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[Xen-devel] Re-reading domain configs on domain restart

  • To: Xen-devel <Xen-devel@xxxxxxxxxxxxxxxxxxx>
  • From: Andy Smith <andy@xxxxxxxxxxxxxx>
  • Date: Tue, 7 Feb 2012 17:04:43 +0000
  • Delivery-date: Tue, 07 Feb 2012 17:05:19 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>
  • Openpgp: id=BF15490B; url=http://strugglers.net/~andy/pubkey.asc


I spoke briefly to Ians C and J about this at FOSDEM and they
suggested I send an email about it.

Behaviour through at least Xen 3.3 to 4.0 on domU restart (i.e.,
when root inside domU does "restart" or "shutdown -r") is to restart
the domU without re-reading the domU config file. As a result, domU
will start up again with only the RAM, block devices, network
routes, etc. that it had when it was last started.

A common sequence of events here is:

- User submits support ticket asking for an extra block device, a
  block of IP addresses routed to them, etc.

- We give them that without shutting their domU down, because there
  is no need to shut it down. We tell them, "next time you need to
  reboot, please take care to actually shut it down and boot it
  again instead, otherwise you'll lose what we just gave you."

- Later -- often much later -- when they need to reboot for some
  reason, the above comment has gone completely out of their heads
  and they reboot.

- domU comes back without the disk, route, whatever they got used to
  having, and completely breaks things for them.

- More support tickets and disruption because of a reboot that
  didn't go as they expected.

No amount of documentation or teaching seems to help as this is
unintuitive behaviour compared to what normally happens when an
admin on bare metal does a reboot.

We're actually on the verge of forcing a shut down just to avoid
this misunderstanding, without any technical requirement for one.
Either that or some horrible script that watches logs for domain
startup and then checks they have all the things they are meant to

As far as I am aware I can set on_restart to something other than
"restart" but there is currently no option that makes it re-read the
config file and restart.

I understand this will never be fixed in xend/xm, but would it be
possible to have xl re-read the domU's config when the domU


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