cannot restore a virtual machine and a Trouble writing files, error -3956 appears when using VMware Data Recovery

Cannot restore a virtual machine and a Trouble writing files, error -3956 appears when using VMware Data Recovery


You may experience these symptoms:
  • A restore (or a restore rehearsal) of a virtual machine fails
  • You receive the error:

    "Trouble writing files, error -3956 (Operation failed)".
    7/16/2009 16:52:14.000: 0x0a396d90: RMapGetChanges: Reporting changes to 41
    7/16/2009 16:52:14.000: 0xb42df028: $[*20750]Trouble writing files, error -3956 ( operation failed)


Error -3956 usually indicates that Data Recovery failed to create the virtual machine during restore.

Reasons for this kind of failure:

  • The virtual machine had an incorrect device backing info at the time of backup, usually an invalid CDROM configuration.

    Workaround: Manually create a virtual machine with same the CPU, memory, etc. settings and restore just the disks to that virtual machine. This can be done by setting restore configuration to No in the restore wizard.

  • A virtual machine is on a shared storage and its datastore is accessible to multiple hosts. In this case, Data Recovery chooses an incompatible datastore during restore and fails.

    Workaround: Verify that the datastores chosen for the virtual machine by the restore wizard is visible on the ESX host that the restore is being made to. If not, choose a valid datastore manually.

Cannot assign IP address to a NIC after a physical to virtual conversion because a duplicate IP exists

Cannot assign IP address to a NIC after a physical to virtual conversion because a duplicate IP exists


  • After a physical to virtual (P2V) conversion of a machine, you cannot assign an IP address to a NIC
  • A message indicates that the IP already exists in the network, but no other machine in the network is sharing the IP


Sometimes after a P2V conversion, the drivers for the physical NICs are still present and the physical NIC is still present as a device on the machine. These ghost NICs still have the old IP address and the virtual NIC cannot be assigned the same IP address.
To resolve this issue, follow these steps inside the guest operating system:
  1. Click Start > Programs > Accessories > Command Prompt.
  2. Type the following command and press Enter:

    set devmgr_show_nonpresent_devices=1

  3. Type the following command , and press Enter:

    start devmgmt.msc

  4. Click View > Show hidden devices.
  5. Expand Network adapters. The old physical network cards are grayed out.
  6. Right-click on these NICs and click Uninstall.
  7. Once all of the grayed out NICs are uninstalled, assign the IP address to the virtual NIC.

Note: When you close the command prompt window, Windows clears the devmgr_show_nonpresent_devices=1 variable that you set in step 2 and prevents ghosted devices from displaying when you click Show hidden devices. If you are a developer or power user and you want to view devices that are not connected to your computer, set this environment variable globally:

  1. Right-click My Computer.
  2. Click Properties.
  3. Click the Advanced tab.
  4. Click the Environment Variables tab.
  5. Set the variables in the System Variables box.

Configuring disks to use VMware Paravirtual SCSI (PVSCSI) adapters

Configuring disks to use VMware Paravirtual SCSI (PVSCSI) adapters


This article includes supplemental information about configuring and using VMware Paravirtual SCSI (PVSCSI) adapters.
PVSCSI adapters are high-performance storage adapters that can result in greater throughput and lower CPU utilization. PVSCSI adapters are best suited for environments, especially SAN environments, where hardware or applications drive a very high amount of I/O throughput. PVSCSI adapters are not suited for DAS environments.
Paravirtual SCSI adapters are supported on the following guest operating systems:
  • Windows Server 2008
  • Windows Server 2003
  • Red Hat Enterprise Linux (RHEL) 5

Paravirtual SCSI adapters also have the following limitations:

  • Hot add or hot remove requires a bus rescan from within the guest.
  • Disks with snapshots might not experience performance gains when used on Paravirtual SCSI adapters or if memory on the ESX host is overcommitted.
  • If you upgrade from RHEL 5 to an unsupported kernel, you might not be able to access data on the virtual machine's PVSCSI disks. You can run with the kernel-version parameter to regain access.
  • Because the default type of newly hot-added SCSI adapter depends on the type of primary (boot) SCSI controller, hot-adding a PVSCSI adapter is not supported.


To configure a disk to use a PVSCSI adapter:
  1. Launch a vSphere Client and log in to an ESX host system.
  2. Select a virtual machine, or create a new one.
  3. Ensure a guest operating system that supports PVSCSI is installed on the virtual machine.

    Note: Booting from a disk attached to a PVSCSI adapter is not supported. The system software must be installed on a disk attached to an adapter that does support bootable disk.

  4. In the vSphere Client, right-click on the virtual machine and click Edit Settings.
  5. Click the Hardware tab.
  6. Click Add.
  7. Select Hard Disk.
  8. Click Next.
  9. Choose any one of the available options.
  10. Click Next.
  11. Specify the options your require. Options vary depending on which type of disk you chose.
  12. Choose a Virtual Device Node between SCSI (1:0) to SCSI (3:15) and specify whether you want to use Independent mode.
  13. Click Next.
  14. Click Finish to finish the process and exit the Add Hardware wizard. A new disk and controller are created.
  15. Select the newly created controller and click Change Type.
  16. Click VMware Paravirtual and click OK.
  17. Click OK to exit the Virtual Machine Properties dialog.
  18. Power on the virtual machine.
  19. Install VMware Tools. VMware Tools includes the PVSCSI driver.
  20. Scan and format the hard disk.
Note: In some operating system types, to perform this procedure, you need to create a virtual machine with the LSI controller, install VMware Tools, then change to the drives to paravirtualized mode.