Using Ubuntu 64 bit server

[J]

On Mon, Oct 18, 2010 at 18:31, Larry Alkoff <labradley at mindspring.com> wrote:
> I plan to purchase a 64 bit motherboard and memory
> to run as a server under Ubuntu 64 bit AMD (includes Intel) server
> 10.04.1.  The memory is 16 gigabytes.
>
> The same motherboard will run various OS under KVM virtualization including:
>
> Ubuntu Desktop 32 or 64 bit version
> Windows XP
> Windows 7
> Apple OS X (possibly)
>
> My questions are:
> 1.  Will either 32 bit or 64 bit Ubuntu work well?
>        Which is recommended?

Do you mean for the host OS or for a VM?  I know you said above that
you wanted to use Ubuntu 64bit as the host OS, but this question is
ambiguous enough that you could clarify... However:

If you mean host OS: then 64bit is the way to go in your case, IMO.

If you mean as a VM OS, either one should work fine...  Caveat: I've
not fared well with kvm or qemu on Linux... I've run Xen for years in
Red Hat to varying degrees of success and with my current job I use
VirtualBox.  I tried KVM a while back but could never get the VMs to
run with any sort of stability, when they'd run at all.  SO YMMV, it
could very well have been something bone-headed that I did.  FWIW, I
know KVM does work as I also have access to a server at work that
functions as a KVM host...

> 2.  Will other 32 bit OS like XP work?

Yes.  I've run various 32bit VMs on my 64bit Ubuntu systems with no
problem. I've done the same on the KVM host at work.

> 3.  How does the 64 bit server handle 32 bit code?
>        If by thunking, is there any speed degradation?

No idea, unfortunately...  I want to say that there is some thunking
that occurs, but honestly, that's too far into dark kernel magic
territory for me to give you a reliable answer.

I can say that a VM will never run as fast as a bare metal system,
there will always be a degredation in speed.  Especially when you
start looking at HVM vs Paravirtualized.  There's a reason why
virtualization providers started providing paravirt driver packs for
HVM guests... Perhaps someone more in the know about KVM could explain
whether KVM uses hardware virtualization fully (like Xen does) or if
it's all paravirt.

However, I'm willing to bet that you're going to see more of a
bottleneck in disk I/O than you will anywhere else.  Depending on how
many VMs you're running, and where their filesystems are stored, disk
I/O is far more likely to cause performance issues, in my own
experience.  On my uber-laptop I can run only one at a time (two if
I'm really careful about what each is doing) because they're stored on
a single SATA disk.  On servers in the past through Xen, Ive seen
varying degrees of bottlenecking at the disk level depending on
whether the VMs were stored on a local RAID device, a SAN share, iSCSI
or external SCSI RAID.

Of course, how much ram each uses can also cause performance issues
when you're running more than one VM at a time...

But in any case, while I can't directly answer your question, I can at
least pass that bit of knowledge along.

Cheers,

Jeff