[libvirt] Memory

Ryota Ozaki ozaki.ryota at gmail.com
Sat Jul 17 12:43:16 UTC 2010

On Saturday, July 17, 2010, Jaromír Červenka <cervajz at cervajz.com> wrote:
> Hi ozaki-r,
> thank you for answer.
> So, there is no posibility that guest can request for more memory (max up to maxMemory) and libvirt/hypervisor/whatever will automatically increase it? I have to do it manually?

AFAIK yes, it has to be down by you or some other resource control
program. Both libvirt and the hypervisor (qemu)
just provide API to change. (Of course your system can provide such
feature using the API if you implement it.)


> Jaromir.
> Dne 17. července 2010 13:49 Ryota Ozaki <ozaki.ryota at gmail.com> napsal(a):
> Hi Jaromír
> 2010/7/17 Jaromír Červenka <cervajz at cervajz.com>:
>> Hello,
>> Could anybody explain me a little bit what is the difference between
>> <currentMemory> and <maxMemory>. I use qemu-kvm hypervisor. When I have
>> 1024MB maxMemory and 512MB of currentMemory, I can see just 512MB in guest
>> os (free -m). Is it ok? Does it have something to do with ballooning
>> technique? What happened if guest will needs more memory than 512MB?
> In qemu-kvm case, currentMemory means actual amount of memory (or initial amount
> of memory) which you allow the guest to use and maxMemory means the maximum
> amount of memory which you (or admin) can increase up to, through libvirt.
> With your setting, libvirt launches the VM with 1024MB memory and then reduces
> the amount of memory down to 512MB through balloon driver. So 512MB which
> you're seeing in the guest is correct and expected. Because the rest
> of the memory
> has been reclaimed by the hypervisor, the guest cannot gain more memory than
> 512MB. The answer for the question--what happened?--is that the guest OS will
> behave as same as when it has just 512MB memory and gets memory pressure,
> i.e., reclaim memory, do swapping, or something.
> If you want to allow the guest to gain more memory, at first you need
> to increase
> the amount of guest memory through the balloon driver (you know you can change
> the amount up to maxMemory at maximum). You can use virsh setmem for that.
>   ozaki-r
>> Thank you for answers :)
>> Kind regards,
>> Jaromír Červenka
>> Official openSUSE community member
>> Web: http://www.cervajz.com/
>> Jabber: cervajz at cervajz.com
>> MSN: jara.cervenka at seznam.cz
>> Tel.: +420 607 592 687
>> Alt. e-mails:
>> jaromir.cervenka at opensuse.org,
>> jaromir.cervenka at speel.cz
>> --
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