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Druva Documentation

Transport modes

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Phoenix backup proxy can access virtual machine data from datastores using three different methods – NBD, HotAdd, NBDSSL. These methods are referred to as VMware Transport modes. This article gives a brief insight about these transport modes and the known limitations around them.


In this mode, the ESX/ESXi host reads data from storage and sends it across a network to the backup proxy.  With NBD transport, large virtual disks can take a long time to transmit as the data is transferred over the network to the backup proxy.


NBDSSL is similar to NBD transport mode, but uses SSL to encrypt all data passed over the TCP connection. 


This mode takes advantage of the SCSI Hot-Add capability of the ESX/ESXi server to mount the VMDKs of a Virtual Machine, which are being backed up to the backup proxy. The data is read directly from the mounted disks directly for the backup. Backups using the HotAdd mode are faster compared to NBD or NBDSSL as it does not require data transfer over the network from the storage to the backup proxy.


For this transport mode to work correctly following are the pre-requisites:

  1. The datastore on which the backup proxy is created must be formatted with the right block size to be able to mount the largest virtual disk of the hot added virtual machines. The block size limitations vary across VMFS, VMFS-2, VMFS-3 and VMFS-5 for a given virtual disk size. For details on the block size requirements, please refer to the VMware kb article.
  2. The proxy being used to backup the virtual machine must be a Virtual Machine on a host that is able to access data store, where the source virtual machine's disk are stored.
  3. In versions 5.1 and older of vSphere, the maximum supported VMDK size is 1.98 TB
  4. The disks that are to be HotAdd must be SCSI. IDE drives are not compatible with HotAdd.
  5. The datastore of the source virtual machine and the datastore of the backup proxy should be of the same blocksize.

Note: If any of the above conditions are not met then the backup will fail over to NBD/NBDSSL.

  Limitations with HotAdd

  1. Hotadd may fail if any disk was created with a newer hardware version than the virtual machine being backed up.
  2. For example, if a disk was moved from a hardware version 8 virtual machine to a hardware version 7 virtual machine. To resolve, upgrade the hardware version of the virtual machine.
  3. 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 that is responsible for hot adding the disks. (
  4. HotAdd may fail if you are trying to back up the virtual machine through the host added as a standalone server, but actually being managed by vCenter.
  5. Hot Add may fail if the virtual machine you are trying to back up and the backup proxy are in different clusters.