SIRIS, ALTO, and NAS: Comparing agentless backup transport methods



This article discusses best practices for optimizing backup speeds when backing up a physical Datto device to a VMware environment.


  • Datto SIRIS


Agentless systems: the HotAdd transport method

The most efficient transport method currently available for Datto Agentless backups is the HotAdd transport method. 

  • Currently this transport is available only on vSIRIS virtual appliances.
  • HotAdd uses the native ESXi I/O stack, and the transport itself doesn't traverse the network.
  • VMware snapshots of the protected VM's virtual disks are attached directly to the vSIRIS VM for agentless backups.

Physical Datto devices: the NBD-SSL transport method

  • The NBD-SSL transport method for agentless backups of VMware VMs destined for a physical SIRIS.
  • Transport speed is affected by I/O performance on both the VMware datastores and the Datto appliance, as well as the network data path through which the backups travel.
  • Backup traffic flows from the VMkernel port, via port 902 to the physical SIRIS device.
  • If the VMs are located on network storage, the backup data path flows from the SAN to the ESXi host, and then to the physical SIRIS via port 902.

Increasing NBD-SSL Throughput

  • Use 10Gbit networking hardware for the VMkernel management ports.
  • Ensure the entire data-path between the VMkernel ports and the physical SIRIS, including the link to the SIRIS itself, is 10Gbit.
  • Enable jumbo frames across the entire data-path between the VMkernel ports and the physical SIRIS.
  • If the ESXi hosts have both 1Gbit and 10Gbit management NICs, ensure that the backup traffic is flowing over the 10Gbit NICs, not the 1Gbit NICs.
  • If possible, push all other management traffic (for example, provisioning or vMotion) over to a different VMkernel NIC than the one being used for agentless backup traffic.


  • Per VMware's documentation, NBD backups should not be performed on virtual disks larger than 1TB each. 
  • For LAN transport, virtual disks cannot be larger than 1TB each. As the name implies, this transport mode is not LAN-free, unlike SAN and HotAdd transport.

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