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Add csi nodeexpandsecret blog article
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Signed-off-by: Humble Chirammal <hchiramm@redhat.com>
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---
layout: blog
title: "Kubernetes 1.25: Use Secrets for Node-Driven Resize of CSI Volumes."
date: 2022-07-26T10:00:00-08:00
slug: kubernetes-1-25-use-secrets-while-expanding-csi-volumes-on-node-alpha
---

**Author:** Humble Chirammal (Red Hat), Louis Koo (deeproute.ai)

Kubernetes v1.25, released earlier this month, introduced a new feature
that lets your cluster expand storage volumes, even when access to those
volumes requires a secret (for example: a credential for accessing a SAN fabric)
to perform node expand operation. This new behavior is in alpha and you
must enable a feature gate (`CSINodeExpandSecret`) to make use of it.
You must also be using [CSI](https://kubernetes-csi.github.io/docs/)
storage; this change isn't relevant to storage drivers that are built in to Kubernetes.

TO turn on this new, alpha feature, you enable the `CSINodeExpandSecret` feature
gate for the kube api server and kubelet, provide a mechanism to receive secretRef
configuration as part of NodeExpansion by the CSI drivers thus make use of
the same to perform node side expansion operation with the underlying
storage system.

## What is this all about?

Before Kubernetes v1.24, you were able to define a cluster-level StorageClass
that made use of [StorageClass Secrets](https://kubernetes-csi.github.io/docs/secrets-and-credentials-storage-class.html),
but you didn't have any mechanism to specify the credentials that would be used for
operations that take place when the storage was mounted onto a node and when
when the volume has to be expanded at node side.
Even-though Kubernetes CSI have implemented similar mechanism for Controller
side operations, ie secretRef field available in the csi PV source and making
use of it while controllerExpand request has been sent to the CSI driver, similar
field was missing in the nodeExpandVolume request. This was causing some trouble
for different CSI drivers to fulfill below scenarios while it perform NodeExpansion
operation.

- At times, the CSI driver needs to check the actual size of the backend block storage (or image)
before proceeding with a node-level filesystem expand operation. This avoids false positive returns
from the backend storage cluster during filesystem expands.

- When a PersistentVolume represents encrypted block storage (for example using LUKS)
you need to provide a passphrase in order to expand the device, and also to make it possible
to grow the filesystem on that device.

- For various validations at time of node expansion the CSI driver has to be connected
to the backend storage cluster, if the secretRef is part of the
nodeExpandVolume request the CSI driver can make use of the same and connect to
the storage cluster to perform the cluster operations.

## How does it work?

To enable this functionality from this version of Kubernetes, Kubernetes Storage SIG have introduced
a new feature gate called `CSINodeExpandSecret`. Once the feature gate is enabled
in the cluster, NodeExpandVolume requests can include a `secretRef` field. The NodeExpandVolume request
is part of CSI; for example, in a request which has been sent from the Kubernetes
control plane to the CSI driver.
The admin can specify these secrets as an opaque parameter in storage class as
he/she specifies other CSI secrets today. The secrets mention in the storage class
has to carry the key name as shown below

```
csi.storage.k8s.io/node-expand-secret-name: test-secret
csi.storage.k8s.io/node-expand-secret-namespace: default`
```

If feature gates are enabled and storage class carries the above secret configuration,
the subject CSI provisioner receives the credentials from the Secret as part of the NodeExpansion request.

CSI volumes that require secrets for online expansion will have NodeExpandSecretRef
field set. If not set, the NodeExpandVolume CSI RPC call will be made without a secret.



## Trying it out


Step 1: enable `CSINodeExpandSecret` feature gate (Please refer to
[Feature Gates](https://kubernetes.io/docs/reference/command-line-tools-reference/feature-gates/))

Step 2: Secret and storage class creation

Ex: Here's an example manifest for a Secret that holds credentials:

```yaml
apiVersion: v1
kind: Secret
metadata:
name: test-secret
namespace: default
data:
username: bXktYXBw
password: Mzk1MjgkdmRnN0pi
```
Here's an example manifest for a StorageClass that refers to those credentials:
```yaml
apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
name: csi-hostpath-sc
parameters:
csi.storage.k8s.io/node-expand-secret-name: test-secret # the name of the Secret
csi.storage.k8s.io/node-expand-secret-namespace: default # the namespace that the Secret is in
provisioner: hostpath.csi.k8s.io
reclaimPolicy: Delete
volumeBindingMode: Immediate
allowVolumeExpansion: true
```
## Example output
If the persistentVolumeClaim(`PVC`) was created successfully, Kubernetes Storage SIGcan see it in the `pv.spec.csi` with command `kubectl get persistentvolume <pv_name> -o yaml`

```yaml
apiVersion: v1
kind: PersistentVolume
metadata:
annotations:
pv.kubernetes.io/provisioned-by: hostpath.csi.k8s.io
creationTimestamp: "2022-07-23T15:14:07Z"
finalizers:
- kubernetes.io/pv-protection
name: pvc-95eb531a-d675-49f6-940b-9bc3fde83eb0
resourceVersion: "420263"
uid: 6fa824d7-8a06-4e0c-b722-d3f897dcbd65
spec:
accessModes:
- ReadWriteOnce
capacity:
storage: 6Gi
claimRef:
apiVersion: v1
kind: PersistentVolumeClaim
name: csi-pvc
namespace: default
resourceVersion: "419862"
uid: 95eb531a-d675-49f6-940b-9bc3fde83eb0
csi:
driver: hostpath.csi.k8s.io
nodeExpandSecretRef:
name: test-secret
namespace: default
volumeAttributes:
storage.kubernetes.io/csiProvisionerIdentity: 1648042783218-8081-hostpath.csi.k8s.io
volumeHandle: e21c7809-aabb-11ec-917a-2e2e254eb4cf
nodeAffinity:
required:
nodeSelectorTerms:
- matchExpressions:
- key: topology.hostpath.csi/node
operator: In
values:
- rook-node01
persistentVolumeReclaimPolicy: Delete
storageClassName: csi-hostpath-sc
volumeMode: Filesystem
status:
phase: Bound
```

If you then trigger online storage expansion, the kubelet passes the appropriate credentials
to the CSI driver, by loading that Secret and passing the data to the storage driver.

Here's an example debug log:

```console
I0330 03:29:51.966241 1 server.go:101] GRPC call: /csi.v1.Node/NodeExpandVolume
I0330 03:29:51.966261 1 server.go:105] GRPC request: {"capacity_range":{"required_bytes":7516192768},"secrets":"***stripped***","staging_target_path":"/var/lib/kubelet/plugins/kubernetes.io/csi/hostpath.csi.k8s.io/f7c62e6e08ce21e9b2a95c841df315ed4c25a15e91d8fcaf20e1c2305e5300ab/globalmount","volume_capability":{"AccessType":{"Mount":{}},"access_mode":{"mode":7}},"volume_id":"e21c7809-aabb-11ec-917a-2e2e254eb4cf","volume_path":"/var/lib/kubelet/pods/bcb1b2c4-5793-425c-acf1-47163a81b4d7/volumes/kubernetes.io~csi/pvc-95eb531a-d675-49f6-940b-9bc3fde83eb0/mount"}
I0330 03:29:51.966360 1 nodeserver.go:459] req:volume_id:"e21c7809-aabb-11ec-917a-2e2e254eb4cf" volume_path:"/var/lib/kubelet/pods/bcb1b2c4-5793-425c-acf1-47163a81b4d7/volumes/kubernetes.io~csi/pvc-95eb531a-d675-49f6-940b-9bc3fde83eb0/mount" capacity_range:<required_bytes:7516192768 > staging_target_path:"/var/lib/kubelet/plugins/kubernetes.io/csi/hostpath.csi.k8s.io/f7c62e6e08ce21e9b2a95c841df315ed4c25a15e91d8fcaf20e1c2305e5300ab/globalmount" volume_capability:<mount:<> access_mode:<mode:SINGLE_NODE_MULTI_WRITER > > secrets:<key:"XXXXXX" value:"XXXXX" > secrets:<key:"XXXXX" value:"XXXXXX" >
```

## The future

As this feature is still in alpha, Kubernetes Storage SIG expect to update or get feedback from CSI driver
authors with more tests and implementation. The community plans to eventually
promote the feature to Beta in upcoming releases.

## Get involved or learn more?

The enhancement proposal includes lots of detail about the history and technical
implementation of this feature.

To know more about Storage Class based dynamic provisioning in Kubernetes please refer:
[Storage Classes](/docs/concepts/storage/storage-classes/) and
[Persistent Volumes](/docs/concepts/storage/persistent-volumes/).

Please get involved by joining the Kubernetes [Storage SIG](https://github.com/kubernetes/community/blob/master/sig-storage/README.md) (Special Interest Group) to help us enhance this feature. There are a lot of good ideas already and we'd be thrilled to have more!

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