VFS Performance

[rmanosuthi]

Now that MO2 importing now works, I decided to test out my old mod list (10G+) and discovered that fuse-overlayfs is awfully slow.

Slow as in "on a very fast NVMe SSD, takes minutes to launch SkyrimSE.exe and pegs CPU0 at 100%".

I'm not sure if this is because of file conflicts, funny symlinks, or something else, but it's not looking good.

[rmanosuthi]

I'm looking at using the system's overlayfs, through a separate binary mofl-vfs, through polkit. If the performance gain's substantial then I'll go with it.

Pros:

• Much better performance (?)
• We no longer rely on a GPL user tool (fuse-overlayfs) so the project doesn't have to also use GPL. Docker's licensed under Apache2 yet they're using the system's overlayfs.

Cons:

• Users will have to install a polkit rule that doesn't prompt for authentication!
• How much do you trust mofl-vfs? All it's supposed to do is
  • Read a game's json file given its path
  • (The potentially dangerous part) Go to the parent dir of the json file, then iterate over mods/
  • Execute mount -t overlay overlay ... based on the mod list and the json file

[rmanosuthi]

I tested the system's overlayfs and it's up to 10x faster than fuse-overlayfs! I'll be going the polkit route then.

[rmanosuthi]

Another alternative is to rewrite syscalls for the game process.

open("Skyrim Special Edition/Data/file_overwritten_by_mod.esp")

vvvvvvvvvv

open("~/.config/mofl/games/SkyrimSE/mods/1000/Data/mod_file.esp")

This will involve libc::ptrace, will require unsafe code, and will be a lot of work.