How to Fix VolSnap 28 Error “The shadow copy could not be created…

This error may occur on physical hosts or on VM hosts. In the case of a VM host, the host logs a misleading entry “0x800423f3”.

Host live backups of virtual machines may fail with a transient Hyper-V VSS Writer error, which typically logs this kind of message in the Event Viewer:

A VSS writer has rejected an event with error 0x800423f3, The writer experienced a transient error.  If the backup process is retried,
the error may not reoccur.
. Changes that the writer made to the writer components while handling the event will not be available to the requester. Check the event log for related events from the application hosting the VSS writer. 

Operation:
   PrepareForSnapshot Event

However, the fault originates in the VM itself as the host requests the VM to prepare for live backup. Our freeware tool VssDiag (available at //backupchain.com) found a a VolSnap Error 28, which looked like this:

The shadow copy of C: could not be created due to a failure in creating the necessary on disk structures.

This VolSnap Error, by the way, may also occur as-is on hosts without VMs.

The Real Reason for VolSnap Error 28

The reason why VolSnap 28 was reported is actually the VM’s disk size.

The above VM has a disk with 10TB attached to it and had only 512MB RAM assigned. Increasing the RAM to 4GB, increasing VSS areas, and increasing the system paging file to 40GB resolved the problem.

The Fix

Unlike what Microsoft wrote about this issue, the problem is due a RAM shortage. You need to increase the RAM inside the VM (or on the host if the VolSnap 28 error occurred on a physical machine).

In addition to that, we increased the VSS diff area using

vssadmin resize shadowstorage  /for=E: /on=E: /MaxSize=Unbounded

for each drive.

Because we like to do a neat job and prevent further outages we also increased the VM’s paging file size to 40 GB.

After all these system changes the backup now works perfectly.