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Xcluster disk image and kernel

Describes howto alter the xcluster kernel and disk image.

The xcluster disk image is shared by all VMs. It is not bootable instead kvm is started with the "-kernel bzImage" option. This makes it possible to select different kernels and it also allows parametes to the kernel to be specified which can be accessed via /proc/cmdline from within the VMs.

Diskim

The disk image and kernel is built using the diskim package. If it is not installed you will get a warning and an instruction when you source Envsettings;

~/xcluster$ . ./Envsettings.k8s 
"diskim" is missing. Install with;

wget -O - -q \
 https://github.com/lgekman/diskim/releases/download/1.0.0/diskim-1.0.0.tar.xz \
 | tar -I xz -C /home/guest/xcluster/workspace -xf -

Extend the image

Extend an existing disk image with new overlays.

Prerequisite; diskim is installed.

The disk image is stored in $XCLUSTER_HOME which defaults to $XCLUSTER_WORKSPACE/xcluster if not set;

$ ls -F $XCLUSTER_WORKSPACE/xcluster
bzImage  cache/  hd.img  hd-k8s.img

You can specify the image with the --image option or the $__image variable. By default hd.img is used. In this example we copy the default image and extend the copy;

export __image=$XCLUSTER_WORKSPACE/xcluster/my-hd.img
cp $XCLUSTER_WORKSPACE/xcluster/hd.img $__image
xc ximage iptools
xc start

Kernel

Prerequisite; diskim is installed.

Download the kernel from https://www.kernel.org/ to $ARCHIVE (default $HOME/Downloads), then;

xc kernel_build

The kernel modules are included in the bzImage in an "init ramfs".

Alter the kernel config

xc kernel_build --menuconfig

New kernel

export __kver=linux-5.18.2
export __kcfg=$MY_CONFIGS/$__kver
cp config/linux-5.18.2 $__kcfg
xc kernel_build --menuconfig

New image

Some programs must be built;

  • BusyBox - The Swiss Army Knife of Embedded Linux
  • Dropbear - Small ssh server+client.
  • iproute2 - For an ip built for the right kernel

The binary release of xcluster contains pre-built versions but if you want to build them do;

xc busybox_build
xc dropbear_build
xc iproute2_build

Now you can build a new image;

Prerequisite; The kernel has been built and diskim is installed.

xc mkimage

Libraries, loader and some programs are included from your host.