Backup transport modes
Posted by Marijan Kozic on 04 February 2021 14:17
Archiware Pure can access Virtual Machine data using two different methods, usually referred as transport modes. This article will explain how they work and what are their differences.

HotAdd transport mode

When Archiware Pure is running as a Virtual Appliance within the same vSphere environment that it is protecting, it is able to leverage the vSphere HotAdd feature to attach and detach SCSI virtual disks without shutting down. During backup, Pure instructs the vSphere Infrastructure server (either a vCenter or a standalone ESXi) to temporarily attach the virtual disks of the target VMs to the Pure Virtual Appliance. This disks then appear as local SCSI drives and Pure is able to access their data directly, using the ESXi I/O stack.
Because the data transfer speed is higher than in network transport mode and because it does not impact LAN bandwidth, the HotAdd mode is preferred and automatically chosen if the requirements are met.

The requirements for the HotAdd transport mode are:
- Pure runs as a Virtual Appliance within the protected vSphere
- ESXi host managing Pure VA has access to datastores where target VMs reside

Because HotAdd only works for SCSI disks and there is a limit of 15 disks that can be attached for each SCSI controller, Pure is limited to a maximum of 45 simultaneously attached virtual disks, meaning that no more than 45 Backup/Verify/Restore/Restore File jobs can be running at a time.

Network transport mode

If the HotAdd requirements are not met, Pure will automatically revert to the Network transport mode. This includes two transport modes that VMware refers to as NBD and NBDSSL, with Pure choosing the SSL protected method by default.
The Network transport mode uses the VMware's closed source implementation of the Network File Copy (NFC) protocol to transfer backup data using the ESXi networking stack. The only requirement is that Archiware Pure has network connectivity to target ESXi hosts on TCP port 902.

Despite Network transport mode typically being much slower than HotAdd, especially on sub 10 GB Ethernet speeds and despite it introducing additional load to the LAN traffic, it is a valid choice as it offers unparalleled freedom by having almost no requirements.

This allows installing Pure on any of the supported platforms such as physical Linux based servers, NAS devices and remote servers as long as there is network connectivity between Pure and the target vSphere. Without the need for a direct storage access, ESXi hosts can use local storage to host VMs and no longer require setting up shared datastores, which is a great advantage for smaller installations with fewer hosts.
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