Using Ubuntu 64 bit server

Christopher Chan christopher.chan at
Tue Oct 19 01:35:13 UTC 2010

On Tuesday, October 19, 2010 08:16 AM, J wrote:
> On Mon, Oct 18, 2010 at 18:31, Larry Alkoff<labradley at>  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.

I have a server that has 16G of RAM too where I will be running multiple 
Windows servers. Oh, the motherboard does not determine whether things 
are 64-bit or not. The server I use has two AMD Opterons supplying 
24-cores in total, two SAS2 host controllers and will eventually have 36 
disks connected (right now there are only 12, two as system disks, 10 in 
a raidz2 array with one as a spare) and a dual-port Intel gbit nic 
onboard and an addon 4 port intel gbit nic.

>> The same motherboard will run various OS under KVM virtualization including:
>> Ubuntu Desktop 32 or 64 bit version

Why Larry? Just curious.

>> Windows XP
>> Windows 7 there a paravirtualized graphic driver for Windows under KVM 
available yet? No? Ah shucks.

>> Apple OS X (possibly)

/me stares. Let us know if you get that working Larry. :-D

>> 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...

KVM is stable when I tried it on Hardy...just don't expect any 
speed/smooth video playback. Xen on the other a very intrusive 
solution compared to KVM. Never found it stable even with Linux guests.

>> 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.

Jeff, you've got things backwards.

KVM must have hardware virtualization available as it is hardware 
virtualization module but it does support paravirtualize stuff like disk 
and nic i/o via virtio. Xen, depending on version, is most certainly NOT 
hardware virtualization. Xen is initially paravirtualization only and 
therefore only Linux guests and other xen-enabled operating systems were 
supported. It has, of course, added hardware virtualization support in 
version 3.0 and so you can run unmodified Windows on Xen with version 
3.0 onwards.

> 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.

If Larry has enough throughput, virtio should help plenty. I have got 
very decent disk performance under KVM with virtio for Windows. No 
virtio then yes, forget it. It will crawl.

> 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.

You could at least verify all your bits of knowledge before passing them on.

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