Knowledge Base

What is VMware vSphere DRS and What Does it Do?

VMware vSphere Distributed Resource Scheduler (DRS) is a feature that balances and optimizes the resource utilization of virtual machines in vSphere clusters. DRS continuously monitors virtual machines and automatically moves them to the most suitable ESXi host (using vMotion) based on their resource requirements. This ensures more efficient use of resources, prevents performance issues, and reduces the workload of system administrators.

• Resource Balancing: DRS monitors the resource utilization of virtual machines, detects imbalances between hosts, and automatically corrects them.
• Automatic Placement: When a new virtual machine is created, DRS determines the most suitable host, optimizing the virtual machine's performance.
• Continuous Optimization: DRS continuously monitors virtual machines and moves them to the most suitable host based on their resource requirements.

Example: Suppose there are two ESXi hosts in a vSphere cluster. Host 1 has 90% CPU utilization, while Host 2 has 30% CPU utilization. DRS will balance the CPU utilization of both hosts by moving some of the virtual machines from Host 1 to Host 2. This prevents performance issues on Host 1 and allows the resources on Host 2 to be used more efficiently.

How Does DRS Work?

DRS works by following these steps:

1. Resource Monitoring: DRS continuously monitors the resource utilization (CPU, memory, disk I/O, network I/O) of all ESXi hosts and virtual machines in the vSphere cluster.
2. Imbalance Detection: DRS detects resource utilization imbalances between hosts. When these imbalances exceed certain threshold values, DRS initiates a balancing operation.
3. Recommendation Generation: For the balancing operation, DRS determines which virtual machines should be moved to which hosts. These recommendations are generated taking into account the resource requirements of the virtual machines, the capacity of the hosts, and the cluster policies.
4. Automatic or Manual Migration: Depending on its configuration, DRS moves virtual machines automatically or with the approval of the system administrator. The migration process is performed using vMotion technology.

Visual Explanation: DRS can be thought of as an orchestra conductor. The conductor monitors the performance (resource utilization) of each musician (ESXi host) in the orchestra and, if necessary, changes the positions of the musicians (moving virtual machines between hosts), optimizing the overall performance of the orchestra (vSphere cluster).

Important Point: DRS moves virtual machines using vMotion technology. Therefore, vMotion must be configured correctly for DRS to work.

How to Install and Configure DRS?

Follow these steps to install and configure DRS:

1. Create a vSphere Cluster: First, create a vSphere cluster that includes the ESXi hosts you want to use DRS with.
2. Enable DRS: Enable DRS from the cluster settings via the vSphere Web Client or vSphere Client.
3. Determine the Automation Level: Determine the automation level of DRS. The automation level determines how automatically DRS will perform balancing operations. There are three different automation levels:
   • Manual: DRS only generates recommendations. The system administrator manually performs the migration operations.
   • Partially Automated: DRS generates recommendations and performs migration operations after receiving approval from the system administrator.
   • Fully Automated: DRS generates recommendations and performs migration operations automatically.
4. Configure Cluster Policies: Configure cluster policies that determine how DRS will operate. These policies determine which hosts virtual machines can run on, resource reservations, and other factors.
5. Adjust Threshold Values: Adjust the threshold values at which DRS will initiate balancing operations. These threshold values can be set separately for resources such as CPU usage, memory usage, disk I/O, and network I/O.

Code Example (PowerCLI):

# Enable DRS
$cluster = Get-Cluster "ClusterName"
$drsConfig = New-Object VMware.Vim.ClusterDrsConfigSpec
$drsConfig.Enabled = $true
$cluster.ExtensionData.ReconfigureComputeResource_Task($drsConfig, $true)

# Set the automation level
$drsConfig.DefaultVmBehavior = "FullyAutomated" # or "PartiallyAutomated" or "Manual"
$cluster.ExtensionData.ReconfigureComputeResource_Task($drsConfig, $true)

Most Common Problems and Solutions Related to DRS

Here are some common problems and solutions that may be encountered while using DRS:

• vMotion Issues: Since DRS uses vMotion technology, vMotion-related issues can prevent DRS from working. In this case, check the vMotion configuration and fix any problems.
• Resource Constraints: Resource constraints on virtual machines can affect DRS balancing operations. For example, if a virtual machine needs to run on a specific host, DRS will take this constraint into account.
• Overload: DRS tries to balance the resource usage of all hosts in the cluster. However, if the hosts in the cluster are overloaded, DRS balancing operations may fail. In this case, add more resources to the hosts in the cluster or reduce the resource requirements of the virtual machines.
• Compatibility Issues: If there are compatibility issues between different ESXi hosts (CPU compatibility, network compatibility, storage compatibility), DRS may not be able to move virtual machines. You can resolve these issues by using the Enhanced vMotion Compatibility (EVC) feature.

Real-Life Example: In the vSphere cluster of an e-commerce company, some ESXi hosts were overloaded during peak traffic periods. Thanks to DRS, virtual machines were automatically moved to less loaded hosts, improving website performance and enhancing user experience.

DRS and Affinity/Anti-Affinity Rules

Affinity and anti-affinity rules are policies that allow virtual machines to run on specific hosts or stay away from specific hosts. DRS performs balancing operations taking these rules into account.

• Affinity Rules: Allows one or more virtual machines to run on the same host. For example, it can be used to ensure that a database server and its associated application server run on the same host. This reduces network latency and increases performance.
• Anti-Affinity Rules: Allows one or more virtual machines to run on different hosts. For example, to ensure high availability (HA), it can be used to ensure that multiple virtual machines of the same application run on different hosts. This ensures that the application continues to run when a host fails.

Important Point: Affinity and anti-affinity rules can restrict DRS balancing operations. Therefore, configure these rules carefully and avoid unnecessary rules.

DRS Reporting and Monitoring

vSphere offers detailed reporting and monitoring features related to DRS. Thanks to these features, you can track how DRS works, which balancing operations it performs, and resource usage.

• DRS Cluster History: Shows the history of balancing operations performed by DRS. This allows you to see which virtual machines were moved to which hosts, when they were moved, and why.
• Resource Usage Charts: Provides charts showing the CPU, memory, disk I/O, and network I/O usage of ESXi hosts and virtual machines. These charts allow you to track trends in resource usage and identify potential performance issues.
• Alarms: You can create alarms when resource usage thresholds are exceeded or when problems occur with DRS. These alarms allow you to proactively resolve issues and optimize the performance of your system.

DRS and Integration with Other vSphere Features

DRS works integrated with other vSphere features, and this integration provides more advanced resource management and optimization features.

• vSphere HA (High Availability): When DRS and HA are used together, if an ESXi host fails, the virtual machines on that host are automatically restarted on other hosts. After this restart, DRS automatically performs balancing operations to balance the resource usage of the virtual machines.
• vSphere DPM (Distributed Power Management): When DRS and DPM are used together, when resource usage in the cluster is low, DRS automatically puts some ESXi hosts into standby mode. This saves energy. When resource needs increase, DRS automatically reactivates the hosts in standby mode.
• Storage DRS (SDRS): While DRS manages compute resources (CPU, memory), SDRS manages storage resources (datastores). SDRS optimizes the use of storage space and improves performance by moving virtual machine disk files between datastores.

Tips and Best Practices for DRS

Consider the following tips and best practices to get the most out of DRS:

• Choose the Right Level of Automation: Determine the level of automation for DRS based on your system's requirements and the skills of your system administrators. Fully automated mode is the best option in most cases, but partially automated or manual mode may be more appropriate in some situations.
• Set Threshold Values Carefully: Adjust the threshold values at which DRS initiates balancing operations based on your system's performance requirements. Threshold values that are too low can cause unnecessary balancing operations, while threshold values that are too high can lead to performance issues.
• Use Affinity and Anti-Affinity Rules When Necessary: Use affinity and anti-affinity rules only when necessary, and remember that these rules can restrict DRS's balancing operations.
• Monitor DRS Reports Regularly: Monitor DRS reports regularly to track how DRS is working, what balancing operations it is performing, and resource utilization. This allows you to identify potential performance issues early and optimize your system.
• Enable EVC: If you have ESXi hosts with different CPU generations, enable the Enhanced vMotion Compatibility (EVC) feature to ensure smooth migration of virtual machines.

DRS and Licensing

DRS is included in some editions of vSphere. It is important to check which vSphere edition includes DRS and which features are supported. You can find a comparison of vSphere editions and their features on the VMware website.

DRS Related Case Studies

Case Study 1: High Traffic E-Commerce Site

An e-commerce company was experiencing high traffic on its website, especially during campaign periods. This caused some ESXi hosts to become overloaded and the website's performance to degrade. After enabling DRS, virtual machines were automatically moved to less loaded hosts. As a result, the website's performance improved significantly and the user experience was enhanced. In addition, DRS eliminated the need for system administrators to intervene manually, saving time and resources.

Case Study 2: Data Center with Different CPU Generations

A data center had ESXi hosts with different CPU generations. This made it difficult to move virtual machines between hosts. By enabling the Enhanced vMotion Compatibility (EVC) feature, compatibility was ensured between hosts with different CPU generations. DRS was able to move virtual machines smoothly thanks to EVC and optimized resource utilization.

Resources for Learning More About DRS

• VMware vSphere Documentation: https://docs.vmware.com/en/VMware-vSphere/index.html
• VMware Knowledge Base: https://kb.vmware.com/
• VMware Communities: https://communities.vmware.com/

Comparison Tables for DRS

DRS Automation Levels Comparison

DRS and SDRS Comparison