Auto install with save partition

Hi there!
Can anyone explain how works the storage: section in user-data file. I want to keep one partition in safe if it was there before installation.
I was trying this config:

  storage:
    config:
    - {ptable: gpt, path: /dev/sda, preserve: true, name: '', grub_device: true, type: disk, id: disk-sda}
    - {device: disk-sda, size: 1048576, flag: bios_grub, number: 1, preserve: true,
      grub_device: false, type: partition, id: partition-sda1}
    - {device: disk-sda, size: 524288000, wipe: superblock, flag: linux, number: 2,
      preserve: true, grub_device: false, type: partition, id: partition-sda2}
    - {fstype: ext4, volume: partition-sda2, preserve: false, type: format, id: format-2}
    - {device: disk-sda, size: 4294967296, wipe: superblock, flag: swap, number: 3,
      preserve: true, grub_device: false, type: partition, id: partition-sda3}
    - {fstype: swap, volume: partition-sda3, preserve: false, type: format, id: format-6}
    - {device: format-6, path: '', type: mount, id: mount-5}
    - {device: disk-sda, size: 10737418240, wipe: superblock, flag: linux, number: 4,
      preserve: true, grub_device: false, type: partition, id: partition-sda4}
    - {fstype: ext4, volume: partition-sda4, preserve: false, type: format, id: format-4}
    - {device: format-4, path: /, type: mount, id: mount-3}
    - {device: disk-sda, size: 5905580032, flag: linux, number: 5, preserve: true,
      grub_device: false, type: partition, id: partition-sda5}
    - {device: format-2, path: /boot, type: mount, id: mount-1}
    - {fstype: ext4, volume: partition-sda5, preserve: true, type: format, id: format-5}
    - {device: format-5, path: /backup, type: mount, id: mount-4}
    swap: {swap: 0}

And it works when /backup exist.
But in fresh install the autoinstall is fail. Is it possible to create partition if its not exist and save it if part exist?