High snapshot stun times

Products

• VMware vSphere ESXi

Issue/Introduction

This document talks about Snapshot Operation limitations directly correlated to number of VMDKs  

Verification Steps 

• Test VM that was used for test is "EXAMPLEVM"
• "EXAMPLEVM" VM has 4 disks attached to it

scsi0:0 = "EXAMPLEVM.vmdk"
scsi0:1 = "EXAMPLEVM_1.vmdk"
scsi0:2 = "EXAMPLEVM_2.vmdk"
scsi0:3 = "EXAMPLEVM_3.vmdk"

• Normal snapshot creation task

[YYYY-MM-DDTHH:MM:SS Z]  Db(167) Hostd[2099500]: [Originator@6876 sub=Vmsvc.vm:/vmfs/volumes/65ee966c-9691bbb8-3dce-34800ddc1c28/EXAMPLEVM/EXAMPLEVM.vmx opID=m3qm7jxe-580851-auto-cg6s-h5:70123203-39-4a-a692 sid=526f6b4a user=vpxuser:admin] Create Snapshot: Test, memory=false, quiescent=false state=5 

• Total Stun time was 245ms 

[YYYY-MM-DDTHH:MM:SS Z]  In(05) vcpu-0 m3qm7jxe-580851-auto-cg6s-h5:70123203-39-4a-a692 Checkpoint_Unstun: vm stopped for 245344 us 

• Network ping drops observed. 

Environment

VMware vSphere ESXi 8.x
VMware vSphere ESXi 7.x
VMware vSphere ESXi 6.x

 

Cause

• To create a VM snapshot, the VM undergoes a temporary “stun” process to:
  • Serialize the device state to disk
  • Close the active disk and establish a snapshot point
• This procedure remains consistent across both Windows and Linux environments.
• The snapshot workflow follows a linear structure, where the duration of the VM stun operation for snapshot creation is directly correlated with the number of VMDKs associated with the virtual machine. 

Resolution

VM snapshot stun times correlate with the number of virtual disks

The above KB explains the concept in detail and point it to the best practices.