config.yml

# butane < config.yml > ignition.json
---
variant: flatcar
version: 1.0.0
storage:
  disks:
    - device: /dev/vda
      partitions:
        - number: 9
          label: ROOT
          resize: true
          # 12Gb ~
          size_mib: 11444.09
  filesystems:
    - device: /dev/vda9
      wipe_filesystem: true
      format: ext4
      label: ROOT
systemd:
  units:
    - name: extend-filesystems.service
      mask: true

ignition.json

{
  "ignition": {
    "version": "3.3.0"
  },
  "storage": {
    "disks": [
      {
        "device": "/dev/vda",
        "partitions": [
          {
            "label": "ROOT",
            "number": 9,
            "resize": true,
            "sizeMiB": 11444
          }
        ]
      }
    ],
    "filesystems": [
      {
        "device": "/dev/vda9",
        "format": "ext4",
        "label": "ROOT",
        "wipeFilesystem": true
      }
    ]
  },
  "systemd": {
    "units": [
      {
        "mask": true,
        "name": "extend-filesystems.service"
      }
    ]
  }
}