Best Practices for Cloning Large (Multi-Terabyte) Virtual Machines

Products

• VMware vSphere ESXi

Issue/Introduction

When attempting to clone exceptionally large Virtual Machines (e.g., 20TB), administrators often encounter constraints regarding storage space on a single datastore, uncertainty around completion timelines, and questions regarding task management (such as canceling an in-progress clone). This article addresses how to distribute a cloned VM across multiple datastores, estimates the time required for a massive clone and outlines best practices for managing the task safely.

Environment

• VMware vSphere ESXi 
• VMware vCenter Server 

Resolution

• Cloning a VM Across Multiple Datastores:

Clone a single Virtual Machine and distribute its virtual disks across multiple datastores. To configure this during the cloning wizard:

1. 1. Initiate the clone by selecting the Clone to Virtual Machine option.

   2. Proceed through the wizard to the Select Storage page.

   3. At the top of the page, choose the Configure per disk option.

   4. Select individual disks and click Configure. This allows you to assign a specific, separate datastore to each selected disk.

   5. Repeat this process for any remaining disks to distribute the large storage load.

• Estimating Clone Time for a large VM:

Providing an exact duration for a cloning operation is not possible due to fluctuating environmental behaviors and active loads. However, you can estimate the timeline by understanding your storage limits.

Key factors impacting this duration include:

• • Hardware Acceleration (VAAI): If the clone is occurring within the same storage array, hardware acceleration (if enabled in your environment) offloads the work to the storage array directly. This significantly reduces the time required to complete the clone.
    
    
  • Current Host/Storage Load: You can use the esxtop command-line utility to monitor how many commands per second (cmds/sec) are currently being executed. This provides real-time insight into the storage load and helps contextualize the transfer speed.
    
    
  • Important Note on VM Power State: Because the exact duration cannot be guaranteed and involves an extended timeline, do not power off the source VM prior to cloning. While powering it off would prevent active read/writes and streamline the copy process.
    
    
• Canceling an Active Clone Task:

It is not recommended to cancel a running clone task. Interrupting a clone operation mid-flight can corrupt or negatively impact the virtual machine files on both the source and the target datastores. Once initiated, you should allow the task to run to its natural completion.

Additional Information

Clone an Existing Virtual Machine