Tips for measuring Windows 7 VDI IO Requirements

When sizing your storage subsystem for a VDI implementation, it’s extremely critical to understand how your VMs will behave and the resulting IO load. Miscalculate and you will suffer poor performance and angry users. Oversize your array and you will waste money. However, measuring your VM performance may not be as straight forward as you think.

A few months ago I posted a script here that let you dump basic IO performance stats for a VM on vSphere 4 and 5. But as you will see, the applications you load into your image and when you measure the performance has a significant impact on the collected metrics.

For my first round of tests I wanted to focus on the boot performance of Windows 7 64-bit. Booting can be one of the most taxing events (aside from full virus scans) on your VDI storage subsystem. Even if you stagger your VM boots over a few hours as a normal practice, what if you have a power outage or significant hardware failure and you need to rapidly power on hundreds of VMs? Will your storage array melt under the load? Will Windows boot so slowly that it will blue screen (hint: Windows 7 VMs should boot under 5 minutes to avoid problems.) SLAs play an important role here and you need to be mindful of them and verify they can be met.

The test environment is pretty basic and includes vSphere ESXi 5.0, XenDesktop 5.5, and Windows 7 64-bit. The IO measurements were performed over a five minute period after powering on the VM, and metrics were collected via my script. The measurements are only for boot IOs, as no user logged into the VM during the collection process. Tests were performed four times for each scenario and the results averaged. Five scenarios were tested:

  • Base Image: Windows 7 64-bit, Office 2010, VMware tools, joined to a domain
  • VDA Only: XenDesktop 5.5 Virtual Desktop Agent
  • VDA/Symantec: Citrix VDA and Symantec End Point Protection 12.1
  • Optimized: Quest vWorkspace Desktop Optimizer applied with all settings enabled except 15, 26, 27, 30; most VMware Windows 7 optimizations applied.
  • XenDesktop VM: VM created with XenDesktop 5.5 MCS from the optimized template

Drum roll for the results please!

As you can see in the table above, the base Win7 image required an average of nearly 15,000 IOs to boot. 15% of those IOs were writes, while the remainder were reads. Simply installing the XenDesktop VDA decreased the number of write IOs, but increased overall IOs by 17% over the base image. Next up is installing Symantec 12.1, and wow look at those numbers jump! 212% increase in IOs over the base image. Using the Quest and VMware recommended optimizations IOs dropped a bit, but nothing substantial.

What I found to be very interesting is what happened to the IOs when the optimized VM template was cloned by XenDesktop MCS and booted as part of a desktop pool. Zero changes were made to the VM, so the only difference is how the VM behaves when under the control of the Citrix Desktop Studio. Approximately 8000 more IOs are required during the boot process, and a lot more writes are taking place. I would not have guess that large of a delta, so this is an interesting find. The read/write ratio also drops to approximately 80/20.

So what does all of this mean? First, every environment is very unique and you should not use my results, or anyone elses, to estimate the IO load for your environment. Second, take your metrics from a provisioned VDI VM (VMware View, XenDesktop, etc.) and don’t just take measurements from your VM template. Third, booting a VM is very IO intensive and if you only size your storage for steady-state IOPS, then boot storms will cause you major headaches.

Depending on the script/method you use to gather the VM IOPS stats, VMware may not always return the read/write stats in the same fashion resulting in the same order, so you may see inverted data. From my observation this happens on a per-VM basis, even through reboots and power on/off cycles. So if your data looks odd, question it, don’t assume everything is legit.

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