Re: problems with understanding of the memory parameters in the xml file

to briefly summarize what those three knobs do: 1) memory - this is the initial memory size for a VM. qemu grants this amount of memory to the VM on start. This is also the memory the guest is able to use if the balloon driver is not loaded as the balloon driver voluntarily gives up memory from the guest OS to the host. 2) currentmemory - in case the guest is using the balloon driver this is the actual memory size it's using. This field is dynamically updated to reflect the size reported by the balloon driver 3) maxMemory - This knob controls the maximum size of the memory when memory hotplug is used. This basically sets the amount of address space and memory slots the VM has so that new memory can be plugged in later.

The above can also be added during runtime e.g. using virsh attach-device. Hence hotplug. It can also be unplugged during runtime but that requires guest cooperation and there are a few caveats of this. Namely to successfully unplug the memory the guest must not write any non-movable pages into it so that it can give up the memory later. On linux that means that no memory-mapped I/O regions can be created there which may lead to weird guest behaviour if the memory is onlined as movable. I'm not sure how windows behaves in this regard though, but AFAIK it supports memory hotplug just fine. > What i find concerning ballooning is that it doesn't work automatically but has > to be adjusted > manually. Is that right ? No, unfortunately none of this works automatically. > Is my idea right, does that work basically ? If yes how do i have to set the > parameters ? > Is the memory released after the guest has e.g. finished his calculation ? > Does that work automatically or do i have to adjust that manually ? When using the balloon driver you can set the 'currentMemory' size down to some reasonable value and the balloon driver will then return the memory to the host. There were some attempts to make this automatic, but I don't remember how they went. One other caveat is that any memory returned by the balloon driver to the host may be available to the guest again e.g. on reboot when the balloon driver is removed. For a 1 NUMA node guest the memory hotplug an balloon can theoretically be combined but unplugging of the memory might not work while the ballon is inflated. I hope this clarified it somehwat.