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RE: Building domains as a lesser user (was Re: [Xen-devel] boot loaders for domain != 0)

On Thu, 3 Feb 2005, Jeremy Katz wrote:

> Linux really seems like a very heavy hammer for something like this.
> Even just thinking from a resource perspective, why boot up a whole
> kernel to do nothing more than read an fs and mount another kernel.

Because it's actually fast, it keeps things simple, and as root file 
systems get more and more complex you're going to be faced with putting 
more and more crud into the boot loader to read the file system, or just 
bite the bullet and use the os to boot the os. 

On our 1700 node opteron cluster, and our 1024 node cluster, and our 256
node clusters, and our many 128 node clusters, we use linux as a bootstrap
all the time. We boot linux from linuxbios and then that first linux boots
the real real linux. And, interestingly enough, it's faster than dedicated
bootstraps like etherboot. Having linux boot linux has been shown, for our 
cases, to be same or faster way to boot than conventional bootstraps. 

For systems like Plan 9, the primary file system (e.g. fossil) is run by a
process, the format is complex (fossil is a write cache for another file
system called venti), and you're just not going to get a simple bootstrap
to parse a fossil partition. So I expect at some point that I'll boot the
domU plan 9 kernel with xm create and I can use that kernel to boot the
real kernel for plan 9 guests. 

(first I gotta catch up with xen 3 and get smp guests working, yikes, you 
xen guys make me work hard to keep up!)

So, I have no idea if I agree with you or not, but the decision is not 
simple, to say the least. 


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