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Answers to your burning DRS load-balancing questions

VMware's DRS is able to move virtual machines from an overloaded host to one with more space -- as long as there are no hiccups. Administrators can prevent any miscues by being prepared.

When it comes to load balancing, administrators and users have different options to help keep hosts running smoothly. VMware's Distributed Resource Scheduler (DRS) balances workloads with available resources inside a virtualized environment. It is up to users if they want to use automatic or manual load balancing with DRS to redistribute virtual machines properly. For those looking for more information, here are five frequently asked questions about VMware DRS.

How can DRS improve resource allocation?

VMware DRS and DPM use vMotion to help move VMs among physical servers to improve resource allocation. DRS also helps improve efficiency and cuts back on power consumption. Users must enable DRS to balance workloads and users can then configure DRS for manual or automatic control. When a cluster is built, DRS can be set up and enabled by selecting a checkbox in the cluster’s settings. It's up to the user to select a level of automation. Fully Automated is one of the more popular choices, but users can also chose Manual if they want to approve resource allocation recommendations before they occur.

What can VMware DRS and vMotion accomplish working together?

VMware DRS and vMotion are able to get things done as standalone products, but when used together, a smooth virtual infrastructure can be achieved. DRS ensures that your resource requirements are enforced by balancing the workload across the resources you presented to the cluster. VMware's vMotion will move a VM while it is running to another ESX host. With DRS and vMotion working together, active VMs can be moved either automatically or manually from one ESX host to another.

What will DRS do when a new host joins the resource pool?

If a new host is introduced, VMware DRS would redistribute the VMs to take advantage of the host. According to the DRS algorithms, DRS resource pools would accept new host resources and redistribute VMs. The specified rules and policies that are already set up for the resource pool will determine which VMs move to the new host that was introduced. However, one potential drawback is licensing issues. Using DRS could vastly increase your server licensing requirements because some application licensing models have not kept up with the data center changes that virtualization enables.

How do I fix a DRS load-balancing failure?

When a host is overloaded or if there are no destination servers available, VMware DRS load balancing won't migrate VMs. One reason there might not be able destination servers available is because all options are out of CPU, memory or other resources. One way to try to fix this is by migrating the troubled VM to a spare server. Another issue that could prevent migration is a dependency. If a VM has a dependency on a certain device, it might be unable to move to the destination server if it doesn't exist there. Users and administrators should check on any dependencies if they experience any problems.

How does VMware DRS differ from Microsoft SCVMM's PRO?

When it comes to load balancing, there is a common way of going about it. When it comes to specific products, VMware DRS uses different metrics than Microsoft SCVMM's Performance and Resource Optimization for load balancing. DRS looks at the current host load standard deviation number and if it is larger than the ideal amount, it will recognize it's unbalanced. DRS usually uses vMotion to migrate the VMs of that unbalanced host. Microsoft's PRO takes a different approach and only considers overutilization on individual hosts. In order for PRO to migrate VMs, System Center Operations Manager must be enabled.

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This was last published in June 2015

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