Virtual Appliance Mode

As the default setting, virtual appliance mode (hot-add) has become quite popular for all-in-one deployments of Veeam Backup & Replication within virtual machines (for details, see the Deployment Scenarios section of the User Guide). It is also often used, when Veeam is deployed in branch office configurations (ROBO).

This mode supports a 100% virtual deployment, and uses the VMware ESXi storage I/O stack, providing very efficient backups and having very little overhead in terms of throughput. During backup or replication, while the original VM is running off of a VM snapshot, the original virtual machine disks (VMDK) are mounted via SCSI hot-add to the backup proxy server. Once the backup or replication job finishes, the disks are unmounted from the proxy server, and the VM snapshot is committed.

Note: For more information on how it works, refer to the section "Data Backup and Restore in Virtual Appliance Mode" in Veeam Help Center.

As an example, virtual appliance mode is a good choice for highly dynamic environments, where it can be difficult for backup administrators to maintain access to newly created datastores for Direct Storage Access. Prerequisites for using virtual appliance mode are described in the following knowledge base article: Appliance Mode (Hotadd) Requirements and Troubleshooting

When planning for the Virtual Appliance mode for a backup proxy consider the time required for actual hot-add operations (such as adding and removing VM disks from the source virtual machine) it can add up to 1-2 minutes per VM. For a backup job containing 100 virtual machines this could result in more than two hours of adding and removing disks with no actual data processing. To mitigate the issue enable parallel processing and process multiple disks from the same virtual machine simultaneously (using this transport mode).

Tip: It is recommended to benchmark how such operations affect the backup window by monitoring a test job in the vSphere console.

Veeam developed Direct Storage Access for NFS based datastores to overcome the problems with disk hot-add and release which causes significant stuns for NFS based VMs). Direct Storage Access should be used for all virtual and physical proxy deployment to backup and restore NFS datastore based VMs.


  • Using the Virtual Appliance mode for proxy servers enables a fully virtual deployment.

  • As the proxy will perform source side data deduplication and compression, this mode will provide satisfactory performance in environments running 1 GbE configurations.

  • Virtual appliance mode utilizes Veeam Advanced Data Fetcher (ADF), providing significant increase in throughput for enterprise class storage.


  • If working in this mode the backup proxy will occupy the virtual infrastructure resources impacting consolidation ratio. This could ultimately require additional physical ESXi hosts and licensing.

  • This mode requires additional planning and configuration in the enterprise environments because of the additional large disk Hot-Add processes in VMware vSphere.

  • In situations with a high number of VMware clusters with individual datastores a minimum of one proxy per cluster is needed, this can increase management overhead.

Considerations and Limitations

Additional load is put on the vCenter Server and ESXi hosts as each disk is mapped and unmapped (disk hot-add) at the backup proxies.

Note: For more information see vCenter Server connection overview in the "Veeam Backup & Replication Server" section of this guide.

It may occur that VMware API reports that unmap and snapshot commit were done correctly but a snapshot file still remains on disk. These "orphaned snapshots" will grow over time and can fill up the datastore leading to downtime. To mitigate the issue, Veeam implemented the following functionality:

  • Veeam Snapshot Hunter. This feature automatically initiates disk consolidation for VMs in the "Virtual machine disks consolidation is needed" state. For more information please see Snapshot Hunter section

  • Bypassing Virtual Disk Development Kit (VDDK) processing to overcome some limitations and performance challenges, in particular:

    • Veeam can back up multiple disks of VM in parallel on same proxy (default number is 4).
    • Typical "hot-add I/O bursts" during hot-add operations are mitigated by bypassing VMware VDDK during restores and replication.
    • When performing writes via hot-add and VDDK, excessive metadata updates on the VMFS datastore will occur. This significantly impacts performance for other workloads on the datastore, and slows down restore throughput. Bypassing VDDK helps overcoming this limitation
  • To avoid some VMware issues related to NFS datastore and hot-add processing (described at ) enable a specific setting that will process VM backups only on backup proxies that run on the same host. For details see . To avoid this completely we highly recommend you to use the Direct NFS backup mode for backup and restore of NFS datastore based VMs.

Note: For additional tips refer to the “Impact of Snapshot Operations” section of this guide.

vSphere 6.5 and encryption

Virtual appliance mode is typically the best choice to ensure data availability for vSphere 6.5 clusters with encrypted virtual machines. In order to support backup of encrypted virtual machines, the virtual backup proxy must be encrypted within the same encryption domain (using the same KMIP server).

Backup modes Direct Storage Access and Backup from Storage Snapshots are unavailable for encrypted virtual machines, and NBD will not be as performant. vSphere 6.5 also enforces SSL/TLS encryption for network mode (NBD), rendering virtual appliance mode a much more performant alternative, and will reduce host CPU usage.


  • Virtual appliance mode should be used when it is not possible to leverage Direct Storage Access, for example in the case of local datastores, Virtual Volumes (VVOL) or vSAN.

  • You will need at least one type of (virtual) SCSI controller added to Proxy Server VM that is used somewhere at the VMs in your infrastructure to allow VMware to HotAdd the VM disks at backup.

  • Add an extra SCSI controller to allow for more VM disks processing in parallel (check the corresponding Veeam proxy settings, default value is 4). The limit for a single controller is the maximum number of devices per SCSI controller (15). Max SCSI controllers per VM is 4 = 60 disks max. Adding one additional SCSI controller is usually sufficient.

  • When deploying hot-add backup proxies avoid cloning existing VMs as this may lead to identical UUIDs and cause hot-add operations to fail.

  • You may re-use any existing Windows server VM (to save on licensing). The Veeam data mover process runs with ‘below normal’ priority by default.

    Note: Changed block tracking (CBT) will be disabled for these hot-add proxies. Consider that it may impact the backup window in case the said virtual machines should be included in backup or replication jobs.

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