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RE: [Xen-users] Xen properties

To: tim.post@xxxxxxxxxxxxxxx, "Gaertner, Avilash" <avilash.gaertner@xxxxxxx>
Subject: RE: [Xen-users] Xen properties
From: "Petersson, Mats" <Mats.Petersson@xxxxxxx>
Date: Mon, 25 Sep 2006 11:49:49 +0200
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Thread-topic: [Xen-users] Xen properties

> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Tim Post
> Sent: 25 September 2006 08:20
> To: Gaertner, Avilash
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxx; xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-users] Xen properties
> Probably best to avoid cross posting of general questions :)
> Xen is not Virtuozzo. If you allocate a resource to a guest, that
> resource is for the exclusive use of that guest, unless you're using
> some kind of network block device and shared file system.
> > - Max actice VMs per Host?

There is no hard limit in Xen for the number of VMs per Host. It will
depend on the size of the Xen heap, which is build-time configurable,
although there are side-effects from changing this configuration. In
default configuration, the max number of VMs per host is a few over 100
(32-bit). In 64-bit Xen there is a xenheap_megabytes option that allows
you to set the xen heap size in megabytes, and you can literally have as
many VMs as you have enough memory for - if you only use 1MB per guest,
you could have thousands on a machine with a couple of gigabyte of

Of course, when you say "active", it depends on what you mean. Only one
guest per CPU core will be able to run, and if you try to run five CPU
intensive tasks on 1 cpu, you'll probably see some difficulties in
getting any reasonable amount of work done - because each active domain
would get at most 20% of the CPU-time. No surprise here, Xen doesn't
magicaly create more cpu-performance - at best, it steals a small amount
from the guests... ;-)

The realisticly practical limit would probably be in the range 0.1-3
VM's per CPU in the system, unless there's some particular reason you
KNOW that many domains will almost always be idle, and further that they
are not even remotely likely to be active together for any noticable
amount of time. There are some scenarios where this would be known to be
true, but for other scenarios where the workload per VM is relatively
unknown, the "common worst case" scenario would be determining how many
VM's per CPU you could possibly run.

> > - Max virtual CPUs?

Xen supports 32 physical CPU's, and any guest can have any number of
virtual CPU's from 1 to 32 [whether there is actually any physical CPU
to match it or not - you can actually "pretend" to have 32 CPU's in a
single CPU machine - but it won't be very efficient to do that...]

> > - Max virtual Memory

As much as union of the memory mapping, OS and CPU architecture allows -
but each guest VM will have a set amount of PHYSICAL memory allocated.

> > - Max Virtual Disk Size

Same as Max Virtual memory - there's no limit in Xen (as Xen doesn't
even know what a disk is at the hypervisor level) for how big a disk is
- although Dom0 being Linux will have some physical disk size limit, and
disks being mapped from Dom0 into the guest would obviously depend on
the max physical disk size. 

> > - Max Virtual Disks

Again, I don't believe there's any fixed limit, rather that if you run a
huge number of disks, either CPU, disk interface or memory limits will
cause one or another form of limitation. 
> However you want to divvy up the resources of the host node 
> (dom-0) and
> export any network attached storage present.
> I think reading the Xen documentation would help you more on 
> the basics
> than asking the list :)
> > - flexible configuration for RAM Memory (Scheduler)?
> You can't turn Xen into virtuozzo. You can't turn lead into gold (or
> gold into lead). I fear, however the efforts extolled on both 
> endeavors
> will be disturbingly equal.
> There is a credit scheduler available in unstable that is 
> quite nice and
> preferred if you want to use Xen to re-sell dom-u's (which looks like
> your purpose due to the questions).
> There is no practical way to make Xen more like Virtuozzo, 
> you can *not*
> over-sell resources. Any attempts to do so simply retards 
> Xen. Some have
> reported success with OpenVZ running as a fully virtualized Xen guest,
> but no real feedback on performance. 
> > 
> > Exept the Management Console, is there any different 
> between Xen and Xen
> > Enterpise?
> > 
> You really need to read up on Xen Enterprise, 
> www.xensource.com is your
> best bet with plenty of FAQ's regarding XenEnt as well as a handy link
> to the wiki :) 
> > 
> > Best regards,
> > Avilash
> Best,
> -Tim
> > _______________________________________________
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> > Xen-users@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-users
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