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.
VMware vSAN 7.x, 8.x
Architecture: Original Storage Architecture (OSA)
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.
Use one of the following methods to address SSD congestion.
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 the VMware vSAN Support for large cache tier capacity guide to adjust advanced configurations.
If hardware changes are not possible, you can 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. You must use the PowerShell script attached to this KB for the settings to persist.