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February 06, 2014

Best Practices: Configuration of VMware vSphere DRS with VDI

Jim Moyle - Atlantis

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VMware vSphere Distributed Resource Scheduler (DRS) is commonly used in todays virtualized infrastructures.  It is considered to be best practice in server virtualization to enable DRS. I commonly face the question whether this best practice applies to a VDI cluster.

Let’s take a quick look at the individual features DRS gives us in vSphere 5.x

  • Initial placement
  • Load balancing
  • Power management (Integration with Distributed Power Management)
  • Cluster Maintenance mode (support for VMware Update Manager)
  • Constraint correction (user initiated maintenance mode)
  • Support for agent virtual machines

The key to understanding the impact of DRS on a desktop infrastructure is the Load Balancing feature.  Load Balancing takes into account how much CPU and memory is consumed on each of the hosts in the cluster and moves VMs, using vMotion, to correct any imbalance that may happen. This move process has an overhead in terms of additional consumed server resources to move the VM, but this overhead is accommodated in the wider performance gain for a well-balanced server load.

In the world of server virtualization, the VM density is much less than in VDI deployments and the associated resource consumption of VMs is relatively static. The same cannot be said of a desktop VM: its consumption will spike heavily over a period of a few seconds and then decrease to a low level almost instantly. As the default polling interval for DRS is 300 seconds (5 mins), this sample size is too large to account for the short bursts of a desktop demand. In addition, the overhead of moving a desktop VM does not necessarily deliver a well-balanced load across the hosts: indeed it can impact the user experience on your VDI hosts.

So, how do we keep all the useful features of DRS for VDI, but turn off load-balancing based on resources?  Luckily VMware gives us a mechanism to do just that.  When you turn on DRS, there is a slider entitled ‘Migration Threshold’ this is available in both types of admin consoles.

To turn off Load Balancing based on resource usage, slide the bar all the way to the left, Conservative. This means DRS will action moves based on everything except resource imbalances between physical hosts.

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