VM Backup Fails with Error: "VM backup failed for virtual machine because one of the disks has a size that is not a multiple of 1 MB"

Products

• VMware vSphere ESXi
• VMware vCenter Server

Issue/Introduction

This article provides guidance on troubleshooting and resolving virtual machine (VM) backup failures caused by incorrectly sized virtual disks. Specifically, it addresses errors related to disk sizes that are not exact multiples of 1 MB, which can prevent successful backups and snapshots.

You may encounter the following symptoms:

• VM backup operations fail.

• Snapshots cannot be taken for the affected virtual machine.

• The following error message is observed in the backup software or task logs:

“VM backup failed for virtual machine because one of the disks has a size that is not a multiple of 1 MB.”

Environment

VMware vSphere ESXi

VMware vCenter Server

 

Cause

This issue occurs when one or more virtual disks in a VM are provisioned with sizes that are not exact multiples of 1 MB. Many backup and snapshot solutions require that all virtual disk sizes be aligned to 1 MB boundaries to function correctly.

For example:

• A virtual disk size of 1.318 TB translates to:

  1.318 × 1024 (GB) × 1024 (MB) = 1,382,023.168 MB — which is not a whole number.

• A properly aligned disk size such as 1.375 TB results in:

  1.375 × 1024 × 1024 = 1,441,792 MB — which is a valid multiple of 1 MB.

Valid disk sizes must result in an integer number of megabytes. Recommended sizes include:

• x.000 TB

• x.125 TB

• x.250 TB

• x.375 TB

• x.500 TB

• x.625 TB

• x.750 TB

• x.875 TB

Disks that deviate from these values may not be compatible with backup or snapshot operations.

Resolution

To restore backup functionality and resolve the issue, follow these steps:

1. Power off the virtual machine
   Shut down the affected virtual machine to ensure a safe environment for modifying disk size parameters.

2. Resize the virtual disk
   Using the vSphere Client or a supported management tool, increase the size of the affected virtual disk to a valid value (e.g., 1.375 TB).

   • Note: If the vSphere Client UI does not allow specifying the desired size as a whole number, use the command-line interface (CLI) to perform the disk size increase. For detailed instructions, refer to the Broadcom Knowledge Base article: Increasing the disk size on a virtual machine.

   • Important: Disk size can only be increased—reducing the size is not supported.

3. Update the guest operating system
   Once the virtual disk has been resized, use appropriate tools within the guest operating system to expand the partition and make use of the newly allocated space. For further guidance, refer to the Broadcom Knowledge Base article on [increasing the size of a disk partition].

Additional Information

Aligning virtual disk sizes to exact multiples of 1 MB is essential for compatibility with snapshot and backup solutions. Misaligned disk sizes can lead to recurring errors and operational failures during data protection processes.

Proactively validating and correcting disk sizes ensures a more stable and reliable virtual infrastructure.