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Based on the score of each VM running on a particular host, the DRS can move the VM to another host, where the score might be better and the workload would be "happier"—yes, the workload's actual happiness, meaning that the workloads can consume the resources they are entitled to.
The DRS calculates intelligent workload placement and balancing across clusters by vMotioning VMs to hosts that can ensure better DRS scores. Internally, the DRS uses a placement decision and evaluates current performance. The DRS calculates a what-if scenario on a different host and verifies the cost of VM migration. If the result is positive and the VM could benefit from a better host that is run more efficiently, the DRS performs a vMotion to the new host. The new focus of these calculations is for the highest VM DRS score—the highest instance on which the VM's resource requirements are being met.
vSphere 6.x and earlier releases used a different model that was cluster-centric. The focus was on hosts and the utilization of host resources.
Cluster DRS score vs. VM DRS score
In fact, vSphere 7 has two notions. There is a VM DRS score and a cluster DRS score. What is the cluster DRS score? It is the average DRS score of all the virtual machines in the cluster. You can see an overview of the cluster DRS score and the VM DRS score in the summary of each of your clusters.
vSphere 7 VM DRS Score
VM DRS score details
The VM DRS score is calculated every minute (compared to every 5 min in previous releases of vSphere). It takes a more granular approach to balancing workloads because it considers other hosts that are able to provide a better score for a particular VM.
The VM DRS score uses metrics such as CPU %ready time, memory swap metrics, and good CPU cache behavior. It uses goodness modeling, which uses an ideal throughput and actual throughput for CPU, memory, and network. During periods of no contention, the ideal throughput of a particular VM is equal to the actual throughput.
Then there are resource costs that lower the VM throughput. The VM DRS score is a combination of how efficient each resource is. The resource cost is used to determine efficiency. The costs of each resource, such as CPU, memory, or network, are added to the cost. One last cost that is considered is the migration cost—when a VM is migrated to another host, it will use some CPU cycles for the operation.
You may not know this, but vMotion can be an "expensive" operation, depending on how many VM memory pages have to be copied to the destination host. vMotion usually consumes a large amount of CPU, memory, and network resources. And don't forget, this is on both the source and destination hosts.
Note: After you migrate from vSphere 6.x to vSphere 7, you might see more vMotion operations due to the new DRS behavior.
vSphere 7 DRS configuration
The three different vSphere DRS automation levels look the same in vSphere 7, but let's quickly recap what they're actually used for. Here are the DRS automation levels that are accessible via the drop-down menu:
- Fully automated—vSphere fully automates the VM placement and migrations. The DRS automatically places virtual machines onto hosts at VM power-on, and virtual machines are automatically migrated from one host to another to optimize resource utilization.
- Partially automated—vSphere places the VMs, but you must click a button to initiate vMotion and PowerON. The DRS automatically places virtual machines onto hosts at VM power-on. Migration recommendations need to be manually applied or ignored.
- Manual—vSphere shows notifications only of recommendations. The DRS generates both power-on placement recommendations and migration recommendations for virtual machines. Recommendations need to be manually applied or ignored.
vSphere 7 DRS automation levels
Configuring the migration threshold
The threshold can be set from 1 to 5. Just drag the bar from Conservative to Aggressive mode. The default (3) is in the middle.
- The DRS will only apply recommendations that must be accepted to satisfy cluster constraints, such as affinity rules and host maintenance. The DRS will not try to correct host imbalance at this threshold.
- The DRS only gives recommendations when workloads are extremely imbalanced or virtual machine demand is not being satisfied on the current host.
- Default—The DRS provides recommendations when workloads are moderately imbalanced. This threshold is suggested for environments with stable workloads.
- The DRS provides recommendations when workloads are fairly imbalanced. This threshold is suggested for environments with bursty workloads.
- The DRS provides recommendations when workloads are even slightly imbalanced and marginal improvement may be realized. For dynamic workloads, this may generate frequent vMotion recommendations.
Configure the DRS migration threshold in vSphere 7
Viewing the VM DRS score
I was wondering whether there was some detailed VM DRS score information visible within the vSphere UI when navigating via the vSphere client. Yes, there are.
You must go and select your cluster. Then select Monitor > VM DRS score under the vSphere DRS section. There is a column that shows the DRS score for the VMs.
Note: The vCLS VMs in my example do not use the DRS and vMotion, so they do not show VM DRS scores at all.
Where to find the VM DRS score for a particular VM
While a VM DRS score close to 0 means poor efficiency, a VM DRS score between 80 and 100% means that there is almost no resource contention.
The VM DRS score is transparent, and you don't need to make any adjustments or special configuration compared to vSphere 6.7 or earlier.
Final words
vSphere 6.7 does not benefit from this new technology. It is only starting with vSphere 7 that the VM DRS score brought this more granular approach to the DRS. There is nothing you need to change on your configuration after you migrate to vSphere 7. The DRS will just have a more granular way to act on workloads within your VMs.
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In vSphere 7, it is the VM that becomes the "first-class citizen". It's not the host, as was the experience with previous releases of vSphere. We always privileged the host's benefit and the consumption of the host's resources. The VM's workloads weren't taken into consideration.
