Deploy and Run a System in a Ramdisk

Abstract

This page provides further information for handling oem images built with KIWI NG and references the following articles:

• Build an Expandable Disk Image

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.

Note

brd support

For this functionality to work, brd (Block Ramdisk Driver) must be supported. If brd is available as a kernel module (as opposed to being compiled in the kernel), you must include the driver into the initrd. To do this, create (or edit) the config.sh file and add the following command to it:

echo 'add_drivers+=" brd "' > /etc/dracut.conf.d/10-brd.conf

KIWI NG allows you to create a bootable ISO image that 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="rd.kiwi.ramdisk">
    <bootloader name="grub2" timeout="1"/>
    <oemconfig>
        <oem-skip-verify>true</oem-skip-verify>
        <oem-unattended>true</oem-unattended>
        <oem-unattended-id>/dev/ram1</oem-unattended-id>
        <oem-swap>false</oem-swap>
        <oem-multipath-scan>false</oem-multipath-scan>
        <oem-ramdisk-size>2048000</oem-ramdisk-size>
     </oemconfig>
 </type>

The type specification above builds an installation ISO image that deploys the System Image into the specified ramdisk device (/dev/ram1). The setup of the ISO image boots with a short boot timeout of 1 second 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 via oem-ramdisk-size specifies the kB size of the OS disk, and it must be at least of the size of the System Image. The disk size can be configured dynamically 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_64-1.15.6.install.iso \
      -serial stdio

Note

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 that gets deployed into the RAM space. Subtract 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 the case of a virtual machine, attach enough main memory to fit this calculation. In the case of QEMU, this can be done with the -m option.

Like all other oem KIWI NG images, the ramdisk setup also supports all the deployment 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.

Note

Limitations Of Ramdisk Deployments

Only standard images that 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 dracut initrd created for the image will boot the image. In the case of a RAM-only system, kexec does not work because it would lose 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.