[Gllug] Virtual disk allocation advice requested

[Bruce Richardson]

On Mon, Jun 30, 2008 at 08:11:20PM +0100, Richard wrote:
> On Mon, Jun 30, 2008 at 01:43:29PM +0100, Bruce Richardson wrote:
> > For example,
> > it is perfectly possible to ensure that one domU comes up before any
> > others (you simply have to add to or modify the standard Xen start-up
> > scripts) and run the NFS server from there.
> 
> It's quite hard to ensure this.  On my server I start the guests in a
> known order, with extra sleeps between the core guests (NFS, mail,
> DNS) and the rest.  But there could still be a case where a guest has
> to do an fsck or whatever where the services wouldn't be up by the
> time the other guests start.  The next step beyond that is to start
> the NFS guest and test that the NFS service is responding before
> starting the other guests.  I don't know of anything you can download
> which actually does this though.

This is a problem faced by environments not based on Xen, though.
There's no magic answer and you usually have to rely on effective
monitoring to tell you when you do encounter such a failure.

With your entire environment based in the one Xen box (something that
should normally only be happening for a test environment) you do have
some options, though; you could start the key infrastructure domains
using 'xm create -c' and scrape the output, looking for a successful
service startup, or I suppose you could divert the domU serial port to a
pty and have it send a specific success signal over that.

Where you have an environment that uses Xen extensively but it is not a
test environment, the answer is simply not to put your eggs in the one
hypervisor basket.

-- 
Bruce

A problem shared brings the consolation that someone else is now
feeling as miserable as you.
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