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Re: [Xen-devel] disk throttling

> In this setup, where would stuff like the net filter be placed? In the
> network driver domain? How would one register the IP address of a new
> domain?

The driver is probably the best place to put filtering from a performance 
standpoint.  IP setup could be done by defining some new control messages and 
sending them directly to the driver domain.

You can (in theory) run a net frontend and a backend in the same domain, which 
would allow you to construct something like:

(user VM [vif front])
([vif back] net filtering VM [vif front])
([vif back] driver domain [hardware])

So there's a intermediary third VM doing the filtering.  This would 
unfortunately cost you some performance, although I don't have any numbers.  
I think some groundwork has been laid for doing this already but I'm not sure 
if it can actually be done yet.

> It would be really cool to be able to run without a full Linux just for
> drivers.  How much work do you think it would be to port one of the
> pre-NGIO Xen drivers to run in a separate VM?

Well, the pre-NGIO drivers are basically just Linux drivers with some (varying 
quantity of) tweaks.  Xen provided them with a Linux-like environment to make 
porting easy.  There was talk of modifying the mini-OS to pull in enough of a 
Linux env to support some drivers in a very small memory footprint, which 
might be a good way of tackling this.

Driver domains could run in 3meg anyhow if they don't have any userspace, so 
this isn't going to get huge savings.

> I know you guys focus primarily on server-class machines, but I am
> dreaming of running Xen on a 1000+ node cluster we have here, and with
> the current mem usage of dom0 (according to postings on this list it has
> a hard time coping on 32megs or less), this would waste at least 32
> gigabytes of mem, just for dom0s.

Woah!  Really cool ;-)  And yes, the 32 gig memory footprint is scary.  OTOH, 
you've probably got a couple of terabytes of ram!  I guess the best way if 
you really want to save memory would be a lightweight (possibly more 
specialised) Xend, to enable much smaller dom0s.


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