HotAdd Transport Mode does not Work when Attempting to Back Up a VMware VM

Symptom

The HotAdd transport mode is not supported for this VM and as a result the backup is failing over to the NBDSSL (encrypted network block device) mode. (Backup job is running slow). The backup of a VMware VM is not using HotAdd Transport. For more details about HotAdd transport, see link.

Verify the following HotAdd prerequisites:

• The HotAdd backup proxy must be a virtual machine. HotAdd involves attaching a virtual disk to the backup proxy, like attaching disk to a virtual machine.
• The HotAdd proxy must have access to the same datastore as the target virtual machine.
• The VMFS version and data block sizes for the target VM must be the same as the datastore where the HotAdd proxy resides. If the HotAdd proxy is a virtual machine that resides on a VMFS-3 volume, choose a volume with block size appropriate for the maximum virtual disk size of virtual machines that customers want to back up, as shown in VMFS-3 Block Size for HotAdd Backup Proxy. This caveat does not apply to VMFS-5 volumes, which always have 1MB file block size.
• The following table displays the VMFS-3 Block Size for HotAdd Backup Proxy:

  VMFS Block Size	Maximum Target Disk Size
  1 MB	256 GB
  2 MB	512 GB
  4 MB	1024 GB
  8 MB	2048 GB

• In vSphere 5.1 and older, the maximum supported VMDK size is 1.98 TB.
• The disks that are to be HotAdd must be SCSI. IDE drives are not compatible with HotAdd.
• VMware Tools must be installed and up-to-date on the VM and the backup proxy.
• Datastore needs sufficient space for a VM snapshot.
• HotAdd may fail if any disk was created with a newer hardware version than the VM being backed up. For example, if a disk was moved from a hardware version 8 VM to a hardware version 7 VM. To resolve, upgrade the hardware version of the VM.
• A single SCSI controller can have a maximum of 15 disks attached. To run multiple concurrent jobs with more than 15 disks, you need to add more SCSI controllers to your backup proxy machine.
• In case of standalone ESX connection (ESX server is not managed by vCenter), you can only HotAdd disks of VMs which are located on the same ESX as the backup proxy machine.
• HotAdd may fail if you are trying to back up the VM through the ESX added as a standalone server into UDP but actually being managed by vCenter.
• Hot Add may fail if the VM you are trying to back up and the proxy server are in different clusters.

Solution

Disable “automount” on the backup proxy machine using “diskpart” utility.