Re: [patch 2/6] [Network namespace] Network device sharing by view

[Daniel Lezcano <dlezcano@fr.ibm.com>]

> > > My point is that if you make namespace tagging at routing time, and
> > > your packets are being routed only once, you lose the ability
> > > to have separate routing tables in each namespace.
> >
> > Right. What is the advantage of having separate the routing tables ?
>
>
> Routing is everything.
> For example, I want namespaces to have their private tunnel devices.
> It means that namespaces should be allowed have private routes of local type,
> private default routes, and so on...

Ok, we are talking about the same things. We do it only in a different way:

	* separate routing table :
		 namespace
			|
			\--- route_tables
				|
				\---routes

	* tagged routing table :
		route_tables
			|
			\---routes
				|
				\---namespace

When using routes private to the namespace, globally the logic of the ip 
stack is not changed, it manipulates only differents variables. It is 
more clean than tagging the route for the reasons mentioned by Eric.

When using route tagging, the logic is changed because when doing lookup 
on the routes table which is global, the namespace is used to match the 
route and make it visible.

I use the second method, because I think it is more effecient and reduce 
the overhead. But the isolation is minimalist and only aims to avoid the 
application using ressources outside of the container (aka namespace) 
without taking care of the system. For example, I didn't take care of 
network devices, because as far as see I can't imagine an administrator 
wanting to change the network device name while there are hundred of 
containers running. Concerning tunnel devices for example, they should 
be created inside the container.

I think, private network ressources method is more elegant and involves 
more network ressources, but there is probably a significant overhead 
and some difficulties to have __lightweight__ container (aka application 
container), make nfs working well, etc... I did some tests with tbench 
and the loopback with the private namespace and there is roughly an 
overhead of 4 % without the isolation since with the tagging method 
there is 1 % with the isolation.

The network namespace aims the isolation for now, but the container 
based on the namespaces will probably need checkpoint/restart and 
migration ability. The migration is needed not only for servers but for 
HPC jobs too.

So I don't know what level of isolation/virtualization is really needed 
by users, what should be acceptable (strong isolation and overhead / 
weak isolation and efficiency). I don't know if people wanting strong 
isolation will not prefer Xen (cleary with much more overhead than your 
patches ;) )



Regards
	-- Daniel









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