Compact the VHDX Disk of a Docker Container or WSL

The size of the file C:\Users\Realdevtipsuser\AppData\Local\Docker\wsl\data\ext4.vhdx was 51 GB. I couldn’t determine who was using that file or why.

Since “Docker” was in the path, it made sense to check Docker settings. And I found the answer: Docker Desktop uses WSL2 storage.

Check container sizes

I use three containers, and their total size is less than 3 GB, not 50! You can check this with the PowerShell command:

docker ps -a

Probably, I once installed something big and the drive increased. Usually, this is a one-way process—the disk never shrinks on its own.

Compact the VHDX

The command to optimize the disk is:

Optimize-VHD -Path "C:\Users\Realdevtipsuser\AppData\Local\Docker\wsl\data\ext4.vhdx" -Mode Full

But you must ensure Docker and its dependencies are not running. When I executed this command, I got an error because the file was being used by another process (0x80070020):

The system failed to compact ‘C:\Users\Realdevtipsuser\AppData\Local\Docker\wsl\data\ext4.vhdx’.
Failed to compact the virtual disk.
The system failed to compact ‘C:\Users\Realdevtipsuser\AppData\Local\Docker\wsl\data\ext4.vhdx’: The process cannot access the file because it is being used by another process. (0x80070020).

Free the file

To release the file, you need to:

1. Stop the Docker desktop,
2. Stop the Docker service (Windows services)
3. Restart the WSL Service (‘WSLService’ or ‘LxssManager’ in dependency of the Windows version).

To find the correct service name for step 3, use PowerShell

Get-Service *wsl*

Then run

restart-Service WSLService
wsl –shutdown

After that execute optimize command Optimize-VHD -Path

Optimize-VHD -Path "C:\Users\Realdevtipsuser\AppData\Local\Docker\wsl\data\ext4.vhdx" -Mode Full

After the compacting the file shrank to 11Gb.

Alternative: Diskpart

As an alternative you can use a console utility ‘Diskpart’. The commands are similar.

How to compact the VHDX with Diskpart

Execute the utility with diskpart command

Run > diskpart

Then inside Diskpart:

select vdisk file="C:\Users\Realdevtipsuser\AppData\Local\Docker\wsl\data\ext4.vhdx"
compact vdisk

Note: WSL2 must be stopped before compacting.