Article number
Affected Versions
Source Hypervisor
Target Hypervisor

Error: “I/O error to recovery disk {disk_name} of virtual machine {vm_name}”

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An administrator receives an alert that IOs to a recovery disk for a protected VM are failing.

Root Cause

This alert appears for the following reasons:

  • The underlying storage is out of space, has very little free space, or there is actually an ongoing performance issue (this refers to the actual storage hardware, not virtual datastore within vCenter or SCVMM).

  • A disk was either resized or that the disks' sizes on the production and recovery are not the same. As an example, expanding a disk on a protected VM from the Guest OS itself. This will lead to the Hypervisor not being aware of the change in size, thus the reflection remains outdated and the mirror is not automatically expanded.

  • If a protected RDM is not resized per this article this will also cause the reflection to not update as mentioned in the point above.


The below alert is seen in the Zerto GUI:

I/O error to recovery disk {disk_name} of virtual machine {vm_name} is triggered

This can lead to syncs becoming stuck and not being able to Failover with a current checkpoint as checkpoints are not created during a sync.


Based on the scenario encountered, to resolve this alert:

  1. Check for storage free space at the storage hardware level (array). If there is ample space available for the datastore, review host/storage logs for any indication of Check Conditions returned by the array or Performance Degradation warnings in the host logs.

  2. Compare the size of the recovery disk with the size of the production VMs VMDK or VHDX. Do these match in provisioned size, at the OS level?

    • In case the disk was resized:

      • Note:Due to technical limitation, we can only resize volumes with a whole (integer) number of KBs. Please make sure this disk is expanded to a size that is a whole number of KBs which should allow the VM to be protected.

      • To locate the problematic disk, navigate to the VRA > Edit Settings of the VM > locate the corresponding disk > resize the disk size.

        • For more information on how to increase the size of a VMware virtual disk (VMDK), refer to VMware's KB 1004047

        • To find the corresponding disks, export the VPG settings under the VPG tab in the ZVR GUI, and get a detailed list of the VPG information.

      • If the disk in question is an RDM, be sure to follow the steps in this article so Zerto is aware of the capacity change.