Reclaiming unused disk space from WSL2

A few days ago I opened File Explorer and saw that my C: drive only have about 17 GB storage left. I found my virtual disk used by WSL2 has used 22GB of space. But when I ran df -h / in WSL, it only used 11GB. So there was a mysterious 11GB of memory neither used by WSL nor given back to Windows.

I consulted StackOverflow and this is the best answer that summarizes all alternatives available to try from many sources, including the long-standing issue on WSL’s Github repo.

These are procedures that work for me:

1. Locate ext4.vhdx Paste %USERPROFILE%\AppData\Local\Packages\CanonicalGroupLimited.UbuntuonWindows_79rhkp1fndgsc\LocalState\ to File Explorer address bar. (In my case, I’ve migrated it to another folder.)

2. Run df -h / in WSL terminal. If there’s a large discrepancy between the size of ext4.vhdx and the actual used space in WSL, it needs to be manually reclaimed because it doesn’t shrink automatically. I did diskpart but it made no difference until I finally found the better solution.

3. Fist clean up WSL storage:

   1. remove .cache and /tmp
   2. remove repos you no longer use or at least delete their node_modules

4. Now the key step: fill the unused spaces with zeroes

   dd if=/dev/zero of=~/zeroes

   This process can take hours.

5. Run sudo sync then remove the zeroes rm ~/zeroes

6. not sure what sudo fstrim / do, but I run it last time

7. Now back to administrator command prompt and run diskpart

   wsl --shutdown
   diskpart # open window Diskpart
   select vdisk file="C:\WSL-Distros\…\ext4.vhdx"
   attach vdisk readonly
   compact vdisk
   detach vdisk
   exit # exit diskpart

   (Source)

That’s it. Now the size of the ext4.vhdx file should have shrunk to the actual volume used by WSL2.