Automation levels for individual virtual machines can be customized to override the DRS cluster automation level. There are five automation level modes:
Fully Automated
Partially Automated
Manual
Default (cluster automation level)
Disabled
Each automation level behaves differently:
The default automation level is not listed in the table above as it aligns with the cluster automation level. When the automation level of the cluster is modified, the individual automation level is modified as well.
Disabled automation level
If the automation level of a virtual machine is set to disabled, then DRS operations are disabled for that specific virtual machine. DRS does not generate a migration recommendation or generate an initial placement recommendation. The virtual machine will be powered-on on its registered host. A powered-on virtual machine with its automation level set to disabled will still impact the DRS load balancing calculation as its consumes cluster resources. During the
recommendation calculation, DRS ignores the virtual machines set to disabled automation level and selects other virtual machines on that host. If DRS must choose between virtual machines set to the automatic automation levels and the manual automation level, DRS chooses the virtual machines set to automatic as it prefers them over virtual machines set to manual.
Manual automation level
When a virtual machine is configured with the manual automation level, DRS generate both initial placement and load balancing migration recommendations, however the user needs to manual approve these recommendations.
Partially automation level
DRS automatically places a virtual machine with a partially automation level, however it will generate a migration recommendation which requires manual approval.
The impact of manual and partially automation level on cluster load balance
When selecting any other automation level than disabled, DRS assumes that the user will manual apply the migration recommendation it recommends. This means that DRS will continue to include the virtual machines in the analysis of cluster balance and resource utilization. During the analysis DRS simulates virtual machine moves inside the cluster, every virtual machine that is not disabled will be included in the selection process of migration recommendations. If a particular move of a virtual machine offers the highest benefit and the least amount of cost and lowest risk, DRS generates a migration recommendation for this move. Because DRS is limited to a specific number of migrations, it might drop a
recommendation of a virtual machine that provide almost similar goodness. Now the problem with this scenario is, that the recommended migration might be a virtual machine configured with a manual automation level, while the virtual machine with near-level goodness is
configured with the default automation level. This should not matter if the user monitors each and every DRS invocation and reviews the migration recommendations when issued. This is unrealistic to expect as DRS runs each 5 minutes.
We have seen a scenario where a group of the virtual machines where configured with manual mode. It resulted in a host becoming a “trap” for the virtual machines during an overcommitted state. The user did not monitor the DRS tab in vCenter and was missing the migration
recommendations. This resulted in resource starvation for the virtual machines itself but even worse, it impacted multiple virtual machines inside the cluster. Because DRS generated
migration recommendations, it dropped other suitable moves and could not achieve an optimal balance.
Disabled versus partially and manual automatic levels
Disabling DRS on a virtual machines have some negative impact on other operation processes or resource availability, such as placing a host into maintenance mode or powering up a virtual machine after maintenance itself. As it selects the registered host, it might be possible that the virtual machine is powered on a host with ample available resources while more suitable hosts are available. However disabled automation level avoids the scenario described in the previous paragraph.
Partially automatic level automatically places the virtual machine on the most suitable host, while manual mode recommends placing the host on the most suitable host available. Partially automated offers the least operational overhead during placement, but can together with manual automation level introduce lots of overhead during normal operations.
Risk versus reward
Selecting an automation level is almost a risk versus reward game. Setting the automation level
to disabled might impact some operation procedures, but allows DRS to neglect the virtual machines when generating migration recommendations and come up with alternative solutions that provide cluster balance as well. Setting the automation level to partially or manual will offer you better initial placement recommendations and a more simplified maintenance mode process, but will create the risk of unbalance or resource starvation when the DRS tab in vCenter is left unmonitored.
Thoughts on Virtualized vCenter
Most admins like to keep track of the virtualized vCenter server in case a disaster happens.
After a disaster occurs, for example a datacenter-wide outage, you only need to up the ESXi host on which the vCenter virtual machine is registered and then manually power-up the vCenter virtual machine.
An alternative to this method is keeping track of the datastore vCenter is placed on and then registering and powering on the virtual machine on a (random) ESXi host after the disaster. This is slightly more work than disabling DRS for the vCenter Server virtual machine, but it may offer better performance of vCenter during normal operations.
Due to expanding virtual infrastructures and additional features, vCenter is becoming more and more important for day-to-day operational management. Assuring good performance
outweighs any additional effort necessary after a (hopefully) rare occasion, but both methods have merits.
Overall, you should select the automation level in accordance with your environment and level of comfort. Try to have virtual machines in DRS Fully Automated mode as much as possible, as DRS considers these virtual machines for cluster load balance migrations before the virtual machines not in Fully Automated mode.
Impact of VM Automation Level on DRS Load Balancing Calculation
Contrary to popular belief, a virtual machine set to Disabled automation level still impacts the calculation of the Current Host Load Standard Deviation as the sum of the active workload is divided through the capacity of the host. DRS does not need to be aware of virtual machine automation levels at that stage. During the recommendation calculation, DRS ignores virtual machines set to Disabled automation level and selects other virtual machines on that host. If DRS must choose between virtual machines set to the automatic automation levels and the Manual automation level, it chooses the virtual machines set to automatic as it prefers them over virtual machines set to Manual.