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Can virtual SAN performance create VM performance issues?

Monitor the performance of your VSAN and VMs to ensure one isn't affecting the other. Performance metrics and alerts can make it easy to spot VSAN issues before they affect VMs.

Virtual SANs and VMs have numerous dependencies, so performance problems in one can cause problems in the other....

You should monitor virtual SAN performance to catch problems before they cause VM performance issues, including the inability to create new VMs.

The performance of VMs can suggest potential virtual storage area network (VSAN) problems. You should routinely monitor key virtual SAN performance indicators to help ensure important VMs are free of VM performance issues. These metrics can also serve as early warnings of possible VSAN problems.

For example, you can monitor the rate of I/O operations on the data store, as well as memory use, processor use, and network throughput and bandwidth on each VM.

VSAN platforms often offer integrated tools to assist with monitoring and troubleshooting. For example, VMware vSAN offers the vSAN Performance Service, which enables you to monitor vSAN cluster performance, vSAN host node performance and vSAN VM performance.

For example, if the VMware vSAN Performance Service is on, you can use it to oversee hosts within the vSAN cluster. This provides performance metrics for each host and displays charts for IOPS, latency, throughput and congestion. It can also provide performance metrics for disk groups or individual disks. In addition, you can use the service to monitor performance charts for each VM, including IOPS, throughput and latency.

By monitoring these metrics and comparing them to a baseline, you can spot possible VSAN problems that might lead to VM performance issues. In addition, varied alarms can help you quickly detect potential trouble. For example, the VSAN platform can monitor errors on VSAN devices, oversee VSAN cluster health -- along with the health of cluster data -- watch for VSAN cluster limits and alert you to troubled physical devices in the cluster.

VSAN configuration can limit VM creation

Sometimes, you'll get an error that indicates an operation failed or a VM can't be created when you try to create a VM on a VSAN. There are several possible problems to consider here.

First, the VSAN might simply not have enough physical storage due to inadequate capacity. For example, the SAN might need more disks added, or some portion of the SAN's capacity may have become unavailable, such as failed or disconnected disks.

VSAN performance relies heavily on policies that determine the amount and placement of storage allocated to the request.

A second common issue occurs when the SAN is unable to provision the requested space. VSAN performance relies heavily on policies that determine the amount and placement of storage allocated to the request. If the request exceeds VSAN policies, the VSAN might refuse to provision the storage, which would result in an error.

For example, if the storage policy demands multiple copies of a virtual disk -- each on a different host -- but there aren't enough hosts available to store the required number of copies, the provision request might fail. As another example, a VSAN can't migrate content between disks or hosts to free space, so a provisioning request that can't receive adequate storage on all hosts -- according to policy -- might fail, even if there is enough raw capacity to accommodate the request.

Make the most of your VSAN

A VSAN isn't a single thing, but is instead a complex amalgamation of hardware and software. To get full performance from it, you need to perform thorough compatibility checks. Take advantage of compatibility guides and diagnostic tools to ensure your system is fully functional.

VSAN performance can also suffer from network configuration problems. Check for drivers that aren't updated and security products that might block connections. Monitoring tools can automate this process and make it easier to maintain steady connections.

You can investigate these virtual SAN performance and configuration problems by reviewing the physical disk capacity and health status in the VSAN cluster. You can then add capacity or resolve disk problems to restore the ability to create VMs on the VSAN.

A similar issue can occur when a created VM has no VM performance issues, but becomes inaccessible or noncompliant. These are two distinctly different states in a VSAN. For example, a VM becomes inaccessible when the VSAN can't find more than 50% of the replicas of a VM object. A VM becomes noncompliant when it no longer fits within the associated storage policy.

Generally, the VSAN cluster will work to rebuild corrupted or damaged VM files that result in the inaccessible or noncompliant state. This requires adequate resources in the VSAN cluster. Otherwise, the cluster space will need expansion.

The complex dependencies between VMs and VSANs mean that virtual SAN performance can create significant VM performance issues. Use monitoring tools to check the health of the VSAN and head off problems that can affect VM performance. In particular, watch out for configuration and storage problems that can limit your ability to create VMs at all.

This was last published in June 2018

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