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Writable layer

Overlaybd writable/container layer can be used for image build/conversion, and container runtime.

There are two kinds of writable layer, append-only based and sparse-file based:

  • append-only

    It is a writable layer in the form of log-structure that data is appended to the end of the data file with index update. It has high performance but the size will increase without gc (not implemented yet). It is suitable for image conversion and image building.

  • sparse-file

    Is is a writable layer based on sparse file. It is suitable for container runtime.

    This feature is incomming.

There are two kinds of mount mode, dir mod and dev mod:

  • dir

    It returns a bind mount when calling snapshotter Mounts, the image with writable layer will be mount to a dir before Mounts return. This dir can be used as rootfs directly.

  • dev

    It returns a mount of a writable device with fs type parameter when calling snapshotter Mounts. It is up to the caller to decide when and where to mount the device. This device can be passed to a secure container or a vm runtime through virtio-blk.

Usage

  • for containerd/ctr 1.6+

    Just pass the parameter through --snapshotter-label, by setting dev or dir to containerd.io/snapshot/overlaybd.writable. #5660

    ctr run --net-host --snapshotter=overlaybd --rm -t --snapshotter-label containerd.io/snapshot/overlaybd.writable=dev registry.hub.docker.com/overlaybd/redis:6.2.1_obd test_rw

  • for containerd api

    snOpts := snapshots.WithLabels(map[string]string{"containerd.io/snapshot/overlaybd.writable": "dev"})
redis, err := client.NewContainer(context, "redis-master",
	containerd.WithSnapshotter("overlaybd"),
	containerd.WithNewSnapshot("redis-rootfs", image, snOpts),
	containerd.WithNewSpec(oci.WithImageConfig(image)),
)

  • for proxy snapshotter plugin config

    {
    "root": "/var/lib/containerd/io.containerd.snapshotter.v1.overlaybd",
    "address": "/run/overlaybd-snapshotter/overlaybd.sock",
    "rwMode": "dev", // overlayfs, dir or dev
    "writableLayerType": "sparse" // append or sparse
}