Why do we need rebalance in VSAN cluster?

When any capacity device in your cluster reaches 80 percent utilization, Virtual SAN automatically rebalances the cluster, until the utilization of all capacity devices is below the threshold.

Cluster rebalancing evenly distributes resources across the cluster to maintain consistent performance and availability.

Other operations can initiate cluster rebalancing:

  • If Virtual SAN detects hardware failures on the cluster
  • If Virtual SAN hosts are placed in maintenance mode with the Full data migration option
  • If Virtual SAN hosts are placed in maintenance mode with Ensure accessibility when objects assigned FTT=0 reside on the host.

Note:

To provide enough space for maintenance and reprotection, and to minimize automatic rebalancing events in the Virtual SAN cluster, consider keeping 30-percent capacity available at all times.

Cluster rebalance can be done in two ways :

1: Automatic Rebalance

2: Manual Rebalance

Let us know how the Automatic Rebalance works:

By default, Virtual SAN automatically rebalances the Virtual SAN cluster when a capacity device reaches 80 percent utilization. Rebalancing also occurs when you place a Virtual SAN host in maintenance mode.

Run the following RVC commands to monitor the rebalance operation in the cluster:

  • vsan.check_limits. Verifies whether the disk space utilization is balanced in the cluster.
  • vsan.whatif_host_failures. Analyzes the current capacity utilization per host, interprets whether a single host failure can force the cluster to run out of space for reprotection, and analyzes how a host failure might impact cluster capacity, cache reservation, and cluster components.
    The physical capacity usage shown as the command output is the average usage of all devices in the Virtual SAN cluster.
  • vsan.resync_dashboard. Monitors any rebuild tasks in the cluster.   

You can Manually rebalance through the cluster health check, or by using RVC commands.

If the Virtual SAN disk balance health check fails, you can initiate a manual rebalance in the vSphere Web Client. Under Cluster health, access the Virtual SAN Disk Balance health check, and click the Rebalance Disks button.

Use the following RVC commands to manually rebalance the cluster:

  • vsan.check_limits. Verifies whether any capacity device in the Virtual SAN cluster is approaching the 80 percent threshold limit.
  • vsan.proactive_rebalance [opts]<Path to ClusterComputeResource> –start. Manually starts the rebalance operation. When you run the command, Virtual SAN scans the cluster for the current distribution of components and begins to balance the distribution of components in the cluster. Use the command options to specify how long to run the rebalance operation in the cluster, and how much data to move each hour for each Virtual SAN host. For more information about the command options for managing the rebalance operation in the Virtual SAN cluster, see the RVC Command Reference Guide.
    Because cluster rebalancing generates substantial I/O operations, it can be time-consuming and can affect the performance of virtual machines.