Storage vMotion fails with "Fatal Internal Error" and "File System Timeout" on NFS Datastores

Products

• VMware vSphere ESXi

Issue/Introduction

Symptoms

• Storage vMotion tasks for virtual machines fail during Phase 8 (Disk Copy Phase).

2026-04-09T22:18:07.261Z In(05) vmx - MigrateVMXdrToSpec: type: 2 srcIp=<127.#.#.1> dstIp=<127.#.#.1> mid=505f1e8f1d710c74 uuid=4c4c4544-0058-5310-8032-############ priority=no checksumMemory=no maxDowntime=0 latencyAware=yes diskOpFile= srcLogIp=<<unknown>> dstLogIp=<<unknown>> ftPrimaryIp=<<unknown>> ftSecondaryIp=<<unknown>> 2026-04-09T22:18:07.504Z In(05) worker-76894761 - SVMotion: Enter Phase 8

• vCenter Server reports
  
  A fatal internal error occurred. Failed to copy one or more disks.

• Virtual machine vmware.log (VMX) reveals repeated warnings: 
  
  2026-04-09T22:19:17.542Z Wa(03) vmx - SVMotion: scsi0:0: Disk transfer rate slow: 0 kB/s over the last 10.00 seconds, copied total 5120 MB at 74858 kB/s.
2026-03-09T22:19:37.549Z Wa(03) vmx - SVMotion: scsi0:0: Disk transfer rate slow: 0 kB/s over the last 10.00 seconds, copied total 5184 MB at 58953 kB/s.

• Migration terminates with the error: SVMotionMirroredModeThreadDiskCopy: Found internal error when woken up on diskCopySemaphore. Aborting storage vmotion. 

2026-04-09T22:48:19.120Z Wa(03) worker-76894761 - SVMotionMirroredModeThreadDiskCopy: Found internal error when woken up on diskCopySemaphore. Aborting storage vmotion. 2026-04-09T22:48:19.123Z In(05) vmx - [msg.svmotion.fail.internal] A fatal internal error occurred.

Environment

VMware vSphere ESXi 8.x 
VMware vSphere ESX  9.x 

 

Cause

The issue is caused by Extreme NFS RPC Latency and  Backend Storage Contention.
During a Storage vMotion, ESXi uses "Mirroring Mode" to write data to both the source and destination disks simultaneously. If the Source NFS datastore delays the  Read requests within the hypervisor's timeout window, the I/O throughput "flatlines" (0 kB/s). 

 

Resolution

Storage vendor should be investigating the issue further for IO latency seen during copy process.