Article number
Affected Versions
Source Hypervisor
Target Hypervisor

Problem with VPG in ‘Needs Configuration’ Status When Protecting RDM(s) with an Odd Number of Blocks

Viewed 1052 times


An administrator may need to utilize RDM type disks for specific applications to function properly. It must be noted the number of blocks that make up the RDM so as to ensure Zerto can properly replicate the disk and not enter a 'Needs Configuration' state.

Root Cause

Zerto does have limitations with regards to RDMs in the following ways:

  1. RDM disks with an even number of blocks can replicate to RDM disks of the same size with an even number of blocks and to VMDKs.

  2. RDM disks with an odd number of blocks can only replicate to RDM disks of the same size with an odd number of blocks and not to VMDKs.


When protecting a VM with RDM LUNs in a Zerto Virtual Replication VPG, an administrator may see that the VPG unexpectedly drops into a “Needs Configuration” status with subtext similar to the following:

The RDM {0} for the VM {1} in VPG {2} has an odd number of blocks, which can only be replicated to a target RDM and not to a VMDK.

This behavior will also occur when attempting to protect a VM with such an RDM, resulting in an error similar to the following:

Protected RDM with odd blocks: vm = [VM {1}, server {2}, size in bytes = {3}, internal name = {4}]


If a VPG enters Needs Configuration status where RDMs are utilized, the administrator will need to change the RDM LUN sizing to contain an even number of blocks. In order to identify which RDM LUN is at fault, the following procedure can be followed by utilizing the vCenter’s managed object browser (MOB):

  1. Open the vCenter MOB by browsing to https://vcenterIP/mob.
  2. Once inside the vCenter MOB browse to the relevant VM configuration by changing the URL in the browser to https://vcenterIP/mob/?moid=vm-#### (for the vm-#### you will need to enter the VM’s MOREF ID.)
  3. At the VM level, choose Config > Hardware.
  4. At the hardware level, located the RDM device and select the device[####] field.User-added image
  5. After selecting the device and verifying it is the RDM, take note of the capacityInKB field, and select the backing value to progress to the backing screen.User-added image
  6. At the backing level record the lunUuid value.
  7. After recording the lunUuid value, go back up to the VM level, and select runtime.User-added image
  8. Within the runtime you will see the host this VM is running on. Select the host-xx and enter the host level.User-added image
  9. Within the host level, select config, and then within config select storageDevice.
  10. From within storage device, search (via ctrl + F) for the lunUuid recorded previously - you should see the ID in the following form within a link starting with scsiLun:User-added image
  11. Select the link, and look for and select the capacity value.User-added image
  12. Within the capacity menu, you should see the values needed.User-added image
  13. If the block length is an odd number, the calculation of capacityInKB (step 5) will be off by one block.
  14. To correct this, change the RDM size such that the block length is an even number.