After reverting to snapshot of a VM, the first power on task is slow - subsequent power on operations are much faster.

Products

• VMware vSphere ESXi

Issue/Introduction

VM slow to power on after reverting to snapshot.

Power on task will complete - no errors/warnings in either the GUI or logs.

Power on task will pause for a period of time at around 66% and then complete.

The larger the disk the longer the pause at this stage.

All subsequent power on operations will complete much quicker than the first.

 

 

Environment

CBT is enabled on the VM.

There is frequent reverting to snapshot on the VM's - possibly in an automated or testing/dev environment.

Cause

When you revert to a snapshot for a virtual machine that has Changed Block Tracking (CBT) enabled, it will automatically disable and enable CBT.

When powering on the VM, CBT is re-initialized which will take some time to retrieve the allocated blocks.

The first power-on after reverting to a snapshot for a CBT enabled VM is expected to take more time as a result.

A full backup is always needed after CBT is re-initialized.

 

Resolution

Do not use CBT on VM's if there is expected to be frequent operations like this (reverting to snapshot) especially if the power on operations needs to be very quick immediately after.

If CBT is enabled and there is expected to be a high frequency of  reverting to snapshots, be aware of the expected longer first boot time.

Additional Information

Changed Block Tracking (CBT) on virtual machines  https://knowledge.broadcom.com/external/article/320557 

Enabling or disabling Changed Block Tracking (CBT) on virtual machines https://knowledge.broadcom.com/external/article/315370