vProxy: VM backup fails with "Failed to finalize writer. Incomplete synthesis"

Summary: VM disk configuration issue

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Symptoms

vProxy VM backup fails with:

Backup has failed: Backup has FAILED. Failed to backup virtual disk "Hard disk 2". Failed to finalize writer. Incomplete synthesis, only moved 131072 of 135397376 bytes, from previous data file from offset 0 to target data file
Note: The disk number that it reports is usually the last disk in the VM.

 

Cause

The issue is seen when a VM has a mix of thin and thick provisioned disks on the same VM.

Guest OS: Windows Server 2016 or later (64-bit)
Compatibility: ESXi 6.0 and later (VM version 11)

Hard disk 1
Capacity 100 GB
Type Thin Provision

Hard disk 2
Capacity 100 GB
Type Thick Provision Lazy zeroed

 

Resolution

Re-configure VM so that all disks are the same type, e.g: Thin provision.

Workaround:

  1. Disable VMware Change Block Tracking (CBT) for the VM

  2. Reboot the VM.

  3. Migrate (VMware vSphere vMotion) VM to a new datastore.
    Selecting Migrate menu

  4. Take a VMware snapshot of the VM.
    Take a VMware snapshot

  5. Delete the VMware snapshot.
    Delete snapshot menu

  6. Clean "last vproxy backup session" information.
    Edit Custom Attributes wizard

  7. Perform a level full backup of the VM.

 

Affected Products

NetWorker
Article Properties
Article Number: 000182661
Article Type: Solution
Last Modified: 12 Nov 2025
Version:  4
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