Is the hotplug/udev stuff worth the troubles?

Is the hotplug/udev stuff worth the troubles?

[Michael Holzt]

I've joined this list (again) about 10 days ago, because i have severe
problems getting Xen running on an AMD64 system. Unfortunately nobody
answered to my problem description and i was not able to figure it out
myself yet too.

After following the other threads on this list it is, however, my 
impression that i'm only one of many users who experience severe problems 
with the hotplug scripts. Now, Xen 2.0 does really fine without hotplug, 
so i'm wondering if hotplug is really needed.

I'm totally against the concept of having to run hotplug on a server,
especially on a Xen Domain 0. I always strive to make my server setups
rock-solid, and i do not want to have any unnecessary dynamic stuff on 
them. Having dynamically changing setups is in my opinion a major pain 
in the a** when it comes down to server administration and or having to 
fix problems. I also fail to see any advantage of hotplug: I normally 
start a domain 0 once and then it should run forever. There is nothing 
changing in my setup during this, so why do i need hotplug? Now, i don't 
know why you have opted to use hotplug in Xen 3. Maybe it has big 
advantages which i fail to see. Maybe someone who knows can explain 
why hotplug is needed.

But what i really would like to see would be an option to run Xen 3 /
unstable without having to use hotplug, even if this would require a 
little more work when setting up a new domU. I want to have my dom0 
as small (and secure) as possible and being forced to run hotplug is 
in my opionion a big downside of the new version of Xen.

Just my EUR 0.02.

Regards
Michael

-- 
      It's an insane world, but i'm proud to be a part of it. -- Bill Hicks

Re: Is the hotplug/udev stuff worth the troubles?

[Keir Fraser]

On 4 Dec 2005, at 13:57, Michael Holzt wrote:

> There is nothing
> changing in my setup during this, so why do i need hotplug? Now, i 
> don't
> know why you have opted to use hotplug in Xen 3. Maybe it has big
> advantages which i fail to see. Maybe someone who knows can explain
> why hotplug is needed.

Main reason is for dynamic creation of virtual devices. The best 
example is that every guest network interface needs a 'backend' virtual 
interface to be created in domain0, and that needs to be configured 
after creation. The callback for userspace to do that setup (e.g., 
attach to a bridge) is via hotplug. In 2.0 the config (attach to bridge 
xen-br0) was hardcoded in the kernel. Which is a nice simple solution 
but hardly flexible.

For simple setups that really don't want to be running full 
hotplug/udev scripts perhaps a solution is to hijack the hotplug 
callback utility -- e.g., install a very simple script or even C 
program, point /proc/sys/kernel/hotplug at it, and implement fairly 
'dumb' setup policies in there. It's probably less effort to just get 
hotplug running though. :-)

  -- Keir