Build an ISO Hybrid Live Image

A Live ISO image is a system on a removable media, e.g CD/DVD or USB stick. Once built and deployed it boots off from this media without interfering with other system storage components making it a useful pocket system for testing and demo- and debugging-purposes.

To add a live ISO build to your appliance, create a type element with image set to iso in your config.xml (see Build Types) as shown below:

<image schemaversion="6.9" name="JeOS-Tumbleweed">
  <!-- snip -->
  <preferences>
    <type image="iso" primary="true" flags="overlay" hybridpersistent_filesystem="ext4" hybridpersistent="true"/>
    <!-- additional preferences -->
  </preferences>
  <!-- snip -->
</image>

The following attributes of the type element are relevant when building live ISO images:

  • flags: Specifies the live ISO technology and dracut module to use, can be set to overlay or to dmsquash.

    If set to overlay, the kiwi-live dracut module will be used to support a live ISO system based on squashfs and overlayfs. If set to dmsquash, the dracut standard dmsquash-live module will be used to support a live ISO system based on squashfs and the device mapper. Note, both modules support a different set of live features. For details see Decision for a live ISO technology

  • hybridpersistent: Accepts true or false, if set to true then the resulting image will be created with a COW file to keep data persistent over a reboot

  • hybridpersistent_filesystem: The filesystem used for the COW file. Possible values are ext4 or xfs, with ext4 being the default.

With the appropriate settings present in config.xml KIWI can now build the image:

$ sudo kiwi-ng --type iso system build \
      --description kiwi-descriptions/suse/x86_64/suse-leap-15.1-JeOS \
      --target-dir /tmp/myimage

The resulting image is saved in the folder /tmp/myimage and can be tested with QEMU:

$ qemu -cdrom LimeJeOS-Leap-15.1.x86_64-1.15.1.iso -m 4096

The image is now complete and ready to use. See Deploy ISO Image on an USB Stick and Deploy ISO Image as File on a FAT32 Formated USB Stick for further information concerning deployment.

Decision for a live ISO technology

The decision for the overlay vs. dmsquash dracut module depends on the features one wants to use. From a design perspective the overlay module is conceived for live ISO deployments on disk devices which allows the creation of a write partition or cow file. The dmsquash module is conceived as a generic mapping technology using device-mapper snapshots. The following list describes important live ISO features and their support status compared to the overlay and dmsquash modules.

ISO scan

Usable in the same way with both dracut modules. This feature allows to boot the live ISO as a file from a grub loopback configured bootloader. The live-grub-stick tool is just one example that uses this feature. For details how to setup ISO scan with the overlay module see Deploy ISO Image as File on a FAT32 Formated USB Stick

ISO in RAM completely

Usable with the dmsquash module through rd.live.ram. The overlay module does not support this mode but KIWI supports RAM only systems as OEM deployment into RAM from an install ISO media. For details how to setup RAM only deployments in KIWI see: Deploy and Run System in a RamDisk

Overlay based on overlayfs

Usable with the overlay module. A squashfs compressed readonly root gets overlayed with a readwrite filesystem using the kernel overlayfs filesystem.

Overlay based on device mapper snapshots

Usable with the dmsquash module. A squashfs compressed readonly root gets overlayed with a readwrite filesystem using a device mapper snapshot. This method was the preferred one before overlayfs existed in the Linux kernel.

Media Checksum Verification

Boot the live iso only for ISO checksum verification. This is possible with both modules but the overlay module uses the checkmedia tool whereas the upstream dmsquash module uses checkisomd5. The activation of the verification process is done by passing the kernel option mediacheck for the overlay module and rd.live.check for the dmsquash module.

Live ISO through PXE boot

Boot the live image via the network. This is possible with both modules but uses different technologies. The overlay module supports network boot only in combination with the AoE (Ata Over Ethernet) protocol. For details see Booting a Live ISO Image from Network. The dmsquash module supports network boot by fetching the ISO image into memory from root=live: using the livenet module.

Persistent Data

Keep new data persistent on a writable storage device. This can be done with both modules but in different ways. The overlay module activates persistency with the kernel boot parameter rd.live.overlay.persistent. If the persistent setup cannot be created the fallback to the non persistent mode applies automatically. The overlay module auto detects if it is used on a disk or ISO scan loop booted from a file. If booted as disk, persistency is setup on a new partition of that disk. If loop booted from file, persistency is setup on a new cow file. The cow file/partition setup can be influenced with the kernel boot parameters: rd.live.overlay.cowfs and rd.live.cowfile.mbsize. The dmsquash module configures persistency through the rd.live.overlay option exclusively and does not support the automatic creation of a write partition in disk mode.

dmsquash documentation

Documentation for the upstream dmsquash module can be found here. Options to setup dmsquash are marked with rd.live