caching fs to use over immutable network filesystems

The use-case is something like your Steam Deck has a large directory of roms mounted over NFS, but if you disable wifi, you want the roms you've already played still available.

This basically bind-mounts your NFS share over a path of your choosing, except it caches all file attributes/paths forever, and copies the files you open to your cache directory. When you access that file again it doesn't access the remote server at all. So if your remote (say NFS) server is not available, you can still play the games.

If you add/change files to your remote, you must delete /local/cache/dir/cache-fs.tree and remount.

This assumes you install the cache-fs binary as mount.cachefs, then you can use it in /etc/fstab etc

mount -t cachefs -o remote_dir=/remote/dir/to/cache /local/cache/dir /where/you/want/it/mounted

Or put it in /etc/fstab like:

/local/cache/dir /where/you/want/it/mounted cachefs defaults,ro,allow_other,remote_dir=/remote/dir/to/cache,nofail,_netdev 0     0

If you have rust installed already, run the normal: cargo build --release

If you don't, or if you need to compile for some ancient glibc, and have podman or docker, run: podman run --rm -v "$PWD":/usr/src/myapp -w /usr/src/myapp docker.io/library/rust:1.62.1 bash -c 'apt-get update && apt-get -y install libfuse-dev && cargo build --release && strip target/release/cache-fs'

There are many ways to set this up, personally I run sudo systemctl start sshd on the Deck and run all these commands via ssh from another computer, but that's optional, this is how I did it.

(Optional): To speed first access up, you can pre-cache your filesystem on the NFS server, or from a computer with a faster (perhaps wired) connection by running cache-fs -c /path/to/server/roms/dir/, this will create a file /path/to/server/roms/dir/cache-fs.tree.zst which will be copied to the cache directory on first run instead of made by scanning the NFS share over Deck WiFi.

Switch to desktop mode, install EmuDeck following instructions from there, copy your compiled cache-fs to /home/deck/cache-fs (I run scp target/release/cache-fs deck@steamdeck:/home/deck/) then run these commands, you'll need to re-run them again after any SteamOS update:

# create the directories to be mounted
mkdir -p /home/deck/Emulation/{romsnfs,roms,roms-cache}

# type your password to get a root console
sudo -i

# link the executable so mount.cachefs works
ln -sf /home/deck/cache-fs /usr/bin/mount.cachefs
# disable readonly fs
steamos-readonly disable
# initialize the pacman keyring
pacman-key --init

# these next 3 commands shouldn't be necessary but are, probably a bug to report to Valve...
# this command will fail, press 'N' to not delete the package
pacman -Sy archlinux-keyring
# delete the signature file so we can install it anyway
rm /var/cache/pacman/pkg/archlinux-keyring-*.pkg.tar.zst.sig
# install the keys, press 'Y' to trust them
pacman -U /var/cache/pacman/pkg/archlinux-*.pkg.tar.zst

# finally install what we were after, nfs-utils that provide mount.nfs
pacman -S --overwrite '*' nfs-utils

Now add the following to your /etc/fstab, I use run the command nano /etc/fstab, obviously replace IP and share with yours:

192.168.1.1:/mnt/deck/roms /home/deck/Emulation/romsnfs  nfs  defaults,ro,soft,timeo=100,retrans=0,retry=0,nodev,noexec,nosuid,noatime,async,v4,noauto,nofail,x-systemd.automount,x-systemd.mount-timeout=10,x-systemd.requires=NetworkManager.service,x-systemd.idle-timeout=1min,_netdev 0     0

/home/deck/Emulation/roms-cache /home/deck/Emulation/roms cachefs defaults,ro,allow_other,remote_dir=/home/deck/Emulation/romsnfs,nofail,_netdev 0     0

Now you can reboot, or just run, (as root):

systemctl daemon-reload
systemctl start home-deck-Emulation-romsnfs.automount home-deck-Emulation-roms.mount

You will now see all of your roms (the same network tree) in /home/deck/Emulation/romsnfs and /home/deck/Emulation/roms, but /home/deck/Emulation/roms-cache will be empty until the first file is accessed and it's copied there, start up EmulationStation and enjoy!

Send me other ways you use it, if your roms are accessible over http, running cache-fs over rclone might be a great way to go, what else?