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2020-08-13 Kubernetes

Create A Persistent Volume

Create a Persistent Volume for Kubernetes#

So you have just setup your k8s cluster but now you need persistent storage for your databases etc. (Not for your ephemeral application pods)

There are seemingly infinite options to choose from of storage classes we didn’t even know exist.

  • awsElasticBlockStore
  • azureDisk
  • azureFile
  • cephfs
  • cinder
  • configMap
  • csi
  • downwardAPI
  • emptyDir
  • fc (fibre channel)
  • flexVolume
  • flocker
  • gcePersistentDisk
  • gitRepo (deprecated)
  • glusterfs
  • hostPath
  • iscsi
  • local
  • nfs
  • persistentVolumeClaim
  • projected
  • portworxVolume
  • quobyte
  • rbd
  • scaleIO
  • secret
  • storageos
  • vsphereVolume

Usually we just persistent our data to the SSD of the server. So let us try create a local persistent volume first.

Persistent Volumes#

Just as a refresher:

A persistent volume (PV) is a piece of storage in the Kubernetes cluster, while a persistent volume claim (PVC) is a request for storage.

There is a tutorial on how to create the PV with k3s and using longhorn, but I digress lets just get some empty space.

Get Some Empty#

So for a start let us simply get some empty space for our k8s cluster that is not shared with other things.

So the persistent volume bankend we will use is hostPath - A hostPath volume mounts a file or directory from the host node’s filesystem into your Pod.

  1. Log into your node

  2. View disk space usage

    [root@st2 home]# df Filesystem 1K-blocks Used Available Use% Mounted on /dev/mapper/centos-root 52403200 13602876 38800324 26% / devtmpfs 5023824 0 5023824 0% /dev tmpfs 5035684 0 5035684 0% /dev/shm tmpfs 5035684 223216 4812468 5% /run tmpfs 5035684 0 5035684 0% /sys/fs/cgroup /dev/mapper/centos-home 46172132 33112 46139020 1% /home /dev/sda1 1038336 185224 853112 18% /boot

  3. List block devices

    [root@st2 home]# lsblk NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT sda 8:0 0 100G 0 disk ├─sda1 8:1 0 1G 0 part /boot └─sda2 8:2 0 99G 0 part ├─centos-root 253:0 0 50G 0 lvm / ├─centos-swap 253:1 0 5G 0 lvm [SWAP] └─centos-home 253:2 0 44.1G 0 lvm /home sr0 11:0 1 1024M 0 rom

  4. Display info about volume groups

    [root@st2 home]# vgs VG #PV #LV #SN Attr VSize VFree centos 1 3 0 wz–n- <99.00g 4.00m

    dev/mapper is physical disk /home is the mount point

    you can mount a physical anywhere on physical disk a volume group creates a logical volume

fstab file system table a erfence of all disks

[root@st2 /]# cat /etc/fstab

#
# /etc/fstab
# Created by anaconda on Wed Sep 11 16:42:48 2019
#
# Accessible filesystems, by reference, are maintained under '/dev/disk'
# See man pages fstab(5), findfs(8), mount(8) and/or blkid(8) for more info
#
/dev/mapper/centos-root /                       xfs     defaults        0 0
UUID=59791677-1abf-422c-a183-6186b6117ccf /boot                   xfs     defaults        0 0
/dev/mapper/centos-home /home                   xfs     defaults        0 0
/dev/mapper/centos-swap swap                    swap    defaults        0 0

If not mounted it won’t show in df

Check if there are any processes using files in that path: lsof /home

Unmount: umount /home

REduce: lvreduce -L 4G /dev/mapper/centos-home

Grow: xfs_growfs /home

Mount again: mount -av

But now the superblock is broken:

[root@st2 ~]# mount -av
/                        : ignored
/boot                    : already mounted
mount: /dev/mapper/centos-home: can't read superblock
swap                     : ignored

CHeck amount free

vgs

Create logical volume

lvcreate -L 40G -n persist centos

Name is persist (from centos volume group)

Lists logical volumes

lvs

Make filesystem:

mkfs.xfs /dev/mapper/centos-persist

Mount:

mount /dev/mapper/centos-persist /persist

Ensure filesystem loads at bootime:

vi /etc/fstab
/dev/mapper/centos-persist /persist             xfs     defaults        0 0
  1. Let us create a new volume by shrinking /home and creating a new volume and path

I don’t know what the difference is between a persistent volume and a storage class.

A StorageClass is a way to dynamically provision volumes. They can be referred to in a persistent volume claim.

A PersistentVolume is an independent storage volume.

Get the config of the resource:

kubectl get pv my-persistent-volume -o yaml
apiVersion: v1
kind: PersistentVolume
metadata:
  annotations:
    field.cattle.io/creatorId: user-kmmwg
  creationTimestamp: "2020-01-14T08:36:54Z"
  finalizers:
  - kubernetes.io/pv-protection
  labels:
    cattle.io/creator: norman
  name: my-persistent-volume
  resourceVersion: "7088911"
  selfLink: /api/v1/persistentvolumes/my-persistent-volume
  uid: ed8c625b-5930-4459-b2fc-34e379168c48
spec:
  accessModes:
  - ReadWriteOnce
  capacity:
    storage: 10Gi
  hostPath:
    path: /persist
    type: Directory
  persistentVolumeReclaimPolicy: Retain
  volumeMode: Filesystem
status:
  phase: Available

Put now out application needs a persistent volume claim - it is what the app or deployment uses. It will figure out which persistent volume to us.

One PersistentVolume will be bound to one claim