Guest VM Unresponsiveness and Storage I/O Latency due to Host Resource Constraints

Products

• VMware vSAN

Issue/Introduction

This article addresses critical guest VM unresponsiveness and ESXi host management failures caused by severe host resource starvation. When an ESXi host experiences excessive CPU and memory overcommitment, the vSphere scheduler may become unable to process vital management tasks, leading to the failure of hostd and vpxa agents and eventual management disconnection. In vSAN environments, this resource contention can trigger metadata deadlocks and vMotion memory transfer timeouts, leaving virtual machines in an unmanageable state.

• • One or more Virtual Machines (VMs) become unresponsive and cannot be managed via vCenter or guest OS tools.
  • The ESXi host may show as "Not Responding" in vCenter or experience intermittent management disconnections (SSH failure, UI timeout).
  • High storage I/O latency spikes (e.g., >10ms) are observed on vSAN NVMe or SSD capacity devices.
  • Management agents (hostd, vpxa) fail to restart or exhibit extremely high memory utilization.
  • vMotion operations for large or latency-sensitive VMs fail or time out during the memory transfer phase.
  • Logs indicate vMotion memory related failures or vSAN metadata deadlocks: vMotion memory transfer failed or metadata deadlock in vsanmgmtd

Environment

ESXi (all versions)
vSAN (all versions)
Virtual Machines with High Latency Sensitivity set

Cause

Excessive CPU and memory overcommitment on the ESXi host led to a failure in the scheduler's ability to process vSAN metadata updates and vMotion memory transfers in a timely manner.

Resolution

To resolve this and restore service, please follow these steps:

1. Perform a Host Reboot: A hard reboot of the affected ESXi host is required to clear current service deadlocks (vsanmgmtd/hostd) and restore management connectivity.
2. Review CPU Allocation: After the host is back online, we recommend reviewing the CPU and memory reservations for the affected VMs. High contention can be mitigated by ensuring that total vCPU allocation does not significantly exceed the physical core count.
3. Adjust Latency Sensitivity: Ensure critical VMs are not set to "High" latency sensitivity unless they require and have 100% CPU reservations.
4. Increase TCP/IP Heap Size: We recommend increasing the heap size on the host to better accommodate high-throughput operations like vMotion and vSAN metadata updates.
   Get the current heap size: esxcfg-advcfg -g /Net/TcpipHeapSize
   Set the increased value (50% more recommended): esxcfg-advcfg -s 128 /Net/TcpipHeapSize
5. Monitor Health: Use esxtop to monitor %RDY and %WAIT metrics post-reboot to ensure resources are balanced.

Additional Information

References:

1. Troubleshooting a virtual machine that has stopped responding: VMM and Guest CPU usage comparison
2. Details on common timeouts for memory-heavy VMs.