WSL2 as rung zero — what it proves, and the exact wall it hits

Decision. Spend 3–6 months in WSL2 to retire the userland risk — bash, coreutils, apt, git, Docker, dotnet/bun on Linux — because that part is genuinely identical, and WSL2 now runs a real Microsoft-built Linux 6.18 kernel with systemd[1][2][3]. Then stop: WSL2 cannot teach you a display server, a bootloader, drivers, suspend/resume, or package-manager-owns-the-machine, and it hides the single decision that dominates day-one pain — your repos must live on ext4, not /mnt/c, which is exactly where a Dropbox-synced Windows folder puts them[4][5].

What WSL2 genuinely validates

These transfer 1:1 to bare metal. If they work here, they work there.

WSL itself went open source at Build 2025 (MIT), so the seams are now inspectable: wsl.exe, wslservice.exe, the Linux-side init/gns/localhost daemons and the plan9 server[14][15] — microsoft/WSL ⭐ 33k (Jul 2026)[16]. Still closed: lxcore.sys (WSL1) and p9rdr.sys/p9np.dll, the \\wsl.localhost redirector[15].

The filesystem cliff — the number that decides your setup

Microsoft’s own guidance: “We recommend against working across operating systems with your files… store your files in the WSL file system if you are working in a Linux command line”[4]. The mechanism is a plan9 file server in wslservice.exe reached over a Hyper-V socket, one round-trip per operation[17].

Source: wsl-filesystem-benchmark ⭐ 3 (Jul 2026)[5]. Random read across the boundary is ~34× slower; Windows→WSL writes land under 1% of native. The canonical bug, microsoft/WSL#4197, reports 40.4 MB/s on /mnt under WSL2 vs 442 MB/s under WSL1 and is still open[18].

⚠ 2026 change: virtiofs is now selectable (virtiofs=true under [wsl2], kernel 6.18.26.3-1+), replacing plan9 for /mnt/*; a 27 May 2026 PR gave each virtio device its own DMA pool to remove the last SWIOTLB contention point. It is still opt-in — plan9 remains the default[19][17]. HN reports a Hugo build at 50 s on native ext4, 75 s on virtiofs, and “3–5 m or more” on plan9 — virtiofs closes most of the gap but does not close it[20].

The Dropbox trap. A repo under C:\Users\you\Dropbox\... is doubly penalised: every git status, dotnet build and node_modules walk pays the cross-OS tax above, and the sync client indexes tens of thousands of dependency files it cannot exclude per-directory[21]. The fix is not tuning — it is git clone into ~/code and letting the remote be the sync mechanism. This is a rehearsal for bare metal, where /mnt/c does not exist at all.

What WSL2 structurally hides

Every row here is a real migration risk that WSL2 cannot retire.

2026 sharp edges worth knowing before you start

• Memory ballooning. The Linux page cache grows and Windows can’t see it as reclaimable — Microsoft’s own example shows 2 GB attributed to the VM in Windows while Linux is using 50 MB[31]. autoMemoryReclaim now defaults to dropCache (was previously off); gradual is the gentler option[3].
• VHDX only grows. Default max is 1 TB and deleting files does not shrink the file. Enable sparseVhd=true (or wsl --manage <distro> --set-sparse true) — ⚠ once sparse, Optimize-VHD refuses to compact it, so you’re committed to automatic reclaim[3][32][33].
• inotify does not cross the boundary. File-watching on /mnt/c doesn’t fire, so Vite/Angular/dotnet watch HMR silently dies; the workaround is polling — or, correctly, moving the repo to ext4[34].
• Case sensitivity is inverted per-tree. Windows drives mount case-insensitive by default (case=off); ext4 is case-sensitive. Mixed-case files created in WSL become unopenable from Windows tools, and git config core.ignorecase disagreements produce phantom conflicts[35].
• cgroup v1 is gone. WSL now isolates each distro’s systemd into its own cgroup v2 hierarchy; IsolateDistroCgroup (default true) is the escape hatch for legacy v1 workloads[36][37].
• The 8-second rule. Config edits need the VM fully stopped (wsl --shutdown), not just the terminal closed[3].

A sane %UserProfile%\.wslconfig baseline for a 32 GB dev box:

[wsl2]
memory=16GB
processors=8
networkingMode=mirrored
virtiofs=true

[experimental]
autoMemoryReclaim=gradual
sparseVhd=true
hostAddressLoopback=true

hostAddressLoopback and ignoredPorts only apply under mirrored[3]; virtiofs needs kernel 6.18.26.3-1+[19].

Verdict: which risks WSL2 retires

Stop learning from WSL2 when your friction stops being “how do I do X in Linux” and becomes “I want Linux to be the machine” — the point where WSL2 mistakes are still trivially unwound but the questions it can answer have run out[38]. Concretely: when you have (a) all repos on ext4 with no /mnt/c dependency, (b) systemd units you wrote yourself, and (c) a dotfiles repo you can git clone onto a fresh box. At that point the next rung is a machine WSL2 can’t simulate — and everything remaining on the ✗ list has to be learned on real hardware.