A: Storage DRS, a new feature in VMware vSphere 5, can be tasked with the responsibility of load balancing virtual machine (VM) disk files with the goal of reducing storage I/O latency. This reduction can have a significant impact on overall VM performance. When enabling Storage DRS, however, you must identify a threshold in milliseconds at which its services begin taking action.

Although the default setting for I/O latency is set to 15 milliseconds, your threshold will be different based on storage architecture and the types of disks being used. At a recent VMworld session on storage I/O control, the following thresholds were offered as starting points for where this slider should be set:

  • Solid state disks (SSDs): 10 to 15 milliseconds
  • Fibre Channel and SAS disks: 20 to 40 milliseconds
  • SATA disks: 30 to 50 milliseconds

Obviously, your mileage may vary.