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Hurdish Sub-Environments vs. Virtualization

From: olafBuddenhagen
Subject: Hurdish Sub-Environments vs. Virtualization
Date: Wed, 21 Nov 2007 14:06:58 +0100
User-agent: Mutt/1.5.17 (2007-11-01)


(Sorry for the crosspost. It is mostly for bug-hurd, but it is also
related to things discussed on the other lists in the past.)

The main idea behind the Hurd architecture, setting it apart from the
monolithical mainstream systems, is the ability for users to modify
and/or extend for themselfs as much of the system behaviour as possible,
without affecting the rest of the system.

Now this sounds rather abstract -- which is probably why people
generally have a hard time seeing how this feature is really relevant in
practice. And I believe that this failure to get the main idea across,
is actually the main reason -- or at least one of the main reasons --
for the Hurd's poor success. Consequently, this is something I have been
thinking about quite a bit.

I am now trying to approach it from an new direction, which allows
expressing the idea in terms I believe people can much better relate to:


For your convenience, I'm including the full text below. So, what do you


Everyone is talking about virtualization now. Well, maybe not your mum;
but almost everybody. OK, probably not your aunt either... Well, you get
the point :-)

Now I tend to be just this tiny little bit sceptical about things
everyone talks about, and thus generally quite late in the game when it
comes to crying "me too!". But I think the time has come when I can join
in without risking my great reputation as an antediluvian freak.

So, coolness factor etc. aside: *Why* is everyone talking about
virtualization? I think the reason is that it offers a very simple,
straighforward solution to a bunch of problems related to various kinds
of isolation.

One very prominent kind is related to security: Mainstream operating
systems (both UNIX derivates and Windows) by default allow any process
in the system to communicate with almost anything else in the system.
The concepts of users and file access permissions provide some limits,
but these are unsuitable to enforce any serious security policy: They
only work under the assumption that software is bug-free, and that users
only run software they absolutely trust.

Bolted-on solutions like SELinux allow to restrict the communications
channels in theory; but these are extremely complex to manage, making
them error prone, and unfeasible to use anything else than simple
default policies provided by the OS vendor.

Hardware virtualization on the other hand provides security in a trivial
manner: Basically, it just cuts *any* communication channels -- (almost)
total security through total isolation. They err on the other side:
Usually you *do* want to have *some* communication, and with VMs you
have to jump through hoops to set it up, e.g. using virtual network

A somewhat related use case is isolation in administrative matters: With
a VM, the guest system is completely independent of from the host
system. It can be configured differently; it can be upgraded without
affecting the host system, as well as the other way round. You can have
different user accounts. And so on.

Again, the cost of total isolation is... Well, total isolation :-) It
means that you *have* to manage each VM individually -- sometimes a
desirable property, sometimes a burden. (And most of the time, a bit of

Last but not least, VMs allow total isolation of interfaces: The guest
system only talks to the (virtual) hardware, and is thus totally
independent of the functionality and interfaces of the host OS -- you
can run a totally different system inside the VM.

Here, the downside of independence is a lot of overhead, and very poor
resource utilization. Paravirtualization cuts this down a bit, but
doesn't fundamentally change the situation.

(This is a blessing for hardware vendors of course -- especially as
standard application vendors lately have been slacking a bit with
bloating their software tomake up for recent increases in processing
power and memory sizes...)

All in all, while hardware virtualization provides total isolation in
all regards, it is often total overkill too -- more isolation than
necessary or desired.

Various kinds of container mechanisms (vserver and OpenVZ in Linux for
example) are an interesting alternative in many situations. Here, you
have a single instance of the system, but several isolated user
environments -- so you get isolation of communication channels, and
usually also some administrative independence (at varying degrees), but
without the overhead of hardware virtualization. (The term "lightweight
virtualization" is sometimes used for that; however, it doesn't seem to
be widely adopted: Google gets some relevant hits, but not really that

What these container solutions can't do (apart from being less robust
against security exploits, due to the common system instance), is
running a different system in the subenvironment.

There are also some specific middleground-solutions like User Mode Linux
or lguest, which allow running another instance of the system, but with
less overhead than true hardware virtualization.

Now let's take a look at the Hurd. It's main feature, compared to
traditional (monolithic) UNIX-like systems, is the fact that almost all
system funtionality isprovided by optional layers (servers and
libraries), which can easily be replaced: Any user or program can run
it's own services instead of using the system-provided ones -- thus
creating a different environment, with little or no overhead, and
without affecting the rest of the system. (This is a tribute to the GNU
philosophy, that a user should always have full control over the
software he runs.)

By default, all processes run in a single standard environment; but upon
demand, any process can be put into some different, more or less
independent subenvironment. There are endless variations: You could run
select processes with distinct instances of some default servers, to
increase robustness and scalability; you could set up containers
isolated from the rest of the system; you could use a different variant
of some server, e.g. a different network stack optimized for some
specific use case; you could run another instance of the whole system
(this is called subhurd or neighbour-Hurd); you could run a special
enivronment, with well defined versions of certain components, to be
sure that a certain feature is present independent of the host system,
or to avoid possibleincompatibilities through changes in the host
system; you could even run a totally different system, having little in
common with the main one. All of this can be done on any running Hurd
installation, without any modification to the host system.

We haven't been expressing these Hurd features in terms of
virtualization up till now. But I think it makes perfect sense to do so:
It seems common practice to describe various facilities of this kind by
the term "virtualization"; and saying that the Hurd is designed from
ground up to support fine-grained virtualization, is certainly more
perspicuous to most people than talking about user extensibility.

So, let's be more buzzword compliant :-) Let's call it advanced
lightweight virtualization.

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