DOKK / manpages / debian 11 / nbdkit / nbdkit-cdi-plugin.1.en
nbdkit-cdi-plugin(1) NBDKIT nbdkit-cdi-plugin(1)

nbdkit-cdi-plugin - export a layer from a container image

 nbdkit cdi [name=]NAME[:TAG|@DIGEST] [layer=N]

"nbdkit-cdi-plugin" (Containerized Data Importer plugin) is a plugin for nbdkit(1) which downloads a container image using podman-pull(1) and makes a single layer available over NBD. The plugin is essentially a convenient wrapper around podman(1) and jq(1) and requires both tools to be installed.

Given a Containerized Data Importer (CDI) registry image containing a VM disk, you can export the whole layer using:

 $ nbdkit cdi docker://kubevirt/fedora-cloud-container-disk-demo

The layer exported over NBD is a tar file:

 $ nbdcopy nbd://localhost - | file -
 -: POSIX tar archive

To export only the VM disk image in this layer, combine this plugin with nbdkit-tar-filter(1). In this example when you run file(1) on the tar entry within the layer it shows that it is a QCOW2 file:

 $ nbdkit cdi docker://kubevirt/fedora-cloud-container-disk-demo \
         --filter=tar tar-entry=./disk/downloaded \
         --run 'nbdcopy "$uri" - | file -'
 -: QEMU QCOW2 Image (v2), 4294967296 bytes

[name=]NAME[:TAG|@DIGEST]
The name or URI of the container image. This is passed to podman-pull(1).

"name=" is a magic config key and may be omitted in most cases. See "Magic parameters" in nbdkit(1).

Export the N'th layer (instead of layer 0).

"TMPDIR"
A temporary copy of the layer is created in "TMPDIR". If this environment variable is not set then /var/tmp is used instead.

$plugindir/nbdkit-cdi-plugin.so
The plugin.

Use "nbdkit --dump-config" to find the location of $plugindir.

"nbdkit-cdi-plugin" first appeared in nbdkit 1.22.

nbdkit(1), nbdkit-plugin(3), nbdkit-tar-filter(1), podman(1), podman-pull(1), jq(1), https://github.com/kubevirt/containerized-data-importer/blob/master/doc/image-from-registry.md.

Richard W.M. Jones

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2021-01-20 nbdkit-1.24.1