Optimize Domain-0 and Memory Overhead for XEN Hypervisors

Question

How can I optimise domain-0 and memory overhead for a XEN hypervisor?

Environment

OnApp 2.x/3.x/4.x/5.x

Answer

To configure domain-0 (XEN hypervisor) and memory overhead, use the steps below.  

We've created a speadsheet with some examples to calculate the values for domain-0 and memory overhead, just enter the total physical RAM for the hypervisor in the first column.

1)  Update the value in grub.conf on the hypervisor:

/boot/grub/grub.conf

title CentOS (2.6.18-238.12.1.el5xen)
         root (hd0,0)
         kernel /xen.gz-3.4.3 dom0_mem=1024000  <= UPDATE THIS VALUE
         module /vmlinuz-2.6.18-238.12.1.el5xen ro root=/dev/VolGroup00/LogVol00
         module /initrd-2.6.18-238.12.1.el5xen.img

NOTE: this value is in kilobytes, so be careful to use the correct units.

2)  Reboot the hypervisor (generally, after migrating any running VMs to another HV or simply stopping any VMs running there)

shutdown -r now

 (Pre-3.x)

3)  Update the memory overhead value in the UI with the following formula:

dom0_ram + (total_ram * .024) + 128 = memory_overhead

NOTE: The RAM values in the equation need to be in MB.

So, for example, a HV with 24GB of physical RAM: 1536 + (24576 * 0.024) + 128 = 2253