Starting to Learn Kubernetes a Step Behind - 11. config&storage Part 2 -

Story

Last time

In Starting to Learn Kubernetes Late - 10. config&storage Part 1 -, we learned about config. This time, we will learn about storage.

Volume and PresistentVolume

A Volume refers to a predetermined available volume. You cannot delete or create new volumes here. PresistentVolume refers to a persistent volume located externally. Here, you can delete or create new volumes. For stateful things like DB, use PresistentVolume. For temporary things, do you use Volume?

※ PresistentVolumeClaim is a resource for assigning PresistentVolume.

Types of Volume

The types of Volume introduced in the book (Kubernetes Complete Guide) are as follows:

emptyDir
- Uses temporary disk space
- When the pod is deleted, emptyDir is also deleted
- Cannot specify the mount destination
hostPath
- A version of emptyDir where you can specify the mount destination
downwardAPI
- Mounts a file that has placed Pod information as a file
projected
- Creates a directory that combines secret/configMap/downwardAPI/serviceAccountToken and mounts it

※ types-of-volumes

Since you can't keep the Volume, you have to be careful when deleting the Pod. If you save logs as files, is it good to have a temporary Volume? However, you have to regularly move it to external storage, which is a hassle. (After all, it's best to stream logs and flow them to an external service)

As a product, there's not much use...?

Types of PresistentVolume

Use an external persistent volume. For example, the following types are available:

GCE Persistent Disk
AWS Elastic Block Store
NFS
iSCSI
Ceph
OpenStack Cinder
GlusterFS

In Starting to Learn Kubernetes Late - 06. workloads Part 2 -, we used NFS. The method of creating a PersistentVolume varies depending on the external persistent volume, but there seem to be common points, so I'll write about them here.

Label
- Make it easier to specify by labeling the PersistentVolume
Capacity
- The capacity required by the Volume. Assigned from the smallest capacity.
Access Mode
- ReadWriteOnce
  - Read/Write is possible from a single node
- ReadOnlyMany
  - Read is possible from multiple nodes
- ReadWriteMany
  - Read/Write is possible from multiple nodes
Reclaim Policy
- Policy on whether to discard or reuse the Volume after use
  - Delete
    - Delete the entity of the PersistentVolume
  - Retain
    - Keep the entity of the PersistentVolume without leaving it
    - Not mounted again
  - Recycle
    - Delete the data of the PersistentVolume and make it reusable
    - Mounted again
    - (Scheduled to be abolished, use DynamicProvisioning)
StorageClass
- The type of storage provided by each provider
  - Basically, it is created automatically

PersistentVolumeClaim

In fact, to use PresistentVolume, you need to make a request with PresistentVolumeClaim. The necessary items are as follows:

Label Selector
- Filtering by label
Capacity
- The capacity you are asking for
Access Mode
- Refer to the Access Mode of PresistentVolume
StorageClass
- Refer to the StorageClass of PresistentVolume

If the Volume that meets the requirements was RetainPolicy, it will become "Released" when the Claim is deleted.

Finally

This time, it felt like I just wrote the book as it is. What I actually tried is Starting to learn Kubernetes a step behind - 06. workloads part 2 -. Well, I thought it would be better not to get too deeply involved. Next time is here.

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Kubernetes