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overlays.md

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Overlays

In the earliest stage of start all VMs are mounting an iso image (cdrom) that may contain any number of tar-files called "overlays". The files are unpacked on root "/" in sort order and provides a primitive packaging system. The iso image is created from ovl directories. Example;

# Start an xcluster with overlays
xc mkcdrom network-topology iptools; xc start

NOTE; You do not normally install SW on a running xcluster, instead you create an overlay, include it in xc mkcdrom and re-start. This is not what you are used to from other systems but has number of advantages, for instance;

  • You always start from the same point. The system is never "tainted" by some previous action. This is a really big advantage for testing but also for development.

  • The system setup is simple to preserve, re-create and document in the ovl dir.

  • To switch between wastly different setups is done in seconds, for instance switch to ipv6-only for Kubernetes.

An overlay can be created from directory containing a tar script;

> ls -F $($XCLUSTER ovld timezone)
default/ README.md  tar*

The timezone overlay dir is a good template.

Xcluster will search for the ovl directories in the $XCLUSTER_OVLPATH. Copy the timezone dir to some place of your liking and add to the $XCLUSTER_OVLPATH;

mkdir -p $HOME/work/xcluster-ovls
cp -r $($XCLUSTER ovld timezone) $HOME/work/xcluster-ovls/work
export XCLUSTER_OVLPATH="$HOME/work/xcluster-ovls/work:$XCLUSTER_OVLPATH"
# (modify you "work" overlay dir)
xc mkcdrom work; xc start

The "tar" script must create a tar file for the overlay and take the out-file as parameter. The out file may be "-" for 'stdout' which allows a neat trick for checking the contents of an overlay;

$ cd $($XCLUSTER ovld network-topology)
$ ./tar - | tar t
bin/
bin/coredns
etc/
etc/init.d/
etc/init.d/11local-dns.rc
etc/init.d/11hosts.rc
etc/init.d/11xnet.rc
usr/
usr/lib/
usr/lib/network-topology.sh

The SETUP variable

A primitive way to allow variations for overlays. It is up to the "tar" script in the overlay to interpret the SETUP variable (or any other variable). Example;

SETUP=ipv6 xc mkcdrom etcd coredns k8s-config

Overlay cache

Overlays may be cached. This server two purposes;

  1. Speed up overlays that take time to build

  2. Pre-build overlays that are particularly hard to build

Overlays are not cached automatically you must cache them;

xc cache iptools
xc cache --list
xc cache --clear   # Clears the cache

The xcluster binary release contains some cached overlays for Kubernetes setup that you usually should keep.