First, let's summon the video device out of thin air:

sudo modprobe v4l2loopback

Check this extensive documentation[archive] for details on the arguments you can use. A newly created video device is now available. In my case, it is at /dev/video3.

Now, see how ffmpeg takes input from testrc and then writes it to /dev/video3.

ffmpeg -re -f lavfi -i testsrc=size=1280x720:rate=30 -f v4l2 /dev/video3

Similarly, we can use ffplay to show the video.

ffplay /dev/video3

At my workplace, I used ffmpeg and gstreamer to deliver real-time video feeds to our customers. This information might be available online, but it's scattered. While this post serves as my personal note, I hope it will also be useful for you. Ciao!

This problem is not unique:

1. https://stackoverflow.com/questions/69113444/docker-pull-so-slow

2. https://github.com/docker/hub-feedback/issues/2172

3. https://github.com/docker/hub-feedback/issues/2161

4. https://forums.docker.com/t/docker-pull-taking-forever/130113/2

So I decided to set up my local Docker registry as a mirror. First, I need to spin up the Docker registry image from my Portainer:

After that, for each computer I use at home, update /etc/docker/daemon.json:

{
  "registry-mirrors": ["http://192.168.1.20:32776"]
}

Do not forget to restart docker service sudo systemctl restart docker.

The result is that docker pull almost saturates my network speed. Without this mirror, docker pull often capped at hundreds of kbps despite my 150Mbps downlink.

Visit this URL to see what images are cached http://<container_addr>/v2/_catalog:

Follow me for updates.

The problem is that WSL uses a virtual disk as storage, which previously retained its size on the host machine even when files inside the virtual disk were no longer there. Newer WSL releases will use a sparse disk to address this problem automatically. However, the old version requires setting the flag manually.

C:\Users\faruq>wsl --list --verbose
  NAME                   STATE           VERSION
* Ubuntu-22.04           Stopped         2
  docker-desktop-data    Stopped         2
  docker-desktop         Stopped         2

Solution #1: Use diskpart to compact the vdisk size

Microsoft DiskPart version 10.0.22621.1

Copyright (C) Microsoft Corporation.
On computer: BITWISE

DISKPART> select vdisk file=C:\Users\faruq\AppData\Local\Packages\CanonicalGroupLimited.Ubuntu22.04LTS_79rhkp1fndgsc\LocalState\ext4.vhdx

DiskPart successfully selected the virtual disk file.

DISKPART> compact vdisk

  100 percent completed

DiskPart successfully compacted the virtual disk file.

DISKPART>

Below are pictures comparing the size before and after the compact command. However, there is not much difference because before I wrote this article, I had already compacted the vdisk and reclaimed 50GB.

Solution #2: Set vdisk as sparse

Another solution is to simply set the WSL distro to use a sparse vdisk and let the system do the job.

C:\Users\faruq>wsl --manage Ubuntu-22.04 --set-sparse false
Conversion in progress, this may take a few minutes.
The operation completed successfully.

If you try to compact it like the previous solution, you will get an error:

DISKPART> compact vdisk

DiskPart has encountered an error: The requested operation could not be completed due to a virtual disk system limitation.  Virtual hard disk files must be uncompressed and unencrypted and must not be sparse.
See the System Event Log for more information.

See you later!