Unique machine ID for nfs root systems

When multiple hosts running systemd share the same NFS root, they will also share the same machine-id. This can cause issues, especially with journald. Journald logging can be disabled by deleting /var/log/journal which solves most of the problems, but a better solution is to use unique machine-id values on all of the hosts. Modern motherboards provide a unique UUID accessible via DMI, so a reasonable option is to write this out to /run during boot and point systemd at that as the machine-id. Here is a simple way to set that up.

Create the file /etc/initramfs-tools/scripts/init-top/machineid:

#!/bin/sh -e
prereqs() { echo "$PREREQS"; }
case "$1" in
    exit 0
# write sanitized motherboard UUID to /run/machine-id
sed 's/-//g' /sys/class/dmi/id/product_uuid > /run/machine-id
# requires the following symbolic links:
# ln -sf /run/machine-id /etc/machine-id
# ln -sf /run/machine-id /var/lib/dbus/machine-id

Rebuild the initramfs (for example update-initramfs -u) and replace /etc/machine-id and /var/lib/dbus/machine-id with symlinks to /run/machine-id.