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MicroShift Storage Plugin Overview

MicroShift enables dynamic storage provisioning out of the box with the ODF-LVM CSI plugin. This plugin is a downstream Red Hat fork of TopoLVM. This provisioner will create a new LVM logical volume in the rhel volume group for each PersistenVolumeClaim(PVC), and make these volumes available to pods. For more information on ODF-LVM, visit the repo's README.

Design

ODF-LVM manifests can be found in microshift/assets/components/odf-lvm. For an overview of ODF-LVM architecture, see the design doc.

Deployment

ODF-LVM is deployed onto the cluster in the openshift-storage namespace, after MicroShift boots. StorageCapacity tracking is used to ensure that Pods with an ODF-LVM PVC are not scheduled if the requested storage is greater than the volume group's remaining free storage.

System Requirements

Volume Group Name

The default integration of ODF-LVM assumes a volume-group named rhel. ODF-LVM's node-controller expects that volume group to exist prior to launching the service. If the volume group does not exist, the node-controller will fail to start and enter a CrashLoopBackoff state.

Volume Size Increments

ODF-LVM provisions storage in increments of 1Gb. Storage requests are rounded up to the near gigabyte. When a volume group's capacity is less than 1Gb, the PersistentVolumeClaim will register a ProvisioningFailed event:

  Warning  ProvisioningFailed    3s (x2 over 5s)  topolvm.cybozu.
  com_topolvm-controller-858c78d96c-xttzp_0fa83aef-2070-4ae2-bcb9-163f818dcd9f failed to provision volume with 
  StorageClass "topolvm-provisioner": rpc error: code = ResourceExhausted desc = no enough space left on VG: 
  free=(BYTES_INT), requested=(BYTES_INT)

Usage

ODF-LVM's StorageClass is deployed with a default StorageClass. Any PersistentVolumeClaim without a .spec.storageClassName defined will automatically have a PersistentVolume provisioned from the default StorageClass.

Here is a simple example workflow to provision and mount a logical volume to a pod:

cat <<'EOF' | oc apply -f -
kind: PersistentVolumeClaim
apiVersion: v1
metadata:
  name: my-lv-pvc
spec:
  accessModes:
  - ReadWriteOnce
  resources:
    requests:
      storage: 1G
---
apiVersion: v1
kind: Pod
metadata:
  name: my-pod
spec:
  containers:
  - name: nginx
    image: nginx
    command: ["/usr/bin/sh", "-c"]
    args: ["sleep", "1h"]
    volumeMounts:
    - mountPath: /mnt
      name: my-volume
  volumes:
    - name: my-volume
      persistentVolumeClaim:
        claimName: my-lv-pvc
EOF