Add support to CernVM-FS Snapshotter

docs/cvmfs.md

@@ -0,0 +1,89 @@
+# Lazy-pulling using CernVM-FS Snapshotter
+
+CernVM-FS Snapshotter is a containerd snapshotter plugin. It is a specialized component responsible for assembling
+all the layers of container images into a stacked file system that containerd can use. The snapshotter takes as input the list
+of required layers and outputs a directory containing the final file system. It is also responsible to clean up the output
+directory when containers using it are stopped.
+
+See the official [documentation](https://cvmfs.readthedocs.io/en/latest/cpt-containers.html#how-to-use-the-cernvm-fs-snapshotter) to learn further information.
+
+## Prerequisites
+
+- Install containerd remote snapshotter plugin (`cvmfs-snapshotter`) from [here](https://github.com/cvmfs/cvmfs/tree/devel/snapshotter).
+
+- Add the following to `/etc/containerd/config.toml`:
+```toml
+# Ask containerd to use this particular snapshotter
+[plugins."io.containerd.grpc.v1.cri".containerd]
+    snapshotter = "cvmfs-snapshotter"
+    disable_snapshot_annotations = false
+
+# Set the communication endpoint between containerd and the snapshotter
+[proxy_plugins]
+    [proxy_plugins.cvmfs]
+        type = "snapshot"
+        address = "/run/containerd-cvmfs-grpc/containerd-cvmfs-grpc.sock"
+```
+- The default CernVM-FS repository hosting the flat root filesystems of the container images is `unpacked.cern.ch`.
+  The container images are unpacked into the CernVM-FS repository by the [DUCC](https://cvmfs.readthedocs.io/en/latest/cpt-ducc.html)
+  (Daemon that Unpacks Container Images into CernVM-FS) tool.
+  You can change the repository adding the following line to `/etc/containerd-cvmfs-grpc/config.toml`:
+```toml
+repository = "myrepo.mydomain"
+```
+- Launch `containerd` and `cvmfs-snapshotter`:
+```console
+$ systemctl start containerd cvmfs-snapshotter
+```
+
+## Enable CernVM-FS Snapshotter for `nerdctl run` and `nerdctl pull`
+
+| :zap: Requirement | nerdctl >= 1.6.3 |
+| ----------------- | ---------------- |
+
+- Run `nerdctl` with `--snapshotter cvmfs-snapshotter` as in the example below:
+```console
+$ nerdctl run -it --rm --snapshotter cvmfs-snapshotter clelange/cms-higgs-4l-full:latest
+```
+
+- You can also only pull the image with CernVM-FS Snapshotter without running the container:
+```console
+$ nerdctl pull --snapshotter cvmfs-snapshotter clelange/cms-higgs-4l-full:latest
+```
+
+The speedup for pulling this 9 GB (4.3 GB compressed) image is shown below:
+- #### with the snapshotter:
+```console
+$ nerdctl --snapshotter cvmfs-snapshotter pull clelange/cms-higgs-4l-full:latest
+docker.io/clelange/cms-higgs-4l-full:latest:                                      resolved       |++++++++++++++++++++++++++++++++++++++|
+manifest-sha256:b8acbe80629dd28d213c03cf1ffd3d46d39e573f54215a281fabce7494b3d546: done           |++++++++++++++++++++++++++++++++++++++|
+config-sha256:89ef54b6c4fbbedeeeb29b1df2b9916b6d157c87cf1878ea882bff86a3093b5c:   done           |++++++++++++++++++++++++++++++++++++++|
+elapsed: 4.7 s                                                                    total:  19.8 K (4.2 KiB/s)
+
+$ nerdctl images
+REPOSITORY                    TAG       IMAGE ID        CREATED           PLATFORM       SIZE     BLOB SIZE
+clelange/cms-higgs-4l-full    latest    b8acbe80629d    20 seconds ago    linux/amd64    0.0 B    4.3 GiB
+```
+- #### without the snapshotter:
+```console
+$ nerdctl pull clelange/cms-higgs-4l-full:latest
+docker.io/clelange/cms-higgs-4l-full:latest:                                      resolved       |++++++++++++++++++++++++++++++++++++++|
+manifest-sha256:b8acbe80629dd28d213c03cf1ffd3d46d39e573f54215a281fabce7494b3d546: exists         |++++++++++++++++++++++++++++++++++++++|
+config-sha256:89ef54b6c4fbbedeeeb29b1df2b9916b6d157c87cf1878ea882bff86a3093b5c:   exists         |++++++++++++++++++++++++++++++++++++++|
+layer-sha256:e8114d4b0d10b33aaaa4fbc3c6da22bbbcf6f0ef0291170837e7c8092b73840a:    done           |++++++++++++++++++++++++++++++++++++++|
+layer-sha256:a3eda0944a81e87c7a44b117b1c2e707bc8d18e9b7b478e21698c11ce3e8b819:    done           |++++++++++++++++++++++++++++++++++++++|
+layer-sha256:8f3160776e8e8736ea9e3f6c870d14cd104143824bbcabe78697315daca0b9ad:    done           |++++++++++++++++++++++++++++++++++++++|
+layer-sha256:22a5c05baa9db0aa7bba56ffdb2dd21246b9cf3ce938fc6d7bf20e92a067060e:    done           |++++++++++++++++++++++++++++++++++++++|
+layer-sha256:bfcf9d498f92b72426c9d5b73663504d87249d6783c6b58d71fbafc275349ab9:    done           |++++++++++++++++++++++++++++++++++++++|
+layer-sha256:0563e1549926b9c8beac62407bc6a420fa35bcf6f9844e5d8beeb9165325a872:    done           |++++++++++++++++++++++++++++++++++++++|
+layer-sha256:6fff5fd7fb4eeb79a1399d9508614a84191d05e53f094832062d689245599640:    done           |++++++++++++++++++++++++++++++++++++++|
+layer-sha256:25c39bfa66e1157415236703abc512d06cc1db31bd00fe8c3030c6d6d249dc4e:    done           |++++++++++++++++++++++++++++++++++++++|
+layer-sha256:3cc0a0eb55eb3fb7ef0760c6bf1e567dfc56933ba5f11b5415f89228af751b72:    done           |++++++++++++++++++++++++++++++++++++++|
+layer-sha256:a8850244786303e508b94bb31c8569310765e678c9c73bf1199310729209b803:    done           |++++++++++++++++++++++++++++++++++++++|
+layer-sha256:32cdf5fc12485ac061347eb8b5c3b4a28505ce8564a7f3f83ac4241f03911176:    done           |++++++++++++++++++++++++++++++++++++++|
+elapsed: 181.8s                                                                   total:  4.3 Gi (24.2 MiB/s)
+
+$ nerdctl images
+REPOSITORY                    TAG       IMAGE ID        CREATED          PLATFORM       SIZE       BLOB SIZE
+clelange/cms-higgs-4l-full    latest    b8acbe80629d    4 minutes ago    linux/amd64    9.0 GiB    4.3 GiB
+```

pkg/imgutil/snapshotter.go

@@ -33,6 +33,7 @@ const (
 	snapshotterNameStargz    = "stargz"
 	snapshotterNameNydus     = "nydus"
 	snapshotterNameSoci      = "soci"
+	snapshotterNameCvmfs     = "cvmfs-snapshotter"
 
 	// prefetch size for stargz
 	prefetchSize = 10 * 1024 * 1024
@@ -44,6 +45,7 @@ var builtinRemoteSnapshotterOpts = map[string]snapshotterOpts{
 	snapshotterNameStargz:    &remoteSnapshotterOpts{snapshotter: "stargz", extraLabels: stargzExtraLabels},
 	snapshotterNameNydus:     &remoteSnapshotterOpts{snapshotter: "nydus"},
 	snapshotterNameSoci:      &remoteSnapshotterOpts{snapshotter: "soci", extraLabels: sociExtraLabels},
+	snapshotterNameCvmfs:     &remoteSnapshotterOpts{snapshotter: "cvmfs-snapshotter"},
 }
 
 // snapshotterOpts is used to update pull config