[CentOS-virt] Reading the new 6.0 manual - now questions

Fri Jun 17 16:16:03 UTC 2011
Dennis Jacobfeuerborn <dennisml at conversis.de>

On 06/17/2011 05:22 PM, Steve Campbell wrote:
> I'm new to running VMs, so I'm hoping this hasn't been a problem
> question asked before. I've only glazed over the emails from this list
> since I've joined due to not really needing all of the great information
> if provides.
> Eric suggested I wait for Centos 6 to start loading my new VM host, and
> so I grabbed the RH Virtualization PDF and started digging in.
> Firstly, it occurred to me that Centos 6 might not provide the
> virtualization rpms like it did with Centos 5. RH makes this an add-on
> to their license. Does anyone know if the upcoming Centos 6 will provide
> the virtualization packages (right away or in the future)?

As far as I understand it these add-ons are all open-source and the RH 
packaging deals more with product support on the RHEL side rather than 
trying to "hide" something. So all these add-ons should be available in 
some form in Centos although I haven't checked the details. KVM is 
available definitely though.

> Secondly, I'm not sure I understand the CPU allocation stuff. If I have
> 6 cores, it appears I can only create VMs that use 6 cores total. Using
> the GUI for creating a new VM will provide me with a max number I can
> allocate. Does this mean that I can allocate, for example,  3 VMs that
> use 2 cores each and never be able to create any other new VMs or does
> this mean I can create as many VMs as I want but only start VMs  that
> use the max total cores or less?

If you have 6 cores available in the machine that means you can only 
allocate a maximum of 6 cores for one individual VM but you can have as 
many VMs as you like.
That means you can create 10 VMs with 6 cores each but you have to keep in 
mind that this means the VMs will be fighting over the 6 physical cores in 
the machine. Basically every time a VM schedules a process to run the host 
needs to find a physical core to run it on or wait until one gets freed up.

In other words: You can over-commit cores but you have to take care that in 
terms of actual work load you don't exceed the actual phyical capacity of 
the system.

> I should get my shiny new machine next week, the one with real
> virtualization capability,  so some of this may be answered on my own by
> playing around, but until then, thought I'd get a head start on
> rectifying my stupidity.

There's nothing stupid about it. The virtualization space is developing 
rapidly right now and what might have been true yesterday in terms of 
features might no longer be true today. Since Centos uses libvirt for 
virtualization you can pay attention to the libvirt mailing list to stay up 
to date with what's going on: