How to Use External Storage with Podman on AlmaLinux

Podman has gained popularity for managing containers without a daemon process and its ability to run rootless containers, making it secure and reliable. When deploying containers in production or development environments, managing persistent storage is a common requirement. By default, containers are ephemeral, meaning their data is lost once they are stopped or removed. Using external storage with Podman on AlmaLinux ensures that your data persists, even when the container lifecycle ends.

This blog will guide you through setting up and managing external storage with Podman on AlmaLinux.

Introduction to Podman, AlmaLinux, and External Storage

What is Podman?

Podman is an OCI-compliant container management tool designed to run containers without a daemon. Unlike Docker, Podman operates in a rootless mode by default, offering better security. It also supports rootful mode for users requiring elevated privileges.

Why AlmaLinux?

AlmaLinux is a stable, community-driven distribution designed for enterprise workloads. Its compatibility with RHEL ensures that enterprise features like SELinux and robust networking are supported, making it an excellent host for Podman.

Why External Storage?

Containers often need persistent storage to maintain data between container restarts or replacements. External storage allows:

• Persistence: Store data outside of the container lifecycle.
• Scalability: Share storage between multiple containers.
• Flexibility: Use local disks or network-attached storage systems.

Prerequisites

Before proceeding, ensure you have the following:

1. AlmaLinux Installation: A system running AlmaLinux with sudo access.

2. Podman Installed: Install Podman using:

   sudo dnf install -y podman
   

3. Root or Rootless User: Depending on whether you are running containers in rootless or rootful mode.

4. External Storage Prepared: An external disk, NFS share, or a storage directory ready for use.

Types of External Storage Supported by Podman

Podman supports multiple external storage configurations:

1. Bind Mounts:

   • Map a host directory or file directly into the container.
   • Suitable for local storage scenarios.

2. Named Volumes:

   • Managed by Podman.
   • Stored under /var/lib/containers/storage/volumes for rootful containers or $HOME/.local/share/containers/storage/volumes for rootless containers.

3. Network-Attached Storage (NAS):

   • Use NFS, CIFS, or other protocols to mount remote storage.
   • Ideal for shared data across multiple hosts.

4. Block Devices:

   • Attach raw block storage devices directly to containers.
   • Common in scenarios requiring high-performance I/O.

Setting Up External Storage

Example: Setting Up an NFS Share

If you’re using an NFS share as external storage, follow these steps:

1. Install NFS Utilities:

   sudo dnf install -y nfs-utils
   

2. Mount the NFS Share: Mount the NFS share to a directory on your AlmaLinux host:

   sudo mkdir -p /mnt/nfs_share
   sudo mount -t nfs <nfs-server-ip>:/path/to/share /mnt/nfs_share
   

3. Make the Mount Persistent: Add the following entry to /etc/fstab:

   <nfs-server-ip>:/path/to/share /mnt/nfs_share nfs defaults 0 0
   

Mounting External Volumes to Podman Containers

Step 1: Bind Mount a Host Directory

Bind mounts map a host directory to a container. For example, to mount /mnt/nfs_share into a container:

podman run -d --name webserver -v /mnt/nfs_share:/usr/share/nginx/html:Z -p 8080:80 nginx

• -v /mnt/nfs_share:/usr/share/nginx/html: Maps the host directory to the container path.
• :Z: Configures SELinux to allow container access to the directory.

Step 2: Test the Volume

Access the container to verify the volume:

podman exec -it webserver ls /usr/share/nginx/html

Add or remove files in /mnt/nfs_share on the host, and confirm they appear inside the container.

Using Named Volumes

Podman supports named volumes for managing container data. These volumes are managed by Podman itself and are ideal for isolated or portable setups.

Step 1: Create a Named Volume

Create a named volume using:

podman volume create my_volume

Step 2: Attach the Volume to a Container

Use the named volume in a container:

podman run -d --name db -v my_volume:/var/lib/mysql -e MYSQL_ROOT_PASSWORD=root mariadb

Here, my_volume is mounted to /var/lib/mysql inside the container.

Step 3: Inspect the Volume

Inspect the volume’s metadata:

podman volume inspect my_volume

Inspecting and Managing Volumes

List All Volumes

To list all named volumes:

podman volume ls

Remove a Volume

Remove an unused volume:

podman volume rm my_volume

Troubleshooting Common Issues

1. SELinux Permission Denied

If SELinux blocks access to bind-mounted volumes, ensure the directory has the correct SELinux context:

sudo chcon -Rt svirt_sandbox_file_t /mnt/nfs_share

Alternatively, use the :Z or :z option with the -v flag when running the container.

2. Container Cannot Access NFS Share

• Ensure the NFS share is mounted correctly on the host.
• Verify that the container user has permission to access the directory.
• Check the firewall settings on the NFS server and client.

3. Volume Not Persisting

Named volumes are persistent unless explicitly removed. Ensure the container is using the correct volume path.

Conclusion

Using external storage with Podman on AlmaLinux provides flexibility, scalability, and persistence for containerized applications. Whether you’re using bind mounts for local directories, named volumes for portability, or network-attached storage for shared environments, Podman makes it straightforward to integrate external storage.

By following this guide, you can effectively set up and manage external storage for your containers, ensuring data persistence and improved workflows. Experiment with different storage options to find the setup that best fits your environment.

If you have questions or insights, feel free to share them in the comments below. Happy containerizing!