vMotion memory overcommit configuration and failure thresholds

Answer

p>There is no fixed memory overcommit percentage threshold that triggers a vMotion failure. Migration proceeds as long as the destination host has sufficient free memory to accommodate the VM's configured memory (or its reservation, if set) plus a migration overhead—approximately 256 MB. If the host lacks this memory, vSphere aborts the migration with a "Not enough memory resources" error, and the VM remains powered on.

Confirmed Facts

• vSphere does not enforce a hard percentage-based cap for vMotion initiation.
• Failure occurs when destination free memory is less than the VM memory requirement + migration overhead.
• In vSphere 6.5 and later, if vMotion fails due to memory exhaustion, the VM stays powered on on the source host.
• Pre-6.0 releases could potentially place a VM in a suspended state after a rolled-back migration, but this is deprecated in modern versions.
• The memory overhead for vMotion is typically around 256 MB per VM but can increase with larger VMs or complex NUMA configurations.

Likely Explanation

The vSphere Client’s "Memory overcommit" warning is purely informational and reflects that the host's overall overcommit ratio exceeds 100%. It does not determine vMotion eligibility; the hypervisor makes the decision based on absolute free memory available at the destination.

Steps to verify the threshold for a specific VM

1. Determine the VM’s memory size or reservation (check VM settings or run vim-cmd vmsvc/get.summary <vmid>).
2. Add the estimated migration overhead (start with 256 MB; adjust upward if you observe failures despite seemingly enough space).
3. On the destination host, check free memory via esxtop (press m for memory) or vim-cmd hostsvc/summary.
4. Compare the free memory to the calculated sum. If free memory is lower than the sum, the migration will be rejected.

Missing Diagnostic Detail

To provide a more specific recommendation, does the VM in question have a memory reservation or limit configured? This shifts the required free memory threshold.