Static volume snapshot binding failure

[liyinan926]

I was playing with static volume snapshot binding by first creating a VolumeSnapshotContent object and then a VolumeSnapshot object that is supposed to be bound to the VolumeSnapshotContent object by the external-snapshotter. However, I'm getting the following error:

Failed to check and update snapshot: failed to get input parameters to create snapshot <volume-snapshot-name>: "the snapshot source is not specified."

The VolumeSnapshot was created with the source field unspecified because the intention is to statically bind it to the VolumeSnapshotContent object. My understanding is that the source field is optional in this scenario, according to the comment of the field copied and pasted below.

// If not specified, user can create VolumeSnapshotContent and bind it with VolumeSnapshot manually.

The VolumeSnapshot object looks like the following:

apiVersion: snapshot.storage.k8s.io/v1alpha1
kind: VolumeSnapshot
metadata:
  name: mongo-persistent-storage-mongo-0-vs
  namespace: default
spec:
  snapshotClassName: default-snapshot-class
  snapshotContentName: snapcontent-31dffc80-073a-11ea-830c-42010a800239

I'm using the version v1.1.0 of external-snapshotter.

@xing-yang @jingxu97 @yuxiangqian

[liyinan926]

UPDATE: I had to add a dummy source field to make it work. I think the snapshotter should not fail on an empty source field in this case.

[xing-yang]

@liyinan926 Have you checked the example for importing existing snapshot in this blog? https://kubernetes.io/blog/2018/10/09/introducing-volume-snapshot-alpha-for-kubernetes/

It does need a source with a CSI snapshot handle specified for static binding, otherwise there is no existing snapshot to be imported. The new beta API has these fields more clearly defined and hopefully won't be so confusing.

[liyinan926]

@xing-yang I read that blog post. I meant the source field in the VolumeSnapshot, which is supposed to reference a PersistentVolumeClaim that is to be snapshotted. I did set the CSIVolumeSnapshotSource field in the VolumeSnapshotContent.

[xing-yang]

@liyinan926 that seems to be a bug.

[yuxiangqian]

@liyinan926 Correct me if I am wrong please. You were trying to play with static binding feature without specifying a source? "CSIVolumeSnapshotSource" is removed from the beta version.

[liyinan926]

@yuxiangqian I did specify the CSIVolumeSnapshotSource field in the VolumeSnapshotContent object. The issue is that static binding failed because the field source in the VolumeSnapshot object was not specified, which is supposed to reference a PersistentVolumeClaim to be snapshotted. In the case of static binding, however, this source field should not be required as the intention is not to take a snapshot of a PVC. Like what @xing-yang said, this looks like a bug. BTW: I'm using the alpha version v1.1.0. Also I don't remember I ran into this issue with version v0.4.0. This issue started to show up after I switched to version v1.1.0.

[liyinan926]

Update: actually using a dummy source in the VolumeSnapshotSpec that points to a non-existing PVC does not work, as it resulted in the following error:

Failed to check and update snapshot: failed to get input parameters to create snapshot mongo-persistent-storage-mongo-0-vs: "failed to retrieve PVC mongo-persistent-storage-mongo-0 from the API server: "persistentvolumeclaims mongo-persistent-storage-mongo-0 not found"

So it appears that the external-snapshotter is always trying to check and get the source PersistentVolumeClaim regardlessly, and the field source essentially becomes a required field and it must reference an existing PVC.

I did something that made it appear to work. But it actually doesn't work, and it looks that static binding is broken.

[liyinan926]

@xing-yang @yuxiangqian Looks like the issue I ran into was fixed in #98. Looks like the GCE PD CSI driver I use still uses an older version of external-snapshotter that doesn't contain that fix. Will try it with a newer version of the driver and verify.

[liyinan926]

I tried a newer version of the PD CSI driver with csi-snapshotter version v1.1.0 and verified that static binding works as expected with an empty source field in VolumeSnapshotSpec. Thanks for helping!