Configuring Distributed Resource Scheduler (DRS) in vCenter is a crucial step in optimizing resource utilization and load balancing in VMware clusters. DRS is a feature designed to efficiently distribute virtual machines across ESXi hosts, ensuring optimal performance and resource availability. By automatically migrating virtual machines based on resource demands, DRS helps maintain a balanced workload and prevents resource contention issues. In this article, we will explore the process of configuring DRS from start to finish, covering essential prerequisites and various configuration options.
What is VMware DRS
DRS is a clustering feature designed to efficiently utilize resources and balance the load across ESXi hosts in a cluster. This feature is designed to manage the efficient utilization of the available resources in a cluster to ensure that each VM has enough computational resources for optimal program performance.
If an ESXi host is overloaded with too many virtual machines, it can lead to sluggishness and various issues due to resource limitations. With DRS configuration, virtual machines are moved to other hosts with their own specific resource configuration settings to achieve resource balancing across the entire cluster. The VMware vMotion feature is used to perform the migration of virtual machines transparently from one host to another for DRS operations.
Mandatory prerequisites for setting up DRS
The prerequisites for DRS are the same as the general requirements for a vSphere cluster and include the following:
- Shared storage: This is the storage accessible by all cluster nodes and used for storing VM files.
- Processor compatibility: Since DRS utilizes vMotion, which migrates the running CPU state between different ESXi hosts, the destination hosts must have at least CPU compatibility.
- Hosts in the cluster must be configured in the vMotion section.
How to enable DRS
1. Log in to VMware vCenter.
2. Navigate to your cluster using the Hosts and Clusters view (left-hand side).
3. Click on the vSphere DRS section under the Configure tab. DRS is disabled by default. Click on EDIT.
4. In the displayed window, you can configure the DRS settings.
- From the top section, you can enable or disable DRS.
- Automation: The Automation tab includes DRS automation settings. The automation level can be chosen to determine the extent of control DRS has over migration resources. In the Automation Level section, there are three available modes:
- Fully automated: This mode fully automates the placement and migration of VMs within a cluster. If DRS detects an imbalanced cluster and can improve the situation, VM migration will start automatically.
- Partially automated: Initial VM placement recommendations are automatically applied when powered on. After powering on, when DRS provides migration recommendations to balance the load, you need to manually apply them.
- Manual: In this mode, DRS provides recommendations for initial VM placement and migration. You need to manually apply these recommendations when you agree with them, and no automatic actions are performed.
- Migration Threshold: This parameter is used to adjust the sensitivity level for DRS behavior when conditions for VM migration exist and to define how the cluster tolerates imbalance.
There are five sliding positions for configuring the migration threshold, ranging from the most conservative to the most aggressive (sliding positions from left to right):
1. With the most conservative setting, DRS tolerates imbalance and initiates VM migrations to satisfy affinity rules when entering maintenance mode for an ESXi host.
2. If a vSphere cluster has an imbalanced workload or insufficient resources to meet VM requirements, DRS provides recommendations.
3. The middle position on the slider represents the default value for DRS. It allows you to have a balanced distribution of VM migrations and resource distribution within the cluster. This value is often the optimal threshold value for configuring VMware DRS.
4. DRS provides recommendations when the workload in the cluster is relatively imbalanced. You can use this threshold position for clusters with bursty workloads.
5. The most aggressive migration mode initiates VM migrations even if it achieves minimal load balancing benefits and performance improvements. The most aggressive mode leads to frequent VM migrations within the DRS cluster. DRS does not tolerate any level of imbalance in this mode.
Predictive DRS: This feature is only used with vRealize Operations Manager. It combines Distributed Resource Scheduler and vRealize Operations Manager (with the Dynamic Thresholds feature) to predict cluster-wide resource demands in the future and migrate VMs before the cluster becomes imbalanced. Predictive DRS initiates VM migrations before resource contention occurs. This capability can be enabled or disabled.
Virtual Machine Automation: You can configure custom automation settings for individual virtual machines and override the cluster’s automation settings. A VM can have different automation settings than the cluster’s DRS automation settings. Select the checkbox to enable this option.
Additional Options: Additional options for DRS settings allow you to configure VM distribution, CPU over-commitment, and scalable shares. Useful VMware DRS settings are available in this tab.
VM Distribution: When you turn on VM Distribution, it spreads out the VMs across the ESXi hosts in the cluster. This helps improve availability and can reduce the need to move VMs around too much.
CPU Over-Commitment: This feature allows you to adjust the number of virtual processor cores that you can provision based on the available physical processor cores. By using CPU over-commitment, you can set the maximum processor commitment ratio or the maximum over-commitment ratio. This allows you to set a threshold for the virtual machines on a host. When the maximum commitment exceeds the configured value, DRS initiates the process of migrating VMs to another host. This is done to maintain a balanced utilization of the processor.
Scalable Shares: This feature enables the use of additional shares beyond the share limit assigned to the cluster. For example, when creating a resource pool and defining shares and limits for virtual machines, when the share limit for the resource pool is reached in the cluster, DRS can request additional resources from other clusters. If the resources are accepted, they will be provided.
VM overrides in Vcenter
To apply different DRS settings to a specific VM, you need to overwrite the previous settings. This can be done using the following steps:
- Click on the desired cluster and then select the “Configure” tab from the opened menu.
- Choose the “VM Overrides” section and select the “Add” option.
In the first part, select the specific virtual machine for which you want to apply exception settings. In the second part, check the “overrides” option and apply the desired changes. For example, in this example, I have exempted the WDS virtual machine from DRS settings.
Conclusion
Configuring DRS in vCenter is essential for achieving efficient resource utilization and load balancing in VMware clusters. By enabling DRS and selecting appropriate automation levels, migration thresholds, and other options, administrators can ensure that virtual machines are distributed optimally across ESXi hosts. This results in improved performance, better resource utilization, and enhanced availability. Additionally, the ability to customize DRS settings for individual virtual machines provides flexibility and allows exceptions to be made when necessary. With a well-configured DRS setup, organizations can maximize the benefits of virtualization and create a robust and balanced virtual infrastructure.
