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containerd/docs/snapshotters/README.md
Edgar Lee 34d5878185 Use mount.Target to specify subdirectory of rootfs mount 
- Add Target to mount.Mount.
- Add UnmountMounts to unmount a list of mounts in reverse order.
- Add UnmountRecursive to unmount deepest mount first for a given target, using
moby/sys/mountinfo.

Signed-off-by: Edgar Lee <edgarhinshunlee@gmail.com>
2023-01-27 09:51:58 +08:00

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Snapshotters

Snapshotters manage the snapshots of the container filesystems.

The available snapshotters can be inspected by running ctr plugins ls or nerdctl info.

Core snapshotter plugins

Generic:

  • overlayfs (default): OverlayFS. This driver is akin to Docker/Moby's "overlay2" storage driver, but containerd's implementation is not called "overlay2".
  • native: Native file copying driver. Akin to Docker/Moby's "vfs" driver.

Filesystem-specific:

  • btrfs: btrfs. Needs the plugin root (/var/lib/containerd/io.containerd.snapshotter.v1.btrfs) to be mounted as btrfs.
  • zfs: ZFS. Needs the plugin root (/var/lib/containerd/io.containerd.snapshotter.v1.zfs) to be mounted as ZFS. See also https://github.com/containerd/zfs .
  • devmapper: ext4/xfs device mapper. See devmapper.md.

Deprecated:

Non-core snapshotter plugins

Mount target

Mounts can optionally specify a target to describe submounts in the container's rootfs. For example, if the snapshotter wishes to bind mount to a subdirectory ontop of an overlayfs mount, they can return the following mounts:

[
    {
        "type": "overlay",
        "source": "overlay",
        "options": [
            "workdir=...",
            "upperdir=...",
            "lowerdir=..."
        ]
    },
    {
        "type": "bind",
        "source": "/path/on/host",
        "target": "/path/inside/container",
        "options": [
            "ro",
            "rbind"
        ]
    }
]

However, the mountpoint /path/inside/container needs to exist for the bind mount, so one of the previous mounts must be responsible for providing that directory in the rootfs. In this case, one of the lower dirs of the overlay has that directory to enable the bind mount.