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Re: [Xen-users] How many guest hosts per physical host

To: "Fajar A. Nugraha" <fajar@xxxxxxxxx>
Subject: Re: [Xen-users] How many guest hosts per physical host
From: Grant McWilliams <grantmasterflash@xxxxxxxxx>
Date: Fri, 18 Dec 2009 13:31:28 -0800
Cc: svarghese <vargh@xxxxxxxxx>, xen-users@xxxxxxxxxxxxxxxxxxx
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On Fri, Dec 18, 2009 at 12:51 PM, Fajar A. Nugraha <fajar@xxxxxxxxx> wrote:
On Wed, Dec 16, 2009 at 11:39 AM, svarghese <vargh@xxxxxxxxx> wrote:
> Can I create more than 16 guest hosts in my physical host?

Xen supports running a lot of domUs. Last time I experiment with this
I was able to start over 300 PV domUs with 16 MBs memory each. At that
point xenstored uses lots of CPU, and starting another domU takes some
time, so I stopped it there. Note that I use LVM on dom0 as domU's

If you're looking for maximum number of SUPPORTED domUs, you should
ask your linux vendor (if you're using bundled Xen).

In your particular setup, you'd most likely be memory-bound, so the
answer would be NO. Not if each domU uses 1 GB. You need to reduce the
ammount of memory for each domU. dom0 needs some memory for itself, so
you can't allocate all 16GB for domUs.

> If the total
> number of host with 1GB RAM each exceeds 16, does XEN have any feature to
> mange the memory efficiently so that  memory will be allocated to guest host
> based on the load on guest hosts at a particular point of time?

Not by default. You can try


I think it's important to differentiate the number of VMs you can START and the number that can possibly RUN on the system! I imagine there's a limit to the number you can start since there's always a limit somewhere. As far as the number you can run that depends on the load of each VM.

I have one machine that's an 8 core Xeon (32 GB ram) and am running 42 Full CentOS 5.4 PV DomUs with Guis and 512 MB ea. The first 10 take about 30 seconds to boot each, the next 10 take about a minute to boot each, the next 10 take 2 minutes each etc... If I reboot one VM after the other 40 are up it takes several minutes before it's alive again. They do work OK and provide an interactive environment for an entire classroom but if everyone opens Firefox at the same time you can imagine how little we get done for about 5 minutes.

I'm looking to free up resources everywhere I can but it really looks like I'm CPU bound at this point. Having all 40 VMs running even if they're not being used for anything slows down the others.

I was playing with trying to get Xen to do qcow2 and have a base image in a ramdisk for booting/rebooting purposes but failed miserably. Not only does Xen not have full qcow2 support the code it does have doesn't work properly. Not to mention it won't even load a VM disk image out of a ramdisk anyway even without Qcow2.

I'm testing KVM this week in the same environment. So I guess the answer to your question is "it all depends". :-)

Grant McWilliams

Some people, when confronted with a problem, think "I know, I'll use Windows."
Now they have two problems.

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