When an administrator decides to take the plunge and implement vSphere’s Distributed Resource Scheduler tool, a decision needs to be made regarding the level of automation that will be afforded to DRS. There are three levels from which to choose.
Manual
The DRS cluster will make recommendations to an administrator, but no automated actions will be taken. The administrator must manually carry out any recommendations. This is a good setting if you just want to see what impact DRS might have on your environment.
Partially automated
Partially automated DRS clusters are pretty common. Clusters configured for partial automation will automatically place new virtual machines on existing hosts based on performance and resource criteria. However, after the initial placement event, which may result in recommendations to move other workloads to accommodate the new virtual machine, DRS operates the same way that it does when Manual DRS is use.
Fully automated
Many administrators are loathe to allow DRS to simply work to its will through the fully automated option. When this option is selected, DRS will provide initial placement services as described earlier, but it will also move workloads around the cluster without administrator intervention if there is a chance to better balance workloads running inside the cluster. The administrator is able to specify the level of sensitivity using what is called the Migration Threshold. You can configure DRS to initiate a migration when there is any associated performance improvement or you can choose to be a bit more conservative and wait until DRS finds that an operation will have a significant positive impact.
Thanks Mr Scott, a to-the-point blog!