Full proxy control: EnableSameHostHotaddMode
This setting is a useful tool that helps you optimize and control the Virtual Appliance or HotAdd transport mode. This can be particularly useful in complex or hyper-converged infrastructures (HCI) to take control of how data flows.
What is EnableSameHostHotaddMode?
When Veeam uses HotAdd mode, the virtual disks of a source VM are directly attached to a Veeam Backup Proxy that runs as a VM itself. By default, the Veeam scheduler will attempt to find the most suitable proxy based on load and connectivity. Logically this is a proxy on the same underlying host as the machine you're backing up.
The registry key EnableSameHostHotaddMode forces Veeam to preferentially use a proxy running on the same ESXi host as the VM being backed up. This localizes backup traffic within the host, reducing network load and increasing speed. So there's less room for the Veeam scheduler which proxy will take and perform the task.
Why would you use this?
There's logic performed by Veeam to assign the most optimal Proxy resource. Shouldn't we just leave it to that ? The main reason to use this key is to prevent the infamous “VM stun” or temporary unavailability of virtual machines.
NFS Datastores (HCI): In environments such as Nutanix or Cisco HyperFlex, which use the NFS protocol, a HotAdd operation where the proxy is on a different host than the VM can cause file locking (LCK files) issues. This can result in a VM becoming unresponsive for several minutes while removing a snapshot from the backup action.
Network Optimization: By sending data directly from the host's VMkernel to the local proxy, you avoid having backup data travel across the physical network between hosts.
Storage Bottlenecks: In environments with limited bandwidth between hosts, keeping traffic and be able to force this local, helps ensure overall cluster performance.
Creating the key and the different values
To make optimal use of this feature, the first requirement is to install a Veeam Backup Proxy on each ESXi host in the cluster where the VMs to be protected are located. This can be a Linux or Windows based proxy role.
Setting the Registry Key
Configure the following registry key on the Veeam Backup & Replication server:
- Path: HKLM\SOFTWARE\Veeam\Veeam Backup and Replication
- Name: EnableSameHostHotaddMode
- Type: DWORD (32-bit) This key can have different values (0,1 or 2)
Depending on how strictly you want to enforce the rule, you can set the value on 0,1 or 2
0 (Default): Disabled. Veeam uses any available proxy, regardless of whether it is local or remote.
1 (Preference): Veeam prefers the local proxy. If the local proxy is busy, Veeam waits for availability. However, if the local proxy is completely unavailable, Veeam falls back to a HotAdd proxy on another host. Important remak: this can still cause a stun with NFS.
2 (Strict mode): Veeam enforces the use of the local proxy. If the local proxy is busy, Veeam waits. If the proxy is unavailable, Veeam switches to Network Mode (NBD) instead of using a HotAdd proxy on another host. This completely minimizes the risk of snapshot stuns.
Gerneral remark: After modifying or adding the key, the Veeam Backup Service must be restarted, or you must wait 15 minutes for the automatic scan to pick up the change.
Real-world examples
Must have for Cisco HyperFlex optimization
In a Cisco HyperFlex environment, it is recommended to set the value to 2. Because HyperFlex datastores operate via NFS, this strict setting prevents Veeam from choosing a proxy on another host that would temporarily take ownership of the VM files via the LCK file, which can cause application interruptions.
Getting maximum performance in a 1GbE cluster
An administrator notices that backups are slow because the 1GbE management network between hosts is becoming overloaded by HotAdd traffic. By placing a proxy on each host and setting the value to 1, most data remains within the host. Only in cases of extreme necessity (if a local proxy fails) does the data still travel over the network, drastically reducing the daily load.
Stability preferred over speed
In an environment with highly sensitive SQL servers on NFS storage, the administrator should choose value 2. When an ESXi host undergoes maintenance and the local proxy is down, the administrator prefers that the backup runs slower via Network Mode (NBD) than to risk the SQL server “freezing” during snapshot removal.
To conclude
The EnableSameHostHotaddMode registry key is an interesting setting for any backup administrator working with NFS-based storage or HCI solutions. Although implementing a proxy on each host requires more management, the benefits of a more stable infrastructure and faster backups. From my perspective more advantages than drawbacks.
