains information about the volume group snapshot including the volume group | ||
| snapshot ID. | ||
| This object represents a provisioned resource on the cluster (a group snapshot). | ||
| The VolumeGroupSnapshotContent object binds to the VolumeGroupSnapshot for which it | ||
| was created with a one-to-one mapping. | ||
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| `VolumeGroupSnapshotClass` | ||
| : Created by cluster administrators to describe how volume group snapshots should be | ||
| created. including the driver information, the deletion policy, etc. | ||
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| The Volume Group Snapshot objects are defined as CustomResourceDefinitions (CRDs). | ||
| These CRDs must be installed in a Kubernetes cluster for a CSI Driver to support | ||
| volume group snapshots. | ||
|
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| ## How do I use Kubernetes Volume Group Snapshots | ||
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| Volume Group Snapshot feature is implemented in the | ||
| [external-snapshotter](https://github.com/kubernetes-csi/external-snapshotter) repository. Implementing volume | ||
| group snapshots meant adding or changing several components: | ||
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| * Kubernetes Volume Group Snapshot CRDs | ||
| * 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. | ||
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| The volume snapshot controller, CRDs, and validation webhook are deployed once per | ||
| cluster, while the sidecar is bundled with each CSI driver. | ||
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| 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 | ||
| bundle and deploy the volume snapshot controller, CRDs, and validation webhook as part | ||
| of their Kubernetes cluster management process (independent of any CSI Driver). | ||
|
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| ### Creating a new group snapshot with Kubernetes | ||
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| 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 | ||
| object. | ||
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| 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. | ||
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| * 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. | ||
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| For dynamic provisioning, a selector must be set so that the snapshot controller can | ||
| find PVCs with the matching labels to be snapshotted together. | ||
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| ```yaml | ||
| apiVersion: groupsnapshot.storage.k8s.io/v1alpha1 | ||
| kind: VolumeGroupSnapshot | ||
| metadata: | ||
| name: new-group-snapshot-demo | ||
| namespace: demo-namespace | ||
| spec: | ||
| volumeGroupSnapshotClassName: csi-groupSnapclass | ||
| source: | ||
| selector: | ||
| group: myGroup | ||
| ``` | ||
| In the VolumeGroupSnapshot spec, a user can specify the VolumeGroupSnapshotClass which | ||
| has the information about which CSI driver should be used for creating the group snapshot. | ||
| ### Importing an existing group snapshot with Kubernetes | ||
| You can always import an existing group snapshot to Kubernetes by manually creating | ||
| a VolumeGroupSnapshotContent object to represent the existing group snapshot. | ||
| Because VolumeGroupSnapshotContent is a non-namespace API object, only a system admin | ||
| may have the permission to create it. Once a VolumeGroupSnapshotContent object is | ||
| created, the user can create a VolumeGroupSnapshot object pointing to the | ||
| VolumeGroupSnapshotContent object. | ||
| ```yaml | ||
| apiVersion: groupsnapshot.storage.k8s.io/v1alpha1 | ||
| kind: VolumeGroupSnapshotContent | ||
| metadata: | ||
| name: pre-existing-group-snap-content1 | ||
| spec: | ||
| driver: com.example.csi-driver | ||
| deletionPolicy: Delete | ||
| source: | ||
| volumeGroupSnapshotHandle: group-snap-id | ||
| volumeGroupSnapshotRef: | ||
| kind: VolumeGroupSnapshot | ||
| name: pre-existing-group-snap1 | ||
| namespace: demo-namespace | ||
| ``` | ||
| A VolumeGroupSnapshot object should be created to allow a user to use the group snapshot: | ||
| ```yaml | ||
| apiVersion: groupsnapshot.storage.k8s.io/v1alpha1 | ||
| kind: VolumeGroupSnapshot | ||
| metadata: | ||
| name: pre-existing-group-snap1 | ||
| namespace: demo-namespace | ||
| spec: | ||
| snapshotContentName: pre-existing-group-snap-content1 | ||
| ``` | ||
| Once these objects are created, the snapshot controller will bind them together, | ||
| and set the field `status.ready` to `"True"` to indicate the group snapshot is ready | ||
| to use. | ||
|
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| ### How to use group snapshot for restore in Kubernetes | ||
|
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| At restore time, the user can request a new PersistentVolumeClaim to be created from | ||
| a VolumeSnapshot object that is part of a VolumeGroupSnapshot. This will trigger | ||
| provisioning of a new volume that is pre-populated with data from the specified | ||
| snapshot. The user should repeat this until all volumes are created from all the | ||
| snapshots that are part of a group snapshot. | ||
|
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| ## As a storage vendor, how do I add support for group snapshots to my CSI driver? | ||
|
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| To implement the volume group snapshot feature, a CSI driver MUST: | ||
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| * Implement a new group controller service. | ||
| * Implement group controller RPCs: `CreateVolumeGroupSnapshot`, `DeleteVolumeGroupSnapshot`, and `GetVolumeGroupSnapshot`. | ||
| * Add group controller capability `CREATE_DELETE_GET_VOLUME_GROUP_SNAPSHOT`. | ||
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| See the [CSI spec](https://github.com/container-storage-interface/spec/blob/master/spec.md) | ||
| and the [Kubernetes-CSI Driver Developer Guide](https://kubernetes-csi.github.io/docs/) | ||
| for more details. | ||
|
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| Although Kubernetes poses as little prescriptive on the packaging and deployment of | ||
| a CSI Volume Driver as possible, it provides a suggested mechanism to deploy a | ||
| containerized CSI driver to simplify the process. | ||
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| As part of this recommended deployment process, the Kubernetes team provides a number of | ||
| sidecar (helper) containers, including the | ||
| [external-snapshotter sidecar container](https://kubernetes-csi.github.io/docs/external-snapshotter.html) | ||
| which has been updated to support volume group snapshot. | ||
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| The external-snapshotter watches the Kubernetes API server for the | ||
| `VolumeGroupSnapshotContent` object and triggers `CreateVolumeGroupSnapshot` and | ||
| `DeleteVolumeGroupSnapshot` operations against a CSI endpoint. | ||
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| ## What are the limitations? | ||
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| The alpha implementation of volume group snapshots for Kubernetes has the following | ||
| limitations: | ||
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| * 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). | ||
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| ## What’s next? | ||
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| Depending on feedback and adoption, the Kubernetes team plans to push the CSI | ||
| Group Snapshot implementation to Beta in either 1.28 or 1.29. | ||
| Some of the features we are interested in supporting include volume replication, | ||
| replication group, volume placement, application quiescing, changed block tracking, and more. | ||
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| ## How can I learn more? | ||
|
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| The design spec for the volume group snapshot feature is [here](https://github.com/kubernetes/enhancements/tree/master/keps/sig-storage/3476-volume-group-snapshot). | ||
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| The code repository for volume group snapshot APIs and controller is [here](https://github.com/kubernetes-csi/external-snapshotter). | ||
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| Check out additional documentation on the group snapshot feature [here](https://kubernetes-csi.github.io/docs/). | ||
|
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| ## How do I get involved? | ||
|
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| This project, like all of Kubernetes, is the result of hard work by many contributors | ||
| from diverse backgrounds working together. On behalf of SIG Storage, I would like to | ||
| offer a huge thank you to the contributors who stepped up these last few quarters | ||
| to help the project reach alpha: | ||
|
|
||
| * Alex Meade ([ameade](https://github.com/ameade)) | ||
| * Ben Swartzlander ([bswartz](https://github.com/bswartz)) | ||
| * Humble Devassy Chirammal ([humblec](https://github.com/humblec)) | ||
| * James Defelice ([jdef](https://github.com/jdef)) | ||
| * Jan Šafránek ([jsafrane](https://github.com/jsafrane)) | ||
| * Jing Xu ([jingxu97](https://github.com/jingxu97)) | ||
| * Michelle Au ([msau42](https://github.com/msau42)) | ||
| * Niels de Vos ([nixpanic](https://github.com/nixpanic)) | ||
| * Rakshith R ([Rakshith-R](https://github.com/Rakshith-R)) | ||
| * Raunak Shah ([RaunakShah](https://github.com/RaunakShah)) | ||
| * Saad Ali ([saad-ali](https://github.com/saad-ali)) | ||
| * Thomas Watson ([rbo54](https://github.com/rbo54)) | ||
| * Xing Yang ([xing-yang](https://github.com/xing-yang)) | ||
| * Yati Padia ([yati1998](https://github.com/yati1998)) | ||
|
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| We also want to thank everyone else who has contributed to the project, including others | ||
| who helped review the [KEP](https://github.com/kubernetes/enhancements/pull/1551) | ||
| and the [CSI spec PR](https://github.com/container-storage-interface/spec/pull/519). | ||
|
|
||
| For those interested in getting involved with the design and development of CSI or | ||
| any part of the Kubernetes Storage system, join the | ||
| [Kubernetes Storage Special Interest Group](https://github.com/kubernetes/community/tree/master/sig-storage) (SIG). | ||
| We always welcome new contributors. | ||
|
|
||
| We also hold regular [Data Protection Working Group meetings](https://docs.google.com/document/d/15tLCV3csvjHbKb16DVk-mfUmFry_Rlwo-2uG6KNGsfw/edit#). | ||
| New attendees are welcome to join our discussions. |