Let’s quickly discuss using persistent local volumes with Kubernetes.
First, get yourself a k8s. I have one here running on a packet.net instance. It’s only a single node. Deployed with kubeadm and is running calico as the network plugin.
# k get nodes
NAME STATUS ROLES AGE VERSION
cka Ready master 6h v1.11.1
Let’s look at everything that’s running.
# k get all
NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE
service/kubernetes ClusterIP 172.17.0.1 <none> 443/TCP 6h
# k get all -n kube-system
NAME READY STATUS RESTARTS AGE
pod/calico-etcd-2hcdc 1/1 Running 0 6h
pod/calico-kube-controllers-74b888b647-qr86d 1/1 Running 0 6h
pod/calico-node-5jmrc 2/2 Running 17 6h
pod/coredns-78fcdf6894-4ngmq 1/1 Running 0 6h
pod/coredns-78fcdf6894-gzqcw 1/1 Running 0 6h
pod/etcd-cka 1/1 Running 0 6h
pod/kube-apiserver-cka 1/1 Running 0 6h
pod/kube-controller-manager-cka 1/1 Running 0 6h
pod/kube-proxy-62hp2 1/1 Running 0 6h
pod/kube-scheduler-cka 1/1 Running 0 6h
NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE
service/calico-etcd ClusterIP 172.17.0.136 <none> 6666/TCP 5h
service/kube-dns ClusterIP 172.17.0.10 <none> 53/UDP,53/TCP 6h
NAME DESIRED CURRENT READY UP-TO-DATE AVAILABLE NODE SELECTOR AGE
daemonset.apps/calico-etcd 1 1 1 1 1 node-role.kubernetes.io/master= 6h
daemonset.apps/calico-node 1 1 1 1 1 <none> 6h
daemonset.apps/kube-proxy 1 1 1 1 1 beta.kubernetes.io/arch=amd64 6h
NAME DESIRED CURRENT UP-TO-DATE AVAILABLE AGE
deployment.apps/calico-kube-controllers 1 1 1 1 6h
deployment.apps/calico-policy-controller 0 0 0 0 6h
deployment.apps/coredns 2 2 2 2 6h
NAME DESIRED CURRENT READY AGE
replicaset.apps/calico-kube-controllers-74b888b647 1 1 1 6h
replicaset.apps/calico-policy-controller-55b469c8fd 0 0 0 6h
The c2.medium.x86 (AMD EPYC!) has two extra SSDs.
mkfs.ext4 /dev/sdc
mkfs.ext4 /dev/sdd
mkdir -p /mnt/disks/sdc
mkdir -p /mnt/disks/sdd
mount /dev/sdc /mnt/disks/sdc
mount /dev/sdd /mnt/disks/sdd
Now both are formated and mounted.
# lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sda 8:0 0 111.8G 0 disk
sdb 8:16 0 111.8G 0 disk
├─sdb1 8:17 0 512M 0 part /boot/efi
├─sdb2 8:18 0 1.9G 0 part
└─sdb3 8:19 0 109.4G 0 part /
sdc 8:32 0 447.1G 0 disk /mnt/disks/sdc
sdd 8:48 0 447.1G 0 disk /mnt/disks/sdd
I usually abbreviate kubectl to k.
alias k=kubectl
Setup a storage class.
cat > storage-class.yml <<EOF
kind: StorageClass
apiVersion: storage.k8s.io/v1
metadata:
name: local-storage
provisioner: kubernetes.io/no-provisioner
volumeBindingMode: WaitForFirstConsumer
EOF
kubectl create -f storage-class.yml
List the available storage classes.
# k get sc
NAME PROVISIONER AGE
local-storage kubernetes.io/no-provisioner 4s
Create a persistent volume.
cat > pv-sdc.yml <<EOF
apiVersion: v1
kind: PersistentVolume
metadata:
name: local-pv-sdc
spec:
capacity:
storage: 440Gi
accessModes:
- ReadWriteOnce
persistentVolumeReclaimPolicy: Retain
storageClassName: local-storage
local:
path: /mnt/disks/sdc
nodeAffinity:
required:
nodeSelectorTerms:
- matchExpressions:
- key: kubernetes.io/hostname
operator: In
values:
- cka
EOF
kubectl create -f pv-sdc.yml
Same for sdd.
cat > pv-sdd.yml <<EOF
apiVersion: v1
kind: PersistentVolume
metadata:
name: local-pv-sdd
spec:
capacity:
storage: 440Gi
accessModes:
- ReadWriteOnce
persistentVolumeReclaimPolicy: Retain
storageClassName: local-storage
local:
path: /mnt/disks/sdd
nodeAffinity:
required:
nodeSelectorTerms:
- matchExpressions:
- key: kubernetes.io/hostname
operator: In
values:
- cka
EOF
kubectl create -f pv-sdd.yml
Now I have two PVs.
# k get pv
NAME CAPACITY ACCESS MODES RECLAIM POLICY STATUS CLAIM STORAGECLASS REASON AGE
local-pv-sdc 440Gi RWO Retain Available local-storage 53s
local-pv-sdd 440Gi RWO Retain Available local-storage 4s
Create a persistent volume claim.
cat > pvc1.yml <<EOF
kind: PersistentVolumeClaim
apiVersion: v1
metadata:
name: pvc1
spec:
accessModes:
- ReadWriteOnce
storageClassName: local-storage
resources:
requests:
storage: 100Gi
EOF
kubectl create -f pvc1.yml
PVC will be pending until we create a node that uses it.
# k get pvc
NAME STATUS VOLUME CAPACITY ACCESS MODES STORAGECLASS AGE
pvc1 Pending local-storage 3s
Now a deployment that uses the pvc.
cat > deploy-nginx.yml <<EOF
apiVersion: apps/v1
kind: Deployment
metadata:
name: nginx-deployment
labels:
app: nginx
spec:
replicas: 1
selector:
matchLabels:
app: nginx
template:
metadata:
labels:
app: nginx
spec:
containers:
- name: nginx
image: nginx:1.7.9
ports:
- containerPort: 80
volumeMounts:
- mountPath: "/usr/share/nginx/html"
name: storage
volumes:
- name: storage
persistentVolumeClaim:
claimName: pvc1
EOF
kubectl create -f deploy-nginx.yml
pvc is now bound.
# k get pvc
NAME STATUS VOLUME CAPACITY ACCESS MODES STORAGECLASS AGE
pvc1 Bound local-pv-sdc 440Gi RWO local-storage 21m
Expose nginx.
# kubectl expose deployment nginx-deployment --type=NodePort
service/nginx-deployment exposed
# kubectl get svc nginx-deployment
NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE
nginx-deployment NodePort 172.17.0.157 <none> 80:31813/TCP 7m
Now create an index.html page. I’ll use a uuid just to show it’s not already setup.
# uuid=`uuidgen`
# echo $uuid
b5d3c8cd-a56f-4252-a026-7107790fcd44
# echo $uuid > /mnt/disks/sdc/index.html
# curl 172.17.0.157
b5d3c8cd-a56f-4252-a026-7107790fcd44
Let’s delete the pod.
# k delete pod nginx-deployment-7d869874bc-dlz4s
pod "nginx-deployment-7d869874bc-dlz4s" deleted
A new pod will automatically be created by Kubernetes.
# k get pods
NAME READY STATUS RESTARTS AGE
nginx-deployment-7d869874bc-fkt4m 1/1 Running 0 28s
Run the same curl.
# curl 172.17.0.157
b5d3c8cd-a56f-4252-a026-7107790fcd44
Same UUID.
Lets check from the pod.
# k exec -it nginx-deployment-7d869874bc-fkt4m -- cat /usr/share/nginx/html/index.html
b5d3c8cd-a56f-4252-a026-7107790fcd44
As expected.
Kubernetes.io has a blog post announcing local persistent volumes. It’s worth a read. Anything I say here of value would just be copying from that. :) There are several reasons not to use this model for storage. Seems like it would be best for data heavy applications…data gravity. There is also a simple volume manager available as well, but I didn’t explore that in this blog post.