When generating a memory dump from a VMware vSphere virtual machine for vendor analysis (e.g., Microsoft Support), the resulting file may be significantly smaller than the VM's configured RAM.
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When generating a memory dump from a VMware vSphere virtual machine for vendor analysis (e.g., Microsoft Support), the resulting file may be significantly smaller than the VM's configured RAM.

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Article ID: 447475

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Updated On:

Products

VMware vSphere ESXi

Issue/Introduction

  • A memory snapshot or converted .vmem file is smaller than the configured RAM (e.g., a 32 GB VM producing a 10 GB dump).
  • OS vendors report that the provided dump file is incomplete or lacks the necessary pages for analysis.
  • Guest OS active memory usage is low during the snapshot process.

Environment

VMware vSphere 8.x

VMware vSphere 9.x

Cause

By default, if a VM has no Memory Reservation (set to 0 MB), ESXi allocates physical RAM dynamically on-demand. When a snapshot is taken, ESXi skips unallocated or zeroed pages to save datastore space and reduce the VM "stun" time. This results in a sparse .vmem file that only contains touched memory pages.

Resolution

To capture a complete 1:1 memory dump of the configured RAM, follow these steps to force full physical memory allocation:

  1. Log in to the vSphere Client.
  2. Right-click the affected virtual machine and select Edit Settings.
  3. Navigate to the Virtual Hardware tab and expand the Memory section.
  4. Set the Reservation value to match the Maximum Configured RAM (e.g., for a 32 GB VM, enter 32768 MB).
  5. Enable the checkbox for Reserve all guest memory (All locked).
  6. Click OK to save the changes.
  7. Reboot the Guest OS to ensure the host allocates and backs the entire physical memory immediately at boot.
  8. Reproduce the issue or trigger the memory dump by taking a new snapshot with the Include virtual machine's memory option selected.

Additional Information

Impact / Risks

  • Host Resources: Reserving all guest memory consumes physical RAM on the ESXi host and prevents that memory from being overcommitted or used by other VMs.
  • Storage Space: The resulting snapshot files will occupy the full size of the configured RAM on the datastore (e.g., a 32 GB file for a 32 GB VM).
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