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Re: [LUG] UML

 

William Fidell wrote:
>
> Seeing as it has just been mentioned on the list re servers, what is the 
> difference between User Mode Linux and Xen?

I'm not sure I'm qualified to answer, but I'll take a stab, and anyone
more qualified can correct the misconceptions.

My understanding that that XEN is a virtual machine solution.

In that XEN provides a "virtual x86" server for the guest OS. Of course
it then does clever stuff to step out of the way as much as possible to
minimize the overhead of virtualization. But it still breaks stuff
enough to require guest OS changes, so not a simple x86 emulator.

UML is more like a architecture port for Linux, that runs on a "Linux
system", not within a virtual machine. So instead of compiling Linux
kernel for x86 or SPARC, you compile it to run in Linux (or UML).

I believe the guest OS in XEN needs a certain amount of compatibility
with the hardware. Where as UML is sending calls to the host kernel, so
can run on anything the host kernel can run on. This isn't as big a
bonus as it might sound, as it means that for anything you want the UML
to interact with, it still needs to know a way of asking the host
"Linux" to do it on behalf of the guest. But then for a lot of server
stuff you don't really care if a guest OS can see a USB mouse, or drive
the monitor, you just care it can write to disk and see the network, at
least for things like email, web, database activity.

Whilst there are technical differences, from the user perspective you
get a computer that looks and feels like a Linux server - so what do you
care?

From a practical perspective if you want virtual Linux boxes, I think
what matters is the management interface. i.e. How easy it is to
provision new servers, stop servers treading on each others resources,
etc etc. Performance (especially I/O) may be an issue, but I think if
you configure UML well, it isn't that bad, and let us face it most
servers are not that busy these days.

Since Steve K knows both, and prefers XEN, I assume XEN has the edge
here, but when we set up DCGLUG box XEN was a tad immature, and UML was
tested, ready and willing.

Certainly SuSE seems to have made adding XEN servers very easy, and I
think XEN seems to have caught more interest, so is likely to see more
development. Although I think Redhat were right to be cautious about
XEN, I've seen some comments that suggest it might not be ready for
Enterprise class work. Sure I'd host a web server on it for a club, but
I would be very wary of putting big company stuff on it, but then I
guess big companies have the resources to test it further and find the
weak spots.

For testing, and such like, virtual machines rock. With the UML disk
image as a file in the file system, you can just "cp" to back-up, and
test any thing you like, and run multiple UML to test different things
at the same time.


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