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What factors or VMware DRS rules might stop a host server from powering off during low utilization?
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When properly configured, an infrastructure virtualized with VMware Distributed Resource Scheduler (DRS) can migrate workloads off a lightly used server and power down the unneeded server as a means of conserving energy. As computing demands increase and more resources are required, sleeping servers can be awakened and VMs migrated and rebalanced as needed. However, there are many factors that can prevent a host server from powering down.
One of the most common problems is that the VM simply cannot be migrated off the lightly used server in the first place. This might be because of incompatible hypervisors (no other host server will support the VM). In many cases, no destination server has the resources available to accommodate the additional VM or there are existing resource reservations. It's also possible that VMware DRS rules might prevent migration. DRS might be configured to handle only critical moves (a high-migration threshold setting), or DRS might even be disabled on the troubled VM.
The reason that such periodic consolidation techniques work is because of consistently low computing demands. For example, a workload that is almost idle during overnight hours would often be a good candidate for this kind of migration and power-conservation scheme. When a workload's computing demands are erratic or unpredictable, it might cause the original host to wake and sleep far more often than you intend. Tools such as DRS can often recognize this potential inefficiency and prevent migrating unpredictable workloads -- also preventing the host from sleeping, even during periods of low computing activity.
Other issues often involve hardware and software configuration problems. Start by reviewing VMware DRS rules, such as the "MinPoweredOn" option. Migration tools such as vMotion may need to be enabled or configured in order to move the VMs properly. Distributed Power Management (DPM) software will need to be enabled and configured on hosts that can power down. In terms of hardware, the server itself may be unable to sleep and wake on demand if hardware features such as Wake-on-LAN or an Intelligent Platform Management Interface is not available to DPM. Such hardware limitations are extremely rare in modern servers, but may pose unexpected problems for organizations that choose to extend the working life of some older systems.
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