Re: [Xen-users] how to start VMs in a particular order

[Simon Hobson <linux@xxxxxxxxxxxxxxxx>]

lee <lee@xxxxxxxxxxxxxxx> wrote:

>> Change Required-Start to be "$network $remote_fs $syslog $wait_for_dns", 
>> rerun the init-script setup, and then you have a system that would pause for 
>> the DNS to be available before it started the ntp service.
> 
> You'd also have to specify which other hosts to notify and which of all
> the hosts need to wait for which others.

No, you don't notify anything.
Every system that relies on <something else> does it's own thing - pausing it's 
own boot until the service *IT* requires is available. Absolutely no inter-host 
communication needed other than to detect if the service is available yet.

So with the example I was working with, the system would pause until whatever 
DNS service you've configured it to look for becomes available - regardless of 
whether it's bare metal, a Xen guest (same host or different host), or even an 
external service. That makes it a general purpose tool - unlike trying to get 
Xen to start guests in a particular way which will only work for guests on one 
Xen host.

The downside is that each system needs managing - so potentially every system 
running NTP would need the same mods (but that's where config/automation tools 
can help).
The upside is that you don't have some web of management and notifications 
(which would require the management system to come up before anything else 
could boot) - just a set of boot time scripts on each host that has some 
dependencies on "off host" services.


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