High Disk Latency on vSAN cluster may impact VM performance.

Products

• VMware vSAN

Issue/Introduction

Symptoms:

• Virtual Machines running on the vSAN cluster may stay hung or not responding when launching VM console.

• When checking for the vSAN cluster performance, you may see cluster level latency reported for READ and WRITE.

Environment

• VMware vSAN 7.x
• VMware vSAN 8.x

Cause

• The issue is seen when there are one or more disks used for vSAN experiencing high latency from the physical layer.

• Upon checking the vmkernel logs on ESXi hosts under /var/run/log/vmkernel.log you may see latency reported high.

• Example events as below indicate the physical disk latency is around 3.9 seconds.

2026-03-13T06:25:40.216Z Wa(180) vmkwarning: cpu13:2097946)WARNING: ScsiDeviceIO: 1779: Device naa.#### performance has deteriorated. I/O latency increased from average value of 21153 microseconds to 3965595 microseconds.
2026-03-13T06:25:41.119Z Wa(180) vmkwarning: cpu13:2097946)WARNING: ScsiDeviceIO: 1779: Device naa.#### performance has deteriorated. I/O latency increased from average value of 21153 microseconds to 3710764 microseconds.
2026-03-13T06:25:42.159Z Wa(180) vmkwarning: cpu13:2097943)WARNING: ScsiDeviceIO: 1779: Device naa.#### performance has deteriorated. I/O latency increased from average value of 21153 microseconds to 3590664 microseconds.

• When checking for performance stats under vSAN cluster > Support > Performance for diagnostics for individual disks stats, you may see latency reported for disks under LSOM.

Resolution

• To address to the issue immediately, after identifying the problematic host, place the host into maintenance mode using ensure accessibility or full data migration and monitor the performance on the vSAN cluster.

• If the performance improves, please work with hardware vendor to get the hardware issues resolved.

• The following article can be used for instructions on Requirements when replacing disks in a vSAN cluster