Add the PV backup information design document.

Signed-off-by: Xun Jiang <[email protected]>

changelogs/unreleased/6962-blackpiglet

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+Add the PV backup information design document.

design/pv_backup_info.md

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+# PersistentVolume backup information design
+
+## Abstract
+Create a new metadata file in the backup repository's backup name sub-directory to store the backup-including PVC and PV information. The information includes the way of backing up the PVC and PV data, snapshot information, and status. The needed snapshot status can also be recorded there, but the Velero-Native snapshot plugin doesn't provide a way to get the snapshot size from the API, so it's possible that not all snapshot size information is available.
+
+This new additional metadata file is needed when:
+* Get a summary of the backup's PVC and PV information, including how the data in them is backed up, or whether the data in them is skipped from backup.
+* Find out how the PVC and PV should be restored in the restore process.
+* Retrieve the PV's snapshot information for backup.
+
+## Background
+There is already a [PR](https://github.com/vmware-tanzu/velero/pull/6496) to track the skipped PVC in the backup. This design will depend on it and go further to get a summary of PVC and PV information, then persist into a metadata file in the backup repository.
+
+In the restore process, the Velero server needs to decide how the PV resource should be restored according to how the PV is backed up. The current logic is to check whether it's backed up by Velero-native snapshot, by file-system backup, or having `DeletionPolicy` set as `Delete`.
+
+The checks are made by the backup-generated PVBs or Snapshots. There is no generic way to find this information, and the CSI backup and Snapshot data movement backup are not covered.
+
+Another thing that needs noticing is when describing the backup, there is no generic way to find the PV's snapshot information.
+
+## Goals
+- Create a new metadata file to store backup's PVCs and PVs information and volume data backing up method. The file can be used to let upstream consumers generate a summary.
+- Create a generic way to let the Velero server know how the PV resources are backed up.
+- Create a generic way to let the Velero server find the PV corresponding snapshot information.
+
+## Non Goals
+- Unify how to get snapshot size information for all PV backing-up methods, and all other currently not ready PVs' information.
+
+## High-Level Design
+Create _backup-name_-volumes-info.json metadata file in the backup's repository. This file will be encoded to contain all the PVC and PV information included in the backup. The information covers whether the PV or PVC's data is skipped during backup, how its data is backed up, and the backed-up detail information.
+
+The `restoreItem` function can decode the _backup-name_-volumes-info.json file to determine how to handle the PV resource. 
+
+## Detailed Design
+
+### The VolumeInfo structure
+_backup-name_-volumes-info.json file is an array of structure `VolumeInfo`.
+
+The `VolumeInfo` definition is:
+``` golang
+type VolumeInfo struct {
+    PVCName        string    // The PVC's name. The format should be <namespace-name>/<PVC-name>
+    PVName         string    // The PV name.
+    BackupMethod   string    // The way the volume data is backed up. The valid value includes `VeleroNativeSnapshot`, `PodVolumeBackup`, `CSISnapshot`, `SnapshotDataMover`, and `Skipped`.
+
+    SkippedReason  string    // The reason for the volume is skipped in the backup.
+
+    SnapshotHandle      string       // The actual snapshot ID. It can be the cloud provider's snapshot ID or the file-system uploader's snapshot.
+    Status              string       // The snapshot's final status.
+    Size                int64        // The snapshot corresponding volume size. Some of the volume backup methods cannot retrieve the data by current design, for example, the Velero native snapshot.
+    StartTimestamp      *metav1.Time // Snapshot starts timestamp.
+    CompletionTimestamp *metav1.Time // Snapshot completes timestamp.
+
+    DataMoverInfo       DataMoverInfo
+    NativeSnapshotInfo  VeleroNativeSnapshotInfo
+    PVBInfo             PodVolumeBackupInfo
+}
+
+// DataMoverInfo is used for displaying the snapshot data mover and PodVolumeBackup snapshot status.
+type DataMoverInfo struct {
+    DataMover        string    // The name of the data mover that uploads the snapshot data. The valid values are `kopia` and `restic`. It's useful for file-system backup and snapshot data mover.
+    RetainedSnapshot string    // The ID of the local snapshot.
+}
+
+// VeleroNativeSnapshotInfo is used for displaying the Velero native snapshot status.
+type VeleroNativeSnapshotInfo struct {
+    VolumeType string    // The cloud provider snapshot volume type.
+    VolumeAZ   string    // The cloud provider snapshot volume's availability zones.
+    IOPS       string    // The cloud provider snapshot volume's IOPS.
+}
+
+// PodVolumeBackupInfo is used for displaying the PodVolumeBackup snapshot status.
+type PodVolumeBackupInfo struct {
+    VolumeName string // The PVC's corresponding volume name used by Pod
+    PodName    string // The Pod name mounting this PVC. The format should be <namespace-name>/<pod-name>.
+}
+```
+
+### How the VolumeInfo array is generated.
+
+Two new methods are added to BackupStore for uploading and downloading the VolumeInfo metadata file.
+
+``` golang
+type BackupStore interface {
+    ...
+    PutVolumeInfos(backup string, volumeInfos io.Reader) error
+    GetVolumeInfos(name string) ([]*VolumeInfo, error)
+    ...
+}
+```
+
+There are two phases to handle the VolumeInfo array.
+
+First, the VolumeInfo array is generated before the backup is persisted by this function `persistBackup`. Then the VolumeInfo array will be passed to `persistBackup` function as a parameter. At this phase, the basic information, the skipped information, the Velero native snapshot information, and the PodVolumeBackup information are ready.
+
+Second, in the backup finalize controller, the snapshot data mover and CSI snapshot information will be updated according to the BackupItemOperations result. The VolumeInfo will be updated by the method `PutVolumeInfos`.
+
+### How the VolumeInfo array is used.
+
+#### Generate the PVC backed-up information summary
+The upstream tools can use this VolumeInfo array to format and display their volume information. This is in the scope of this feature.
+
+#### Retrieve volume backed-up information for `velero backup describe` command
+The `velero backup describe` can also use this VolumeInfo array structure to display the volume information. The snapshot data mover volume should use this structure at first, then the Velero native snapshot, CSI snapshot, and PodVolumeBackup can also use this structure. The detailed implementation is also not in this feature's scope.
+
+#### Let restore know how to restore the PV
+In the function `restoreItem`, it will determine whether to restore the PV resource by checking it in the Velero native snapshots list, PodVolumeBackup list, and its DeletionPolicy. This logic is still kept. The logic will be used when the new `VolumeInfo` metadata cannot be found to support backward compatibility.
+
+``` golang
+	if groupResource == kuberesource.PersistentVolumes {
+		switch {
+		case hasSnapshot(name, ctx.volumeSnapshots):
+            ...
+        case hasPodVolumeBackup(obj, ctx):
+            ...
+        case hasDeleteReclaimPolicy(obj.Object):
+            ...
+        default:
+            ...
+```
+
+After introducing the VolumeInfo array, the following logic will be added.
+``` golang
+	if groupResource == kuberesource.PersistentVolumes {
+        volumeInfo := GetVolumeInfo(pvName)
+		switch volumeInfo.BackupMethod {
+		case VeleroNativeSnapshot:
+            ...
+        case PodVolumeBackup:
+            ...
+        case CSISnapshot:
+            ...
+        case SnapshotDataMover:
+            ...
+        case Skipped:
+            // Check whether the Velero server should restore the PV depending on the DeletionPolicy setting.
+        default:
+            // This is not expected, print error.
+```
+
+## Alternatives Considered
+The restore process needs more information about how the PVs are backed up to determine whether this PV should be restored. The released branches also need a similar function, but backporting a new feature into previous releases may not be a good idea, so according to [Anshul Ahuja's suggestion](https://github.com/vmware-tanzu/velero/issues/6595#issuecomment-1731081580), adding more cases here to support checking PV backed-up by CSI plugin and CSI snapshot data mover: https://github.com/vmware-tanzu/velero/blob/5ff5073cc3f364bafcfbd26755e2a92af68ba180/pkg/restore/restore.go#L1206-L1324.
+
+## Security Considerations
+There should be no security impact introduced by this design.
+
+## Compatibility
+After this design is implemented, there should be no impact on the existing [skipped PVC summary feature](https://github.com/vmware-tanzu/velero/pull/6496).
+
+To support older version backup, which doesn't have the VolumeInfo metadata file, the old logic, which is checking the Velero native snapshots list, PodVolumeBackup list, and PVC DeletionPolicy, is still kept, and supporting CSI snapshots and snapshot data mover logic will be added too.
+
+## Implementation
+This will be implemented in the Velero v1.13 development cycle.
+
+## Open Issues
+There are no open issues identified by now.