RE: [Xen-devel] xen-unstable on OL6 (RHEL6 clone) problems

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

Dan Magenheimer writes ("RE: [Xen-devel] xen-unstable on OL6 (RHEL6 clone) 
problems"):
> While I agree with your good intentions, I think what happened is
> that you broke something that was working fine for some subset of
> users/developers (RH/Fedora0-based) in order to unbreak something
> for another subset (Debian-based).

No, that's not the case.  There is nothing RH-vs-Debian in this.
RH/Fedora/etc. and Debian/Ubuntu/etc. are affected by this change in
the same way, to the same extent, and with the same upsides and
downsides.

Both rpm-based distros and Debian-derived distros have facilities for
creating bridges as part of their system configuration.  The fact that
they do it in different ways is somewhat unfortunate but irrelevant.

The old bridge setup code (/etc/xen/scripts/network-bridge) was
completely insane.  It attempted to rename the system's eth0, create a
new bridge called eth0, and transfer all of the configuration done by
other unwitting parts of the dom0 from the real physical eth0 to the
bridge.

This was of course incredibly fragile.  Amongst the problems it caused
were races where different bits of the startup ran in different
orders, causing nondeterministic failures (especially if the dom0
interface was being configured by dhcp, as it increasingly is even for
servers).  Also it led to an ever-increasing amount of ad-hoc code for
transferring interface properties from peth0 to eth0; this code kept
rotting, had distro-specific bugs, needed updating every time the
system grew a new networking feature.

And the whole thing didn't work properly if you wanted to do anything
unusual, in which case the admin would end up fighting the nightmarish
Xen bootup scripts and no doubt cursing our name!

> I think we need to make every effort to make Xen as drop-dead
> easy to install and use as possible for as many developers and
> users as possible, NOT a series of test questions on the
> renew-my-alpha-geek-merit-badge exam.

I think this kind of comment is extremely unhelpful.  I have to say
that I don't appreciate the sarcastic attitude.

Of course we are trying to make Xen easier to install and use.  I'm
personally very conscious of the barriers to casual deployment of Xen.

One of the ways we are working on this problem is by removing crazy
fragile hacks from our bootup scripts and instead reusing existing
functionality that distros provide and document.

This does mean that there is some pain when upgrading.  But the payoff
is less pain for new users (who can just be told to use their distro's
bridging setup) and less pain for all future upgrades (because the
bridging setup will be much less likely to break in the future, as it
often has for an unlucky subset of people during Xen upgrades).

Ian.

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