Things to take into consideration when using SMB/SAMBA protocols with SD-WAN

Products

VMware VeloCloud SD-WAN

Issue/Introduction

Customers that are using the SMB/SAMBA protocols to perform file transfers might notice less Bandwidth being utilized than what they would originally expect. Observation might also be that only one of the available links is being used partly, even though more than 1 link is available to carry the SMB/SAMBA traffic

Cause

SMB/SAMBA protocol is very sensitive to packet loss and implements a very aggressive flow control mechanism. Even if there is packet loss of less than 1% on a link used for the transfer, the impact to the bandwidth utilized being used for a file transfer would be major. The SD-WAN remediation mechanisms will improve the behavior but still the user will not see their transfer utilizing the full of the available Bandwidth of their links.

Resolution

Ensure that the File Transfer flows are using the proper Business Policy and are being treated as Transactional traffic. In that case, if there is packet loss on the path, DMPO of the SD-WAN solution will try to use the proper remediation mechanism in order to improve the performance seen.
Ensure that the available links are able to carry the Bandwidth that was either dynamically measured or manually defined for them. If the user defines a Bandwidth greater than what a link can carry, the SD-WAN solution will try to push more traffic than what the link can carry, which will be translated as packet loss from the recipient of the SMB/SAMBA flow and performance will be greatly impacted. Iperf would be a way to confirm how much traffic would a link be able to carry.
If an Edge has multiple links and the default Business Policies are being used, the Edge might still prefer a link that has some minor packet loss (less than 1%) if it has better latency or more download bandwidth than other available links. This will again have major impact specifically for the SMB/SAMBA protocol; a Business Policy could be used to stir the traffic to a link/group of links that do not have any packet loss, while the Service Provider remediates the packet loss on the affected link. At the time that the Business Policy is in place, only the link/group of links selected on the Business Policy will be used to carry the SMB traffic eg the traffic will not be carried over all available links.

Few things might help to improve customer scenario in high loss and jitter links.

The highest protocol version currently available is SMB 3.1.1.

SMB 3.0 was introduced in Windows Server 2012 and further enhanced in Windows Server 2012 R2 (SMB 3.02) and Windows Server 2016 (SMB 3.1.1). This version introduced technologies such as SMB Multichannel, that may significantly improve performance and availability of the file server.

SMB Multichannel
SMB Multichannel allows file servers to use multiple network connections simultaneously and provides increased throughput.

Performance tuning for SMB file servers
https://learn.microsoft.com/en-us/windows-server/administration/performance-tuning/role/file-server/smb-file-server

Slow SMB files transfer speed
https://learn.microsoft.com/en-us/troubleshoot/windows-server/networking/slow-smb-file-transfer

Please ask the customer to retest with SMB version 3.1.1.