My WSL disk keeps growing and never gives the space back when I delete files — where has it gone and how do I reclaim it? Disk & storage

The gap is real and recoverable, and the reason is that two layers disagree about what "free" means.

The .vhdx is a virtual disk that grows on demand. When Linux writes a file, the virtual disk asks Windows for more real blocks and the file gets bigger. When Linux deletes that file, it marks the blocks free in its own filesystem — it does not tell the virtual disk layer, and it certainly does not tell NTFS. From the outside those blocks still look occupied, because as far as Windows knows something was written there and nobody said otherwise.

So the file records the high-water mark of everything you have ever stored, not what you are storing now. Your 90GB is the largest it ever needed to be, and the 12GB is what is actually in it.

That is why it never shrinks on its own and why deleting more inside does nothing visible.

Reclaiming it is three steps and skipping the first is why people report that compaction "did nothing".

1. Tell the filesystem to release the blocks. Inside the distro:

sudo fstrim -av

This is the step that communicates downward — it discards blocks Linux no longer uses so the virtual disk layer can see they are free. Without it, the compactor looks at the disk, sees data everywhere, and correctly reclaims nothing.

2. Shut everything down, from PowerShell, because the file cannot be in use:

wsl --shutdown

Give it a few seconds. Docker Desktop must be closed too, or it holds its own distro open.

3. Compact the file. diskpart as administrator:

select vdisk file="C:\Users\<you>\AppData\Local\Packages\<distro>\LocalState\ext4.vhdx"
attach vdisk readonly
compact vdisk
detach vdisk

On a 90GB file holding 12GB, expect it to come back close to the real usage.

For question 3 there is now a proper answer rather than a maintenance ritual: sparse mode.

wsl --shutdown
wsl --manage <distro> --set-sparse true

With that on, the disk reclaims space as you delete, instead of only at the high-water mark. It is the setting you want on a laptop that builds containers, which from your description is you.

Two honest caveats. There is a small performance cost, because the disk has to do the bookkeeping as it goes — most people never notice it, but it is not free. And it applies per distro, so if you have several you set it on each.

Check wsl --help on your build for the exact spelling; this area has changed more than once and the flags moved.