subhurds etc. (was: Hiding nodes with unionmount)

[olafBuddenhagen]

Hi,

On Mon, Aug 03, 2009 at 11:14:32AM +0200, Arne Babenhauserheide wrote:
> Am Montag, 3. August 2009 07:25:38 schrieb Sergiu Ivanov:

> > My understanding is that a sub-Hurd is something like another
> > instance of Hurd running on top of the same instance of gnumach.

Right.

> > It seems that a sub-Hurd may reuse some servers from the main Hurd.

Only very specific ones in the current implementation (terminal and root
store).

Zheng Da implemented a method for using arbitrary device emulation
servers. There is still no readily available mechanism for reusing other
servers.

> As soon as the "subhurd needs root" issue is solved, it should allow
> every normal user to boot his own system. 

Indeed.

> And it would be nice to be able to just use the same base Hurd for the
> main hurd and all subhurds - it needs a readonly mounted partition, so
> it might be possible to use the same partition for different subhurds
> and unionmount the specific files into it. 

This should be possible; but I don't think that subhurd is really the
right tool for this. None of the system-wide mechanisms would works in a
useful fashion this way.

In fact pure subhurds are the most boring use of Hurd mechanisms IMHO --
it's not much different than traditional container solutions like Linux
VServer or OpenVZ. Aside from debugging purposes, they are mostly
interesting as a demonstration of what is possible :-)

It's much more interesting to have a partially customized environment
*without* booting a complete extra system instance; but rather accessing
the main system for most stuff. That's what I'm calling "light-weight
subhurd-like environments" -- but it's all very vague, as many of my
ideas :-(

> We should be careful with talking about that, though: VCS filesystems
> have been ridiculed almost as efficiently as the Hurd :) 

Note that while true version controlling filesystems are rather
unpopular, snapshotting filesystems (ZFS, Btrfs) are quickly becoming
standard.

The difference is that snapshotting is done explicitely at certain
points of time, rather than implicitely on certain events.

-antrik-