Sunday, May 01, 2011

RHEL 6 Virtualization, Memory

There are two memory settings presented in virt-manager: Allocation and Maximum. The Allocation setting is what will appear in /proc/meminfo and top. The Maximum is what can be used to boot the VM. Interestingly, the kernel will refuse to boot with out enough RAM, but once booted, will run with significantly less.

The Maximum value needs to be more than 348MB. Any lower, and boot time is noticeably slower due to swap activity.

The Allocation value needs to be more than 148MB. The VM won't crash until the allocation is about 115MB, but there are several factors that could effect that number. Obviously, 148MB may not meet a every VMs needs, but it seems to be the lowest reasonable limit.

Keep in mind, the Maximum is always allocated to the VM at boot time, and them lowers to the Allocations, so don't set it too high. The Allocation can be dynamically changed for a running VM, but cannot exceed the Maximum. Any changes to the Maximum require a reboot.

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