Chapter 6. Dynamic storage using the LVMS plugin
MicroShift enables dynamic storage provisioning that is ready for immediate use with the logical volume manager storage (LVMS) Container Storage Interface (CSI) provider. The LVMS plugin is the Red Hat downstream version of TopoLVM, a CSI plugin for managing logical volume management (LVM) logical volumes (LVs) for Kubernetes.
LVMS provisions new LVM logical volumes for container workloads with appropriately configured persistent volume claims (PVCs). Each PVC references a storage class that represents an LVM Volume Group (VG) on the host node. LVs are only provisioned for scheduled pods.
6.1. LVMS system requirements
Using LVMS in MicroShift requires the following system specifications.
6.1.1. Volume group name
If you did not configure LVMS in an lvmd.yaml file placed in the /etc/microshift/ directory, MicroShift attempts to assign a default volume group (VG) dynamically by running the vgs command.
- MicroShift assigns a default VG when only one VG is found.
-
If more than one VG is present, the VG named
microshiftis assigned as the default. -
If a VG named
microshiftdoes not exist, LVMS is not deployed.
If there are no volume groups on the MicroShift host, LVMS is disabled.
If you want to use a specific VG, LVMS must be configured to select that VG. You can change the default name of the VG in the configuration file. For details, read the "Configuring the LVMS" section of this document.
You can change the default name of the VG in the configuration file. For details, read the "Configuring the LVMS" section of this document.
After MicroShift starts, you can update the lvmd.yaml to include or remove VGs. To implement changes, you must restart MicroShift. If the lvmd.yaml is deleted, MicroShift attempts to find a default VG again.
6.1.2. Volume size increments
The LVMS provisions storage in increments of 1 gigabyte (GB). Storage requests are rounded up to the nearest GB. When the capacity of a VG is less than 1 GB, the PersistentVolumeClaim registers a ProvisioningFailed event, for example:
Example output
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)
6.2. LVMS deployment
LVMS is automatically deployed on to the cluster in the openshift-storage namespace after MicroShift starts.
LVMS uses StorageCapacity tracking to ensure that pods with an LVMS PVC are not scheduled if the requested storage is greater than the free storage of the volume group. For more information about StorageCapacity tracking, read Storage Capacity.
6.3. Limitations to configure the size of the devices used in LVM Storage
The limitations to configure the size of the devices that you can use to provision storage using LVM Storage are as follows:
- The total storage size that you can provision is limited by the size of the underlying Logical Volume Manager (LVM) thin pool and the over-provisioning factor.
The size of the logical volume depends on the size of the Physical Extent (PE) and the Logical Extent (LE).
- You can define the size of PE and LE during the physical and logical device creation.
- The default PE and LE size is 4 MB.
- If the size of the PE is increased, the maximum size of the LVM is determined by the kernel limits and your disk space.
| Architecture | RHEL 6 | RHEL 7 | RHEL 8 | RHEL 9 |
|---|---|---|---|---|
| 32-bit | 16 TB | - | - | - |
| 64-bit | 8 EB [1] 100 TB [2] | 8 EB [1] 500 TB [2] | 8 EB | 8 EB |
- Theoretical size.
- Tested size.
6.4. Creating an LVMS configuration file
When MicroShift runs, it uses LVMS configuration from /etc/microshift/lvmd.yaml, if provided. You must place any configuration files that you create into the /etc/microshift/ directory.
Procedure
To create the
lvmd.yamlconfiguration file, run the following command:$ sudo cp /etc/microshift/lvmd.yaml.default /etc/microshift/lvmd.yaml
6.5. Basic LVMS configuration example
MicroShift supports passing through your LVM configuration and allows you to specify custom volume groups, thin volume provisioning parameters, and reserved unallocated volume group space. You can edit the LVMS configuration file you created at any time. You must restart MicroShift to deploy configuration changes after editing the file.
If you need to take volume snapshots, you must use thin provisioning in your lvmd.conf file. If you do not need to take volume snapshots, you can use thick volumes.
The following lvmd.yaml example file shows a basic LVMS configuration:
LVMS configuration example
socket-name: 1 device-classes: 2 - name: "default" 3 volume-group: "VGNAMEHERE" 4 spare-gb: 0 5 default: 6
- 1
- String. The UNIX domain socket endpoint of gRPC. Defaults to '/run/lvmd/lvmd.socket'.
- 2
- A list of maps for the settings for each
device-class. - 3
- String. The name of the
device-class. - 4
- String. The group where the
device-classcreates the logical volumes. - 5
- Unsigned 64-bit integer. Storage capacity in GB to be left unallocated in the volume group. Defaults to
0. - 6
- Boolean. Indicates that the
device-classis used by default. Defaults tofalse. At least one value must be entered in the YAML file values when this is set totrue.
A race condition prevents LVMS from accurately tracking the allocated space and preserving the spare-gb for a device class when multiple PVCs are created simultaneously. Use separate volume groups and device classes to protect the storage of highly dynamic workloads from each other.
6.6. Using the LVMS
The LVMS StorageClass is deployed with a default StorageClass. Any PersistentVolumeClaim objects without a .spec.storageClassName defined automatically has a PersistentVolume provisioned from the default StorageClass. Use the following procedure to provision and mount a logical volume to a pod.
Procedure
To provision and mount a logical volume to a pod, run the following command:
$ 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 securityContext: allowPrivilegeEscalation: false capabilities: drop: - ALL runAsNonRoot: true seccompProfile: type: RuntimeDefault volumes: - name: my-volume persistentVolumeClaim: claimName: my-lv-pvc EOF
6.6.1. Device classes
You can create custom device classes by adding a device-classes array to your logical volume manager storage (LVMS) configuration. Add the array to the /etc/microshift/lvmd.yaml configuration file. A single device class must be set as the default. You must restart MicroShift for configuration changes to take effect.
Removing a device class while there are still persistent volumes or VolumeSnapshotContent objects connected to that device class breaks both thick and thin provisioning.
You can define multiple device classes in the device-classes array. These classes can be a mix of thick and thin volume configurations.
Example of a mixed device-class array
socket-name: /run/topolvm/lvmd.sock
device-classes:
- name: ssd
volume-group: ssd-vg
spare-gb: 0 1
default: true
- name: hdd
volume-group: hdd-vg
spare-gb: 0
- name: thin
spare-gb: 0
thin-pool:
name: thin
overprovision-ratio: 10
type: thin
volume-group: ssd
- name: striped
volume-group: multi-pv-vg
spare-gb: 0
stripe: 2
stripe-size: "64"
lvcreate-options:2
- 1
- When you set the spare capacity to anything other than
0, more space can be allocated than expected. - 2
- Extra arguments to pass to the
lvcreatecommand, such as--type=<type>. Neither MicroShift nor the LVMS verifieslvcreate-optionsvalues. These optional values are passed as is to thelvcreatecommand. Ensure that the options specified here are correct.
