Large virtual machines have a significant overhead on Red Hat Enterprise Linux hosts on IBM POWER8 hardware, which is not accounted for when scheduling the virtual machine. 1/128th of the virtual machine memory size is required to be available on the host as a consecutive non-swapped block of memory. The end-user effect is that sometimes virtual machines do not run, even though the memory on the host seems to be sufficient.
The default maximum memory size for a Red Hat Enterprise Linux host on IBM POWER8 hardware is 1TB. Memory hot plugging is disabled by default, but can be enabled using the engine-config utility.