vSAN performance insights reports: "SSD congestion detected during resync activity”

Products

• VMware vSAN

Issue/Introduction

This article explains the vSAN performance diagnostics issue: "SSD congestion detected during resync activity." This alert typically triggers during heavy resync operations such as hardware failures, node/disk removals, or cluster rebalancing.

Symptoms:

• High VM disk latency during resync operations.

• vSAN disk group experiencing SSD congestion.

• High VM CPU usage.

• Increased I/O latency when adding a host to the cluster.

Environment

VMware vSAN 7.x, 8.x

Architecture: Original Storage Architecture (OSA)

Cause

SSD congestion occurs when the write ingest rate (from VMs or resync activity) exceeds the de-staging rate from the cache tier to the capacity tier. When the write buffer fills faster than it can be cleared, vSAN throttles I/O at the DOM client layer to prevent the cache from becoming completely exhausted.

Resolution

Use one of the following methods to address SSD congestion.

1. Hardware/Configuration Adjustment (Permanent) Consider increasing the size of the cache tier devices to handle larger write bursts.

   • vSAN 8.x supports cache drives up to 1.6 TB. This is not enabled by default. Refer to VMware vSAN Support for Large Cache Tier Capacity to adjust advanced configurations.

2. Apply Resync Throttling (Software Workaround) If hardware changes are not possible, manually limit the resync bandwidth at the disk group level.

   • Important: Do not use esxcli for this setting, as it is non-persistent and may be overwritten by vCenter. Use the configure-resync-throttle.ps1 script attached to this article for settings to persist.

   Execution Steps:

   1. Download the configure-resync-throttle.ps1 script attached to this article.
   2. Launch PowerCLI on a machine with network access to vCenter.
   3. Connect to vCenter: Connect-VIServer -User [email protected]
   4. Run the script to apply the limit (Example: 100 MB/s limit for a specific cluster): .\configure-resync-throttle.ps1 -ResyncIopsLimit 100 -ClusterName "Your-Cluster-Name"

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