vSAN Skyline Health alarm: Resync Operations Throttling

Products

• VMware vSAN

Issue/Introduction

This article explains the vSAN Skyline Health alarm "Resync Operations Throttling," which triggers when a manual bandwidth limit is active on one or more ESXi hosts. While manual throttling was common in vSAN 6.5 and 6.7, the introduction of Adaptive Resync in version 7.0 and later automates I/O balancing. This article provides the steps to disable manual throttling to resolve the health warning and restore optimal cluster performance.

Symptoms

• Skyline Health triggers the alarm: "vSAN cluster alarm 'Resync Operations Throttling'".
• vSAN Health Check fails with "Hosts with resync operation throttled".
• VMs experience high latency, slowness, or long power-on times during active resynchronization.
• Snapshot deletions take significantly longer than expected.
• Resync operations appear slow or stuck despite available network bandwidth.

Environment

• VMware vSAN 6.x
• VMware vSAN 7.x
• VMware vSAN 8.x
• VMware Cloud Foundation (VCF)

Cause

The alarm indicates that a manual bandwidth limit (typically less than 20 Mbps) is set on a host. In vSAN 7.0+, manual throttling is deprecated. If a manual limit is set via the ESXi command line, vCenter Server may automatically attempt to reset this value to the default (unlimited/512 Mbps) during routine configuration syncs, which can cause the alarm to toggle if the host-level setting persists.

Resolution

To resolve the health warning, manual throttling must be disabled (set to 0).

1. Host-Level (Temporary/Immediate): Log into the affected host via SSH and run: # esxcfg-advcfg -s 0 /VSAN/DomCompResyncThrottle Verify the setting with: # esxcfg-advcfg -g /VSAN/DomCompResyncThrottle Note: Host-level changes may be overwritten by vCenter cluster tasks.

2. Cluster-Level (Persistent): To ensure settings persist across cluster syncs, use PowerCLI to set the throttle at the cluster management layer: Get-Cluster -Name "<ClusterName>" | Get-VsanStatConfiguration | Set-VsanStatConfiguration -ResyncThrottleMbps 0

3. Alternative Command: If the value continues to reset on individual hosts, use the esxcli command: esxcli vsan resync throttle set --level=0

Additional Information

• Performance Mitigation: In specific troubleshooting scenarios where resync traffic saturates the network and impacts production VMs, support may recommend a temporary non-zero throttle (e.g., 100 Mbps) until the environment stabilizes.
• For detailed PowerCLI instructions and advanced scripts, refer to KB 326830.
• If you require further assistance, see Creating and Managing Broadcom Support Cases.