[rhelv6-list] A question about memory ballooning

[Phil Meyer]

On 04/24/2011 03:07 AM, carlopmart wrote:
> Hi all,
>
>  How can I manage automatically memory ballooning under a kvm host 
> (RHEL5 or RHEL6)?? For example if I define a kvm guest to boot up with 
> 512MB of RAM and I have configured 1GB as a maximum memory for this 
> guest, how can I allocate this memory when guest will need it??
>
>  And the opposite question, can memory balloon be deallocated?? And is 
> it possible to do this automatically or is an error to do this??
>
> Thanks.

I can offer observation and research results, but I am not involved in 
the programming for KVM.

First, you will notice two entries for memory in the KVM guest xml file.

In order for memory to balloon up, the 'memory' parameter needs to be 
set to the max value you may ever wish to use.

---
<memory>4194304</memory>
<currentMemory>2097152</currentMemory>
---

In this example, the 2GB VM could expand upwards to 4GB, but no greater.

CAVEAT EMPTOR:

This requires the KVM guest OS to recognize a balloon memory driver!  
Many, do not.

So the observed  limitations are these:

1.  You cannot modify the value of 'memory' without a reboot.
2.  You cannot reduce 'currentMemory' without a reboot.
3.  The KVM guest OS must recognize a balloon driver.

The symptoms we observed of a non functioning balloon driver in the 
guest were these:

The VM guest OS assumed 'memory' as available.
The VM guest OS was limited to 'currentMemory' but did not recognize 
changes to 'currentMemory'.

Guest VMs running Linux kernel 2.6.30 or better seemed to work, with the 
exception of Ubuntu 10.4.  Fedora 12 and up all functioned properly.