[Xen-users] ha cluster with virtual machines as resources

["Sebastian Reitenbach" <sebastia@xxxxxxxxxxxxxxxxxxxx>]

Hi list,

right now I am running a 4 node cluster. The main resources are two NFS 
servers, and two postgresql databases. All 4 nodes are equipped with equal 
hardware. The cluster is managed via linux-ha on sles 10. The linux-ha 
configuration now is a bit complex, and to reduce the complexity, I only 
want to start/stop virtual hosts providing the main resources.

The most promising technologies for the virtual machine are xen and openvz.
Linux-ha provides resource scripts for both, so from that side, it shouldn't 
matter.
Xen has the advantage, it is just included and supported with sles 10, but I 
think openvz has a better resource sharing abilities, due to its nature of 
OS virtualization. 

I tried to start another virtual machine, and the sum of virtual memory then 
was more than physical memory, and the 
   raise VmError(
VmError: I need 3043328 KiB, but dom0 min mem is 200704 and shrinking to 
200704 KiB would leave only 1000220 KiB free.

My problem now is, when the cluster is healthy, then all 4 nodes are 
providing one VM each, where each VM uses e.g. three quarter of available 
RAM. But when then one physical node break, the VM will be unable to start 
on another machine, with the same configuration, because the physical RAM 
will not be enough. Autoballooning, as far as I read and understand, does 
only work for dom0, am I right here?

Is it possible to dynamically adjust RAM of a running xen virtual machine?
As I have read so far, this should work with openvz.

kind regards
Sebastian


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