Backup skipped PriorityClass even when it is referenced by Pod

[phuongatemc]

Steps to reproduce:

• Create a PriorityClass "high-priority" as below:

apiVersion: scheduling.k8s.io/v1
kind: PriorityClass
metadata:
  name: high-priority
value: 1000000
globalDefault: false
description: "This priority class should be used for XYZ service pods only."

• Create a Pod and specified "priorityClassName" as "high-priority"
• Run Backup with include-cluster-resources set to nil. log shows that the priorityclasses being skipped
  time="2021-07-06T23:01:42Z" level=info msg="Getting items for resource" backup=velero-ppdm/mysql-2021-07-06-16-01-40-mysql group=scheduling.k8s.io/v1 logSource="pkg/backup/item_collector.go:165" resource=priorityclasses
  time="2021-07-06T23:01:42Z" level=info msg="Skipping resource because it's cluster-scoped and only specific namespaces are included in the backup" backup=velero-ppdm/mysql-2021-07-06-16-01-40-mysql group=scheduling.k8s.io/v1 logSource="pkg/backup/item_collector.go:192" resource=priorityclasses
• Restore to a different cluster that does not have the PriorityClass "high-priority" ==> Restore failed to restore Pod with error that it cannot find the PriorityClass.

There was an suggestion in Velero community meeting and in the code comments to write a plugin for Pod and return the PriorityClass high-priority as the AdditionalItems to backup. However, I think such plugin would be a workaround because the main problem is the Backup datapath (getResourceItems in specific) should pick up all resources being referenced by the Pod and backup them accordingly.

[reasonerjt]

Per discussion, this can be fixed by introducing a backupItemAction in core velero.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[phuongatemc]

We actually still need this. Instead of writing new plugin, we will enhance the currently internal plugin the backup the PriorityClass if the Pod Spec has reference to it.

[stale]

Closing the stale issue.

[phuongatemc]

This is discussed in the meeting today. We still need it.

[phuongatemc]

Currently, in the internal plugin for Backup's PodAction, https://github.com/vmware-tanzu/velero/blob/main/pkg/backup/pod_action.go we only add PVCs to the "additionalItems" array. We could add PriorityClass as in that array as well.
Similarly, the internal plugin for restore's PodAction, https://github.com/vmware-tanzu/velero/blob/main/pkg/restore/pod_action.go we could also add PriorityClass as well.

[sseago]

@phuongatemc Yes, that makes sense. While "make it a BackupItemAction" was proposed above, I think using an existing BackupItemAction that's already being run for pods probably makes even more sense, as we don't really need to run a completely separate action to add more than one type of resources as additional items.

[phuongatemc]

PR: #4740
I will do a little more tests.