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Every server has a practical I/O limit. When a VM demands extensive storage I/O, it can create a storage bottleneck for other VMs on the same host system, which drives up storage latency and adversely affects the performance of other VMs. VMware vSphere's share-based system limits the number of shares when storage latency becomes too great to ensure that all VMs have access to storage. But VMware Storage I/O Control isn't always perfect, and administrators should monitor workload performance and make regular adjustments to achieve best results.
An administrator must first enable VMware Storage I/O Control (SIOC) and set the latency limit and percentage of peak throughput to configure the congestion threshold. Latency is set to 30 milliseconds by default, and the percentage of peak throughput is set to 90%. Adjust both values to accommodate the performance needs of enterprise VM workloads and available storage resources.
Once you've enabled SIOC on a data store and set the proper configuration, it's possible to compose policy components. For example, the Storage Policy Components dialog box allows you to name and describe the storage policy component. You can then configure three key elements for the policy component. The IOPS limit denotes the maximum IOPS the policy allows, IOPS reservation represents the policy's minimum IOPS and IOPS details the number of shares in the I/O queue -- basically, the relative importance of the associated workload.
After you've created one or more policy components, create a complete VM storage policy. Once you've named and described the policy, it's possible to add policy components to flesh out its nuances. For example, you might choose to add a policy component that outlines the IOPS allocation, and add other components, such as storage encryption, replication and other rules to create the appropriate storage policy.
Now it's time to apply the VM storage policy to a VM. To assign a storage policy to a VM, select the VM, choose VM Policies and select Edit VM Storage Policies. Choose the desired policy from the drop-down list, apply it and click OK. This should apply the selected policy to the VM and its associated disk file.
It's a good idea to monitor storage I/O performance before and after policies are applied to quantify any performance difference produced by the policy or policy changes. Storage policies are dynamic elements and administrators can go back and add, remove or change policies as needed to further optimize workload performance.
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