systemd mstack mounts overlays via symlinks, with caveats

systemd-mstack, introduced in systemd 260 and extended in 261, describes an overlayfs mount as a directory of layer@N symlinks plus a writable rw/ subdirectory. Testing shows writes silently overwrite read-only files, and mounting inside a Docker overlay2 container fails with a cryptic error.

• mstack landed in systemd 260 in March; systemd-nspawn support grew in 261
• Writes to the rw layer overwrite read-only files with no error or warning
• Mounting inside Docker with overlay2 fails: the kernel forbids overlay on overlay
• Mounts are about 1 ms slower than manual mount -t overlay, with no data copying