Virtual machine storage usage showing higher value than allocated disk size

Products

• VMware vSphere ESXi

Issue/Introduction

Virtual machine used space showing in vSphere Client > VM > Summary > Usage is higher when comparing it to the total disk size.

Environment

VMware vSphere ESXi

Cause

This discrepancy is typically caused by one or more of the following factors:

• Snapshots: Each snapshot creates delta files (.vmdk). If snapshots are kept for a long time, these files grow as data changes, potentially doubling the storage footprint.
• Virtual Swap Files (.vswp): When a VM is powered on, ESXi creates a swap file to account for the difference between the assigned RAM and the memory reservation. For example, a VM with 64GB RAM and no reservation will use an additional 64GB of storage.
• Unreclaimed Space (Thin Provisioning): When files are deleted inside the Guest OS, the underlying hypervisor is not immediately notified. As a result, the storage remains "marked as used" on the datastore even though it is "free" inside the VM.
• Zombie Files: Occasionally, orphaned virtual disks or log files from previous sessions may remain in the VM folder.

Resolution

Use following methods to release and reclaim VM used space:

• Consolidate or delete snapshots
• Configure VM memory reservation
  • For more information about "Reserve all guest memory" option, see What is the functionality of the option "reserve all guest memory (all locked) within the virtual machine ?.
  • By default, ESXi creates a .vswp file equal to the amount of "unreserved" RAM when a VM is powered on. The .vswp file size could be reduced if memory reservation is configured.
  • On VM > Edit Settings > Virtual Hardware tab, expand Memory, update the reservation value or check the box "Reserve all guest memory".
• Reclaim space (UNMAP)
  • Run the UNMAP command within the Guest OS (or via ESXi CLI for certain versions) to signal the underlying storage to reclaim unused blocks. For detailed steps, see: Reclaiming VMFS deleted blocks on Thin Provisioned LUNs
• Cleanup orphaned files
  • Browse the datastore and check the VM's folder for any files that are not registered to the VM (e.g., old .log files or leftover -flat.vmdk files).