Why don't the symbolic links in kvm autostart not work

Question

I've deployed 2 kvm guests( db and app) on a Debian7 host.

I can set them autostart by

root@mhost:~# virsh autostart db
root@mhost:~# virsh autostart app

This actually creates 2 symbolic links at the /etc/libvirt/qemu/autostart/ :

root@mhost:~# ls /etc/libvirt/qemu/autostart/
db.xml app.xml

Then I thought I can ensure the two guests starting order by simply changing the links name:

root@mhost:/etc/libvirt/qemu/autostart# mv db.xml 10db.xml
root@mhost:/etc/libvirt/qemu/autostart# mv app.xml 20app.xml

but it turns out this doesn't work. Actually after changing the links name, they even don't autostart anymore. What's the problem?

There is a similiar question with answer, but it didn't explain why the symbolic links method not work.

Also, I've tried to use init start script to ensure the order, but it didn't work either. I'm not a shell script guy, so there might be some problem in my script, i.e., the following:

#! /bin/sh
# /etc/init.d/kvmguests

case "$1" in
    start)
        echo "Starting all kvm guests ..."
        virsh start db
        virsh start app
        echo "Done."
        ;;
    stop)
        echo "Stopping all kvm guests ..."
        virsh shutdown app
        virsh shutdown db
        echo "Done."
        ;;
    *)
        echo "Usage: /etc/init.d/kvmguests {start|stop}"
        exit 1
        ;;
esac
exit 0

Answer 1

IMHO I don't think you can control the boot order by changing the names of the symlinks. but there is a workaround

In /etc/sysconfig/libvirt-guests option START_DELAY you can set the delay between boots (so not all vms will start at the same time)

To (sort of) control the order you can change <emulator> tag in domain XML and point to your script, which will start domain after some delay

e.g. something like this:

<domain type='kvm'>
  <name>test</name>
  ...
  <devices>
    <emulator>/usr/local/bin/qemu-kvm-delay</emulator>
  <devices>
</domain>

the content of /usr/local/bin/qemu-kvm-delay could be as simple as:

sleep 1000
exec /usr/bin/qemu-kvm $*

or something fancy as testing for some service availability and calculating the delay for each vm

Answer 2

It's an application level problem, not in infrastructure level.

Actually boot up 2 KVM instance means you power them on, there's no guarantee on the boot up sequence of the 2 operation systems inside those 2 KVM instance in the KVM perspective.

However, you can make it on your own. here's the logic:

1. boot up VM named db, make sure the database application inside this vm works
2. then boot up VM named app. Maybe you need to make sure the apps connected to your database.

Take mysql for example, here's a simple bash script:

#!/bin/bash

# start the database VM
sudo virsh start db

# make sure the database is started
IS_DB_DOWN=true
while $IS_DB_DOWN; do
  # test if the DB is up
  nc -z <your-db-host> 3306 > /dev/null 2>&1
  if [ $? -eq 0 ]; then
    IS_DB_DOWN=true
  fi
  sleep 5 # use 5 sec as internal
done

# start your VM that runs application
sudo virsh start app