[Openstack] [OpenStack][KVM] Calculation of the number of virtual machines on a single node.

Rahul Sharma rahulsharmaait at gmail.com
Wed Aug 28 12:51:54 UTC 2013

Hi Vladimir,

You might also need to check the flags given in nova.conf for
virtual-memory, virtual-cpu to physical-memory, physical-cpu mapping. If
not specified in nova.conf, default value is taken for them. You can have a
look at table 5.22 on this link:-

For eg, as per the table, "ram_allocation_ratio=1.5", this statement states
that virtual memory used by all the instances combined can be 1.5*(physical
memory) present on the system.

Similarly, the ratio for CPU's is 1:16 but its customizable with those

Thanks and Regards
Rahul Sharma

On Wed, Aug 28, 2013 at 5:14 PM, <josh at kleinpeter.org> wrote:

> That depends on how big the VM is, and what over subscription rate you're
> running out.
> Generally RAM is going to be your limiting factor, it is fairly easy to
> over subscribe CPU, and if you're disk is really unlimited, that isn't
> going to stop you either.
> Assuming you're not oversubscribing RAM, you'll need to account for the
> overhead of your operating system and various services, KVM overhead etc,
> say 8-12GB. Subtract that out of your RAM number.
> Then divide by the amount of RAM your base VM has, and that would be how
> many you could (theoretically) run. Your CPU over subscription rate would
> then play in as a factor of how many of those is really possible.
> More importantly -- try it out and see where it breaks! Nothing like a
> good load test.
> > Hello!
> > I'm starting to deploy a private cloud on OpenStack!  As the hypervisor
> > I use nova+KVM on Centos 6.4.
> > Tell me how to count how many virtual machines can be placed on a single
> > KVM node.
> > How to count the number of virtual machines on single node to get
> > everything working perfectly?
> >
> > Hardware configuration:
> > CPU: 2x  Intel(R) Xeon(R) CPU E5-2609  @ 2.40GHz  - 4 core
> > RAM: 96 GB
> > HDD:Unlimited
> > HW: ProLiant BL460c
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