Mount Azure File Storage in Kubernetes Pod using Symlink 💡

What is Symlink?

In laymen's terms, A symlink or a Symbolic Link is a shortcut to another file or directory. There are two types of links that exist in UNIX based systems
Soft links —It is like a shortcut to a file or directory. Soft links can point to a file or a directory on a different filesystem or partition as well.
Hard links — Hard links are associating two or more file names with the same inode.

What is File storage?

In File Storage(file-based storage), data is stored as a single piece of information inside a folder. Files can be stored in folders, which can then be placed inside other folders in a hierarchical structure. NAS is typical example of File Storage. Azure File Share is example of file storage in Cloud.

What is SMB, CIFS?

IBM developed SMB protocol, to allow systems to read and write files to a remote host over a LAN. The directories on the remote hosts made available via SMB are called smb shares.
CIFS is an implementation of the SMB protocol but created by Microsoft.

Azure file shares can be mounted in Linux distributions using the SMB 3.0

Can we use Linux symlink to mount using CIFS??

mount.cifs mounts a Linux CIFS filesystem. The mount.cifs utility attaches the UNC name (exported network resource) specified as service (using //server/share syntax, where "server" is the server name or IP address and "share" is the name of the share) to the local directory mount-point.

Let's mount Azure Files

What do we need: We need to have an Azure storage account with an Azure file share to be created beforehand for this.
Roadblock: A fact to note that Azure Files does not support Hard Link or Soft Link on Linux using CIFS directly.
Solution: Currently, the Linux client supports another style of symbolic links called Minshall+French symlinks. You can use mfsymlinks mount option.

$ mount -t cifs //<storage-account-name>.file.core.windows.net/<share-name> <mount-point> -o vers=<smb-version>,username=<storage-account-name>,password=<storage-account-key>,dir_mode=0777,file_mode=0777,serverino,mfsymlinks
# This format for storing symlinks on SMB shares 

Container Specifics: We are going to use CIFS to mount the file system so our container should have CIFS libraries as part of its build.

Example Dockerfile for container

FROM ubuntu:20.10
RUN apt-get update -y && apt-get install -y cifs-utils

Kubernetes — I am going to run a container in which I am going to mount Azure File share to a container location created at runtime. The noticeable point is that you need to run the container in privilege mode for this.

apiVersion: v1
kind: Pod
metadata:
  labels:
    os: ubuntu
  name: symlink-example
spec:
   containers:
  - image: singharunk/dev:latest  #container image with cifs package
    name: symlink
    env:
    - name: DEMO_ACC
      value: "xxxxxx"
    - name: DEMO_KEY
      value: "xxxxxxxxxxxxxxxxxxxxxxxxxxxx"
    - name: DEMO_SHARE
      value: "share"
    securityContext:
      privileged: true
      allowPrivilegeEscalation: true
    command: ["/bin/sh"]
    args: ["-c", "while true; do echo hello; sleep 10;done"]
    lifecycle:
      postStart:
        exec:
          command: ["/bin/sh", "-c", "mkdir /opt/share && mount --verbose -t cifs //$DEMO_ACC.file.core.windows.net/$DEMO_SHARE /opt/share -o vers=3.0,username=$DEMO_ACC,password=$DEMO_KEY,dir_mode=0777,file_mode=0777,serverino"]
  restartPolicy: Always

The above YAML file will be able to mount your Azure File Share in your Ubuntu container. The above technique I mentioned is not a standard way to mount storage on the Kubernetes cluster. But there can be use cases where you may have to use this strategy to get storage mounted on Containers as a workaround.

That’s all for this post. Keep learning and Stay healthy.

[SMB AND CIFS] https://docs.microsoft.com/en-us/windows/win32/fileio/microsoft-smb-protocol-and-cifs-protocol-overview
[MORE ON CIFS] https://manpages.debian.org/stretch/cifs-utils/mount.cifs.8.en.html