Allow GC to discard content after successful pull and unpack

This commit adds a config flag for allowing GC to clean layer contents up after
unpacking these contents completed, which leads to deduplication of layer
contents between the snapshotter and the contnet store.

Signed-off-by: Kohei Tokunaga <ktokunaga.mail@gmail.com>

docs/config.md

@@ -106,6 +106,10 @@ version = 2
     # This only works for runtime type "io.containerd.runtime.v1.linux".
     no_pivot = false
 
+    # discard_unpacked_layers allows GC to remove layers from the content store after
+    # successfully unpacking these layers to the snapshotter.
+    discard_unpacked_layers = false
+
     # default_runtime_name is the default runtime name to use.
     default_runtime_name = "runc"
 

pkg/config/config.go

@@ -80,6 +80,11 @@ type ContainerdConfig struct {
 	// related information) to snapshotters. These annotations are required by
 	// stargz snapshotter (https://github.com/containerd/stargz-snapshotter).
 	DisableSnapshotAnnotations bool `toml:"disable_snapshot_annotations" json:"disableSnapshotAnnotations"`
+
+	// DiscardUnpackedLayers is a boolean flag to specify whether to allow GC to
+	// remove layers from the content store after successfully unpacking these
+	// layers to the snapshotter.
+	DiscardUnpackedLayers bool `toml:"discard_unpacked_layers" json:"discardUnpackedLayers"`
 }
 
 // CniConfig contains toml config related to cni

pkg/server/image_pull.go

@@ -127,6 +127,12 @@ func (c *criService) PullImage(ctx context.Context, r *runtime.PullImageRequest)
 			containerd.WithImageHandlerWrapper(appendInfoHandlerWrapper(ref)))
 	}
 
+	if c.config.ContainerdConfig.DiscardUnpackedLayers {
+		// Allows GC to clean layers up from the content store after unpacking
+		pullOpts = append(pullOpts,
+			containerd.WithChildLabelMap(containerdimages.ChildGCLabelsFilterLayers))
+	}
+
 	image, err := c.client.Pull(ctx, ref, pullOpts...)
 	if err != nil {
 		return nil, errors.Wrapf(err, "failed to pull and unpack image %q", ref)