High Write Latency Observed on vSAN Cluster.

Products

• VMware vSAN

Issue/Introduction

• From vSAN cluster > Monitor > vSAN > Performance > Write latency observed.
  
  
  
  
• VMs on the affected vSAN datastore are experiencing slow performance.

Environment

VMware vSAN 8.x

Cause

The issue is caused by a disk having high write latency, which subsequently degrades the performance of Virtual Machines residing on the vSAN cluster.


From ESXi, var/run/log/vsandevicemonitord.log reports disk with "DISK_UNHEALTHY_LATENCY" events.

2025-10-13T09:41:24Z In(14) vsandevicemonitord[2099770]: [779641398784]: WARNING - WRITE Average Latency on VSAN device naa.############### has exceeded threshold value 2500000 us 1 times.
2025-10-13T09:41:24Z In(14) vsandevicemonitord[2099770]: [779641398784]: Latency monitoring interval # is 0 for device
2025-10-13T10:11:30Z In(14) vsandevicemonitord[2099770]: [779641398784]: Device naa.###############  state is DISK_UNHEALTHY_LATENCY SMART.

2026-01-21T02:18:57Z In(14) vsandevicemonitord[2099454]: [604934988480]: WARNING - WRITE Average Latency on VSAN device naa.############## has exceeded threshold value 200000 us 1 times.
2026-01-21T02:18:57Z In(14) vsandevicemonitord[2099454]: [604934988480]: Latency monitoring interval # is 3 for device naa.###############
2026-01-21T02:28:59Z In(14) vsandevicemonitord[2099454]: [604934988480]: WARNING - WRITE Average Latency on VSAN device naa.############### has exceeded threshold value 200000 us 1 times.

Resolution

Engage the hardware vendor to validate the disk's health and replace the faulty disk if necessary.

 

Additional Information

To replace the faulty disk:

When replacing a failed capacity tier disk without Deduplication and Compression make sure to remove the failed disk from the disk group prior to pulling the disk from the host. See Replace a Capacity Device in vSAN Cluster for more details.

When replacing a failed cache tier disk or a capacity tier disk with Deduplication and Compression the entire disk group needs to be removed from vSAN prior to replacing the failed disk. See Replace a Flash Caching Device on a Host in vSAN Cluster & Add or Remove Disks with Deduplication and Compression Enabled for more details.