high load and OOM-killer on domUs while pvmove

Situation

• xen 4.0.1 dom0 (debian squeeze)
• domUs all with LVs as disks: disk = [ 'phy:/dev/vg-00/domu-swap,xvda1,w', 'phy:/dev/vg-00/domu-disk,xvda2,w' ])
• one VG (vg-00) with 2 PVs

Goal

• Move all LVs from one PV to the other (pvmove) and remove the "empty" PV (vgreduce)
• Not disturbing any running machine (domU od Dom0)

Problem(s)

When i start pmove (i even tried to ionice -c3 it) my domUs get very high loads or even get stuck. I think this happens, when pvmove is moving the extends from the domUs LV from one PV to the other. I also saw the domU really freaking out an fireing the OOM-killer. Long story short: i had to interupt the procedure (pvmove --abort), because my domU(s) started to get unusable, major server components were killed or they even died/froze completely.

Questions

• I'm aware of higher IO loads while the transition and can cope with this. But even when ionice -c3d the IO load is so high, that inside the domU tasks get blocked. Why isn't the ionice working here? If i understand this correct, all IO is done by the dom0 (by the blkback driver) so the dom0 should see all IO done by every dom(0|U) and should be able toschedule IO for my renice process - are my assumptions wrong here?

• Why does my domU fire up the OOM-killer? How can this process affect the domU's memory? BTW: when the domUs go crazy my dom0 works fine. High IO, but that's obvious.

• Is there any way to remove one PV without the hassel above? Would it be better to shutdown/pause one domU after the other and pvmove only LVs from this machine?

Thank you in advance for every input - i even would be glad to get some "debugging ideas"!