Reading Time: 3 minutes

VMware vSphere provides a different way to copy the VM data during a backup operation: those modes are called transport modes. There are at least three major transport mode: network mode (or NBD), hot-add mode (or VM proxy mode), SAN mode (of storage offload mode).

Most of the backup products can use those different transport modes depending on the configured infrastructure and the requirements.

Network mode use transport for data access, with NBD (network block device) protocol, with the optionally SSL encryption (in this case is called NBDSSL).

Note that NBDSSL backup is slower than SAN and HotAdd transport modes. Before vSphere 6.0, Network Block Device (NBD) transport was available for somewhat better performance by avoiding SSL connection and data encryption, but for security reasons, NBD is now a synonym for NBDSSL.

More information are available on VMware VDDK documentation:

NBD transport mode

NBD is a Linux-style kernel module that treats storage on a remote host as a block device that has several limitations (documented on the VDDK guide). These NFC session limits do not apply to SAN or HotAdd transport.
NFC Session Connection Limits

Host Platform

When Connecting

Limits You To About

vSphere 4

to an ESX host

9 connections directly, 27 connections through vCenter Server

vSphere 4

to an ESXi host

11 connections directly, 23 connections through vCenter Server

vSphere 5 and vSphere 6.x

to an ESXi host

Limited by a transfer buffer for all NFC connections, enforced by the host; the sum of all NFC connection buffers to an ESXi host cannot exceed 32MB.

52 connections through vCenter Server, including the above per-host limit.

These are host limits, not per process limits. Additionally, vCenter Server imposes a limit of 52 connections.

To improve NBD transfer speed it’s not so easy and documentation does not help so much. For example, it’s possible to increase the vCenter SOAP connection limitation, but the VMware KB 2004663 it’s quite old and not updated to vSphere 6.x.

For the ESXi is possible increasing the NFC buffer size and optimize the Cache Flush interval, but again the related VMware KB 2052302 it’s very old and not updated to vSphere 6.x.

So for Network mode, using vSphere 6.x, there is not much to do unless using a fast NIC for the ESXi management traffic.

As documented in VMware KB 2147768 (Slow backup performance using NBD transport mode), profiling in an ESXi over Gigabit Ethernet found that NBDSSL read performance was about 3.6 megabytes per second, and write performance was about 2.9 megabytes per second.

The best solution could use Hot-Add mode or SAN mode if available, but note that also those modes can have some limitations (see for example the one with hot-add and VSAN datastore).