If an administrator attempt to protect a VM where at least 1 disk was created with a fractional GB amount (i.e. 100.0199 GB), this will result in IO errors to recovery disk during initial sync and effectively halt the sync.

Root Cause

At least 1 disk in a protected VM going through initial sync has a size that is not a full GB (i.e. 100.0199 GB). In VCenter, this is typically not a problem. However, in Hyper-V, there are limitations with utilizing fractional GB provisioned disks. When SCVMM receives the API call from Zerto to create a disk of an exact size, it does not properly create the mirror of the exact same size, instead it rounds up to the nearest GB.

Due to this, the mirror will be of differing size from the associated protected volume and cause this alert found in the index.html of the log bundle.

This same limitation can cause similar issues when a fractional GB sized disk is created directly on a Hyper-V host rather than the SCVMM as the Hyper-V host will allow it. However, SCVMM will round up to the nearest GB in terms of reporting and therefore incorrectly report the size of the disk back to Zerto via reflection collection. Then, at VPG creation, the mirror will be made too large and see a similar behavior. For this reason, it is best to create all disks within SCVMM to avoid this issue.


After creation of VPG/during initial sync, the following alert is seen in the Zerto GUI:

IO error to recovery disk {disk_name} of virtual machine {vm_name}.

The initial sync will not be able to finish due to the above.


To resolve this issue so the sync can complete, the below steps to resize the suspect disk(s) should be followed:

  1. In the protected VM, preferably via SCVMM, expand the suspect disk up to the next whole GB (i.e. 100.0199 GB is resized to 101 GB).

  2. The VPG will go through an Update VPG task automatically which includes sending commands to the Recovery Site to resize the mirrors as well.

  3. Once the Update VPG task completes, this will result in the protected and mirror disks to be the exact same size and the sync should progress forward and eventually complete.