Deploy and Run System in a RamDisk

If a machine should run the OS completely in memory without the need for any persistent storage, the approach to deploy the image into a ramdisk serves this purpose. KIWI NG allows to create a bootable ISO image which deploys the image into a ramdisk and activates that image with the following oem type definition:

<type image="oem" filesystem="ext4" installiso="true" initrd_system="dracut" installboot="install" kernelcmdline=" ramdisk_size=2048000">
    <bootloader name="grub2" timeout="1"/>

The type specification above builds an installation ISO image which deploys the System Image into the specified ramdisk device (/dev/ram1). The setup of the ISO image boots with a short boot timeout of 1sec and just runs through the process without asking any questions. In a ramdisk deployment the optional target verification, swap space and multipath targets are out of scope and therefore disabled.

The configured size of the ramdisk specifies the size of the OS disk and must be at least of the size of the System Image. The disk size can be configured with the following value in the kernelcmdline attribute:

  • ramdisk_size=kbyte-value”

An image built with the above setup can be tested in QEMU as follows:

$ sudo qemu -cdrom \
      {exc_image_base_name}.x86_64-1.15.3.install.iso \
      -serial stdio


Enough Main Memory

The machine, no matter if it’s a virtual machine like QEMU or a real machine, must provide enough RAM to hold the image in the ramdisk as well as have enough RAM available to operate the OS and its applications. The KIWI NG build image with the extension .raw provides the System Image which gets deployed into the RAM space. Substract the size of the System Image from the RAM space the machine offers and make sure the result is still big enough for the use case of the appliance. In case of a virtual machine, attach enough main memory to fit this calculation. In case of QEMU this can be done with the -m option

Like all other oem KIWI NG images, also the ramdisk setup supports all the deployments methods as explained in Deployment Methods This means it’s also possible to dump the ISO image on a USB stick let the system boot from it and unplug the stick from the machine because the system was deployed into RAM


Limitations Of RamDisk Deployments

Only standard images which can be booted by a simple root mount and root switch can be used. Usually KIWI NG calls kexec after deployment such that the correct, for the image created dracut initrd, will boot the image. In case of a RAM only system kexec does not work because it would loose the ramdisk contents. Thus the dracut initrd driving the deployment is also the environment to boot the system. There are cases where this environment is not suitable to boot the system.