A fuse-overlayfs plugin for using CRFS for loading lower layers.

This is still experimental, use at your own risk.

You need to have both the Go toolchain and the C compiler installed. Use make from the upper directory for building crfs-plugin:

$ make

If everything worked correctly and all the needed dependencies were present, the build generates a single crfs-plugin.so artifact that can be directly used from fuse-overlayfs.

You need to use fuse-overlayfs with plugins support from:

https://github.com/containers/fuse-overlayfs/pull/119

Before using crfs-plugin, you need to have a CRFS image.

To create one, you can use:

stargzify.

You can install it with:

$ go get -u github.com/google/crfs/stargz/stargzify

Once stargzify is installed, you can convert a image to the .stargz format with:

$ stargzify docker.io/fedora localhost:5000/myimages/fedora:stargz

There are a few new options needed to pass to fuse-overlays to work with crfs-plugin:

• fast_ino=1: force fuse-overlayfs to stop looking for an inode number after the first one found during the lookup.
• plugins=/path/a.so:/path2/b.so: list of plugins that fuse-overlayfs loads and can be used to handle lower layers.
• lowerdir=//$PLUGIN/$CONF/lowerdirpath: when a lowerdir path starts with //, fuse-overlay uses the plugin $PLUGIN to manage lowerdir with the specified configuration $CONF, that is specific to the plugin. For crfs-plugin, the configuration is the image/layer to use in base64 encoding.

Accepted configuration strings are:

• https://www.example.com/layer.stargz: use directly the layer at the URL.
• docker://docker.io/image:stargz: use the image at the specified registry.
• /path/to/the/file.stargz: use the file as a .stargz layer.

The easiest way run fuse-overlayfs is from a user namespace (or if you like danger, directly as root user).

When an image docker:// is used, fuse-overlayfs will internally create more lower layers depending on how many there are present in the image.

podman unshare can be helpful to create a user+mount namespace that can be used by an unprivileged user.

$ podman unshare
# mkdir lower upper workdir merged
# fuse-overlayfs -o fast_ino=1,plugins=/path/to/crfs-plugin.so,lowerdir=//crfs/$(echo -n docker://localhost:5000/myimages/fedora:stargz | base64 -w0)/$(pwd)/lower,upperdir=upper,workdir=work merged
# podman run --rm -ti --rootfs $(pwd)/merged echo hello

It is possible to use directly a URL for retrieving a .stargz layer. When the lowerdir location starts with the https:// prefix, then the file at that URL will be used directly.

# fuse-overlayfs -o fast_ino=1,plugins=/path/to/crfs-plugin.so,lowerdir=//crfs/$(echo -n https://url/to/stargzified-layer.tar.gz | base64 -w0)/$(pwd)/lower,upperdir=upper,workdir=work merged

Alternatively, it is possible to use a local file:

# fuse-overlayfs -o fast_ino=1,plugins=/path/to/crfs-plugin.so,lowerdir=//crfs/$(echo -n /path/to/stargzified-layer.tar.gz | base64 -w0)/$(pwd)/lower,upperdir=upper,workdir=work merged

make install will install crfs-plugin.so under $PREFIX/libexec/fuse-overlayfs/.

fuse-overlayfs will automatically load the plugins from that location, without requiring the additional -oplugins=... configuration at run time.