add README.md

Author: siscia

README.md

@@ -0,0 +1,72 @@
+# CVMFS Containerd Snapshotter
+
+**It requires containerd >= 1.4.0-beta.1**
+
+This repository contains a containerd snapshotter that exploits the CernVM-FS to provide the filesystem for the containers.
+
+## Background
+
+From version 1.4.0, containerd introduce the concept of remote snapshotter.
+It allows starting containers in which the filesystem is provided externally from the containerd machinery.
+Without the need to download all the layers for each image. And getting rid of the pulling time.
+
+We exploit this new capability to mount OCI layers directly from a filesystem on the local machine.
+We focus on layers provided by CernVM-FS, but with minor changes is possible to mount layers from any filesystem, like NFS.
+
+Overall this new mechanism should bring down the time to start-up a new container image.
+
+If the layers are not in the local filesystem, `containerd` simply follow the standard path and download the tarball.
+
+## Configuration
+
+This remote snapshotter communicates with `containerd` via gRPC over linux socket.
+The default socket is `/run/containerd-cvmfs-grpc/containerd-cvmfs-grpc.sock`.
+The socket is created automatically by the snapshotter if it does not exists.
+
+It is necessary to configure containerd to use this new remote snapshotter.
+A basic configuration would look like:
+
+```
+# tell containerd to use this particular snapshotter
+[plugins."io.containerd.grpc.v1.cri".containerd]
+  snapshotter = "cvmfs-snapshotter"
+
+# tell containerd how to communicate with this snapshotter
+[proxy_plugins]
+  [proxy_plugins.cvmfs-snapshotter]
+    type = "snapshot"
+    address = "/run/containerd-cvmfs-grpc/containerd-cvmfs-grpc.sock"
+```
+
+## Work in progress
+
+This snapshotter is still a work in progress.
+
+Feel free to fill issues and pull requests.
+
+## Testing
+
+This plugin is tested using `kind`.
+
+```
+$ docker build -t cvmfs-kind-node https://github.com/cvmfs/containerd-remote-snapshotter.git
+$ cat kind-mount-cvmfs.yaml
+kind: Cluster
+apiVersion: kind.x-k8s.io/v1alpha4
+nodes:
+- role: control-plane
+  extraMounts:
+    - hostPath: /cvmfs/unpacked.cern.ch
+      containerPath: /cvmfs/unpacked.cern.ch
+
+$ kind create cluster --config kind-mount-cvmfs.yaml --image cvmfs-kind-node
+```
+
+At this point, it is possible to use `kubectl` to start containers.
+If the filesystem of the container is available on the local filesystem used by the plugin, it won't download the tarball, but just mount the local filesystem.
+
+### Many thanks
+
+Thanks to @ktock and the containerd community for the work on a similar plugin and API.
+
+[https://github.com/containerd/stargz-snapshotter/](https://github.com/containerd/stargz-snapshotter/)