Document how to shrink Docker/WSL size on Windows

[patrick-5546]

Purpose

Because the vdisks automatically grow, but do not automatically shrink.

Changes

• Outline problem
• How to shrink vdisk

Resources

• WSL 2 should automatically release disk space back to the host OS microsoft/WSL#4699 (comment)

[polarathene]

My comments on WSL2 experience with this issue may be informative. They're focused around Docker which has some additional gripes with storage management.

• WSL 2 should automatically release disk space back to the host OS microsoft/WSL#4699 (comment) (sparse troubleshooting + Docker Desktop GUI purge method with screenshots)
• WSL 2 should automatically release disk space back to the host OS microsoft/WSL#4699 (comment) (summary comment on the previous one regarding sparse size on disk)
• WSL 2 should automatically release disk space back to the host OS microsoft/WSL#4699 (comment) (insights on resource usage + manual equivalent of wslcompact)
• WSL 2 should automatically release disk space back to the host OS microsoft/WSL#4699 (comment) (advice + observations)

The third comment link responds to a user who used diskpart advice you linked which they said corrupted their disk.

wslcompact works well, but due to the pipe usage is not ideal if you're low on memory (some report corruption but lacking details), regardless you still need enough disk for the exported disk copy to replace the original.

Personally VMware guest seems easier to deal with than WSL2, but you can probably get away with configuring the disk to be sparse VHD (needs to be done early IIRC, otherwise the min size may not drop below the current size). For the most part, when data is removed from the vdisk then it should be reclaimed as free disk space again. It does seem to build up over time/use, so wslcompact or purging via Docker Desktop GUI (if lacking resources for wslcompact) seem to be eventually necessary to fully reclaim disk back.

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Update: New summary that links back to this comment for deeper dive comments.

[patrick-5546]

Thanks @polarathene for the update!