What happens if HotAdd transport mode requirements are not met during a backup job?

If the transport mode is set to Auto, the backup job will automatically fall back to NBD mode for any disks that cannot reserve a SCSI device node. However, if HotAdd is explicitly forced as the transport mode, the backup job will fail when requirements are not met.

When should NAS transport mode be used instead of SAN mode?

NAS transport mode should be used when virtual machine datastores are on NFS storage rather than Fibre Channel or iSCSI. The access node must be a Windows or Linux physical computer, or a Linux virtual machine, and must have access to the NFS exports and ports through any firewall configuration.

Configuring VMware VSA Transport Modes in Commvault

Name: Configuring VMware VSA Transport Modes in Commvault
Uploaded: 2026-03-12T15:38:36-04:00
Duration: 4 min 57 s
Description: TL;DR Transport mode is configured at the VM group level in Commvault Command Center and determines how virtual machine data is read from VMware storage during backup and restore operations. Auto transport mode is the default setting and uses intellige...

Commvault

03/12/2026

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Transcript

The transport mode is set at the VM group level in command center. Navigate to the configuration tab and transport mode is shown in the options pane. By default this is set to auto and the software uses logic to determine the best transport mode for backup based on the type and location of virtual machines, storage infrastructure and the type and location of virtual server access nodes. In most scenarios auto will pick the optimal transport mode but you may need to change this to enhance backup or restore performance or based on specific environmental requirements. Clicking edit and then selecting the drop down menu next to auto will show the available transport modes each with very different characteristics, requirements and use cases. Starting with NBD and NBDSSL. These modes should serve as a fail back when other transport modes are not available. As these are network based they will offer the slowest backup performance. However NBD provides the fastest method of restoring thin provision disks. NBDSSL is similar to NBD mode but data transferred between the access node and the ESX server is encrypted. SAN mode is for when SAN storage is connected to the ESX host. The access node must have access to the datastore LUNs that provide storage for virtual machine disks. Data is read directly from the storage where virtual machines reside without going through the ESX host or transferring data over the network. SAN mode offers the best performance of all transport modes. SAN mode should be used when the host datastores are on Fibre Channel or iSCSI storage and when the access node is a physical server with direct access to the datastore LUNs. NAS transport is similar to SAN and allows the access node to read directly from the network file server without going through an ESX host or transferring data over the local area network. You can only use the NAS transport mode when the access node is on a Windows or Linux physical computer or a Linux virtual machine. If the access node is hosted on a Windows virtual machine you should use HOTAD or NBD mode. Access nodes must have access to the NFS exports from the NFS server and must have access to the NFS ports for the destination NFS server behind any firewall configuration. With HOTAD transport mode virtual disks from the virtual machines being backed up are automatically mounted to the access node so they can be accessed as local disks. HOTAD mode can achieve close to SAN mode performance. When using the HOTAD transport mode the access node must be virtualized. The ESX host the access node is running on must have access to all the datastores for the virtual machine. If the virtual machine and the access node are not on the same host all datastores must be shared between the hosts and the host must be within the same datacenter. The media agent component can be virtualized or physical when using HOTAD. HOTAD mode relies heavily on the SCSI protocol which comes with its own requirements. HOTAD mode does not support IDE disks. With VDDK 6.5 or later VMware recommends the para-virtual SCSI controller. For older versions you should use the LSI controller. A single SCSI controller can have up to 15 disks attached. If you run concurrent backup jobs that include more than 15 disks you might need to add SCSI controllers to the access node that is responsible for HOTAD-ing disks. Having insufficient SCSI device nodes can cause delays for backup or backup copy operations. To ensure that enough SCSI device nodes are available the best option is to load balance backups across multiple access nodes. If the backup operation for a disk cannot reserve a SCSI device node one of the following events occurs. If the transport mode is specified as HOTAD the backup job will fail. If the transport mode is specified as AUTO the backup job will switch to NBD mode for the remaining disks. The ability to automatically fall back to NBD mode in situations where there are not enough SCSI device nodes or another requirement has not been met is the reason why automatic transport mode selection is the default option and you should only force the use of a specific transport mode when it is absolutely required. www.microsoft.com

TL;DR

Transport mode is configured at the VM group level in Commvault Command Center and determines how virtual machine data is read from VMware storage during backup and restore operations.
Auto transport mode is the default setting and uses intelligent logic to select the optimal transport method based on VM location, storage infrastructure, and access node configuration.
SAN mode offers the best backup performance by reading directly from storage, while NBD provides the fastest method for restoring thin-provisioned disks despite slower backup speeds.
HotAdd transport can achieve near-SAN performance but requires virtualized access nodes, shared datastore access, and careful SCSI controller configuration with sufficient device nodes.

This technical tutorial provides a comprehensive walkthrough of VMware VSA transport mode configuration in Commvault, explaining how transport modes determine the method by which virtual machine data is read from VMware storage during backup and restore operations. The video begins by showing where transport mode settings are located within the Commvault Command Center, specifically at the VM group level under the Configuration tab. By default, Commvault uses automatic transport mode selection, which applies intelligent logic to determine the optimal transport method based on virtual machine type and location, storage infrastructure characteristics, and the configuration of virtual server access nodes. The tutorial then examines each available transport mode in detail, starting with NBD and NBD SSL, which are network-based options that offer broader compatibility but slower performance, though NBD excels at restoring thin-provisioned disks. SAN mode delivers the highest performance by reading data directly from storage without traversing the ESX host or network, requiring the access node to have direct LUN access to datastore storage. NAS transport operates similarly for NFS environments, with specific requirements around access node platform and NFS export accessibility. HotAdd transport mode receives particular attention, as it can achieve near-SAN performance by mounting virtual disks directly to the access node, but requires careful attention to SCSI controller configuration, including controller type selection and device node availability. The video emphasizes that automatic mode remains the recommended default because it provides intelligent fallback capabilities when specific transport requirements cannot be met, preventing job failures that would occur if a specific mode were forced without proper infrastructure support.

Chapters

0:00 - Transport Mode Configuration Location
0:54 - NBD and NBD SSL Modes
1:24 - SAN Transport Mode
2:01 - NAS Transport Mode
2:41 - HotAdd Transport Mode
4:14 - Auto Mode Fallback Behavior

Key Quotes

1:45 "SAN mode offers the best performance of all transport modes."
2:53 "HOTAD mode can achieve close to SAN mode performance."
4:42 "The ability to automatically fall back to NBD mode in situations where there are not enough SCSI device nodes or another requirement has not been met is the reason why automatic transport mode selection is the default option."

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