ubuntu overlay虛擬機使用

Ubuntu Overlay File System Usage Guide
OverlayFS is a union file system that combines multiple directories (layers) into a single virtual directory, enabling lightweight file system modifications without altering the underlying data. It is commonly used for containerized environments (e.g., Docker) and sandboxing. Below are the essential steps to set up and use OverlayFS on Ubuntu.

1. Environment Preparation

Before configuring OverlayFS, ensure your system meets the following requirements:

• Kernel Version: OverlayFS requires a Linux kernel version of 3.18 or higher. Verify your kernel version with:

  uname -r
  

• Software Packages: Install the necessary tools using:

  sudo apt-get update
  sudo apt-get install overlayfs aufs-tools
  

2. Create Directory Structure

OverlayFS requires three core directories and one optional directory:

• Lowerdir: Stores the base file system (read-only).
• Upperdir: Stores modifications made to the overlay (read-write).
• Workdir: A temporary workspace required by OverlayFS for operations.
• Mergedir: The virtual directory where the combined file system is mounted (optional if mounting directly to a target).

Create these directories using:

mkdir -p upper lower work merged

3. Add Files to Lowerdir (Optional)

The lowerdir acts as the base layer. You can populate it with files to simulate a read-only file system:

echo "Hello from lower" > lower/hello.txt

4. Mount OverlayFS

Use the mount command to combine the layers into a single virtual directory. The syntax is:

sudo mount -t overlay overlay -o lowerdir=lower,upperdir=upper,workdir=work merged

• lowerdir=lower: Path to the base layer.
• upperdir=upper: Path to the read-write layer.
• workdir=work: Path to the work directory.
• merged: Mount point for the virtual file system.

After mounting, the merged directory will contain files from lowerdir. Any changes made to merged will be stored in upperdir.

5. Access and Modify the Overlay

You can interact with the merged directory like a normal file system:

• Read Files:

  cat merged/hello.txt  # Output: Hello from lower
  

• Modify Files: Changes are written to upperdir:

  echo "Hello from upper" > merged/hello.txt
  cat merged/hello.txt  # Output: Hello from upper
  

• Add New Files: New files are created in upperdir:

  touch merged/newfile.txt
  ls merged/  # Output: hello.txt newfile.txt
  

6. Unmount OverlayFS

When you’re done using the overlay, unmount it to detach the virtual file system:

sudo umount merged

7. Persistent Configuration (Optional)

To automatically mount OverlayFS on system boot, add an entry to /etc/fstab:

sudo nano /etc/fstab

Insert the following line (replace paths as needed):

overlay /merged overlay defaults,lowerdir=/lower,upperdir=/upper,workdir=/work 0 0

Save the file and remount all file systems to apply changes:

sudo mount -a

Key Notes

• Permissions: Ensure you have root privileges (sudo) for mounting/unmounting and modifying directories.
• Data Integrity: Avoid deleting files directly from lowerdir (the base layer). Use upperdir for modifications to prevent corruption.
• Cleanup: To remove the overlay, simply unmount it and delete the upper, work, and merged directories. The lowerdir remains unchanged.

By following these steps, you can effectively use OverlayFS on Ubuntu for lightweight file system virtualization, ideal for containers or testing environments.