Virtualization Lies

January 27th, 2010 by Andrew H.

Virtualization offers advantages that take many forms, but with it can come challenges because virtualization lies. The basic premise of virtualization is that the operating system running on a virtual machine is presented with hardware that doesn’t really exist. Performance monitoring on any platform can be complicated and influenced by a variety of factors, but measuring performance on a system that involves lies is even harder. Luckily, most virtualization products have special ways to gather more accurate system performance. This article will focus on Hyper-V, but similar tools are available for other platforms.

Disk

Since Hyper-V supports dynamically expanding disks, the disk capacity of the virtual machine can be misleading. When dynamically expanding disks are used, plenty of free capacity might appear to be available on the virtual machine, but the host’s free space could be very low. For this reason, it’s important to monitor the available capacity of the Hyper-V host as well as the virtual machines. Disk performance can be monitored accurately in Hyper-V virtual machines the same way as physical machines.

Network

Multiple virtual machines can share the same network adapter. Since each virtual machine can only report its own usage, it’s important to watch the usage of all virtual machines. VMs will also report that they are on a 10gbps connection; however, the bandwidth of the link will always actually equal the bandwidth of the external link.

Processor

Processors in virtual machines don’t equal physical processors. To get the most accurate picture of processor utilization of the Hyper-V server, you’ll need to monitor the Hyper-V Logical Processor performance counters. If this counter is less than 75%, but there still appears to be a resource issue, adding additional virtual processors to the virtual machine could improve performance. The Hyper-V Virtual Processor performance counters can help to determine what virtual processors are using physical processor resources.

Memory

Since Hyper-V doesn’t support memory oversubscription, monitoring memory in Hyper-V is one area that can be monitored the same way as physical machines.

Hopefully this list has given you some insight on what to look out for with your own configurations. By knowing the right performance counters to monitor in virtualized environments, it’s much easier to see through the lies and get the best possible performance out of your virtualized environment.