Troubleshooting › Virtual Machine Backup and Restore Related › Host-based Agentless Backup or Restore Job Uses NBD or NBDSSL Transport Mode even when SAN Mode is Possible

Host-based Agentless Backup or Restore Job Uses NBD or NBDSSL Transport Mode even when SAN Mode is Possible

Valid on Windows platforms. Valid for VMware VM only.

Symptom

Although SAN transport mode is possible, agentless backup or restore job still uses NBD or NBDSSL transport mode.

Solution

The following prerequisites must be completed to utilize the SAN transport mode by agentless backup and restore jobs. For more information, see the VMware KB article.

• The proxy machine must be a physical machine (cannot be a virtual machine).
• The proxy machine must be connected to the SAN LUN in which the VM resides.
• On the proxy machine, SAN Policy of the SAN disk must be configured to OnlineAll.

  To configure the disk, do the following:

  1. Log in to the backup proxy system using an account with administrative privileges.
  2. Open Windows Command Line.
  3. From the command line, type the following commands
     1. Type "diskpart" and then press Enter.
     2. Type "SAN" and then press Enter.

        The current SAN policy displays.

     3. Type "SAN POLICY=OnlineAll" and then press Enter.
• If you want to perform a VM recovery by SAN transport mode, SAN disk must be configured to be writable.

  To clear the readonly flag, do the following:

  1. Log in to the backup proxy system using an account with administrative privileges.
  2. Open Windows Command Line.
  3. From the command line, type the following commands
     1. Type "diskpart" and then press Enter.
     2. Type "list disk" and then press Enter.

        The disks list displays.

     3. Type "select disk xxx" and then press Enter to select the SAN disk that gets be configured to be writable.
     4. Type "attribute disk clear readonly" and then press Enter.
• SAN transport mode offers the best performance on thick disks but the worst performance on thin disks. So, the VM recovery uses NBD or NBDSSL transport mode for thick disks by default. If you want to use SAN transport mode even for thin disks, you can add a string value EnforceTransportForRecovery with value SAN under HKLM\SOFTWARE\Arcserve\Unified Data Protection\Engine\AFRestoreDll (create the key AFRestoreDll if it doesn’t exist).
• When taking snapshot during backup, additional files are generated. So that certain free space is needed in storage device where VM’s VMDK files reside. SAN transport mode needs more free space comparing with NBD/NBDSSL transport mode. So that make sure that there is enough free space in SAN LUN if you want to use SAN transport mode.