Slow Download Speed and File Inflation When Downloading Thin-Provisioned VMDKs from vCenter Datastore Browser

Products

• VMware vCenter Server 8.0

Issue/Introduction

In vSphere 8 environments, users may experience the following issues when attempting to download Thin Provisioned Virtual Machine Disk (VMDK) files using the vCenter Server Datastore Browser:

• Extended Download Duration: The download process takes significantly longer than expected, even for newly created disks with minimal data.
• File Size Inflation: The resulting file on the local machine matches the Provisioned Size of the virtual disk rather than its actual used space on the datastore.
• Network Congestion: High bandwidth utilization is observed as the system transfers unallocated zero blocks over the management network.

Environment

• VMware vCenter Server 8.0.x
• ESXi 8.0.x

Cause

This behavior is by design in vSphere 8.

When a VMDK is downloaded via the vCenter Datastore Browser, the service retrieves both the descriptor file (.vmdk) and the data file (-flat.vmdk). During this transfer process:

1. The Datastore Browser does not preserve the "thin" metadata of the VMFS file system.
2. The -flat.vmdk file is streamed as a continuous data block.
3. Because the file system sees the file's logical size (the provisioned size), the transfer treats the entire capacity as active data, effectively converting the disk to a Thick Provisioned format during transit.

Resolution

To maintain the efficiency of thin provisioning and minimize download time, it is recommended to use the Export OVF Template workflow instead of a direct file download.

1. Power off the Virtual Machine (if applicable).
2. Right-click the Virtual Machine in the vSphere Client.
3. Select Template > Export OVF Template.
4. In the export wizard, verify the details and click OK.

Benefits of this method:

• Data Compression: The OVF export process only packages blocks that contain actual data.
• Faster Transfer: By skipping zero blocks, the total payload sent over the network is drastically reduced.
• Format Preservation: The disk remains optimized for deployment back into a thin-provisioned format later.