Article number
Affected Versions
Source Hypervisor
Target Hypervisor

VM which is a part of a cluster (MSCS or Always On) with RDM or Thick Eager Zero VMDK disks on production site will not load during Failover or Failover test

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Root Cause

Multiwrite disks in clustering setups are required to either be set on an RDM disk or a ThickThin Eager Zero disk. However, Zerto will only create a thick or thin VMKD on the recovery site - which means that vSphere will stop the VM from booting on the DR Site.



A Failover test VM or Failover VM will not turn on in vSphere. A similar error regarding one or more of the VM disks will appear:

"An error was received from the ESX host while powering on VM XXXX - testing recovery.
Failed to start the virtual machine.
Cannot open the disk '/vmfs/volumes/....../XXXX.vmdk' or one of the snapshot disks it depends on. 
Thin/TBZ/Sparse disks cannot be opened in multiwriter mode"


This VPG will need to be removed (please allow the "Keep Recovery Disks" option when deleting the VPG -  it is checked by default) and the recovery preseed disks should be changed to a Thick Eager Zero provisioning.
This can be done with an ESXi command line tool named vmfkstools.

The needed command, which runs on the ESXi host is:
vmkfstools --eagerzero /vmfs/volumes/DatastoreName/VMName/VMName.vmdk

This subject and the remediation method (with vmkfstools) is covered in the following VMware KB:

Once completed, please create the VPG with preseeding.

For a video demonstrating a working clustering setup, where the recovery disks are VMDKs, please see

NOTE: Even if you uncheck the "Thin" checkbox in the "Storage" tab of the VPG Wizard while creating the VPG- Zerto will create Thick Lazy Zeroed disks, while for us to work with MSCS - we must have Thick Eager Zeroed disks on the recovery site.