Slow SMB File Transfer Performance Over HCX extended network

Products

• VMware HCX

Issue/Introduction

• Windows SMB file transfers are capped at approximately 30 Mbps despite a much higher physical underlay.
• Synthetic throughput tests (e.g., iperf3) show expected high speeds, but protocol-specific transfers remain slow.
• Slow performance is observed even for local on-premises inter-portgroup traffic when those networks are extended via HCX.
• No explicit error messages are generated; however, file copy "Time Remaining" estimates are excessively high.

Environment

VMware HCX

Cause

The issue is caused by the Bandwidth Delay Product (BDP) limitation of the SMB protocol when operating over a high-latency extended path. SMB is a synchronous, "chatty" protocol that requires frequent acknowledgments. When SMB packets travel through a WAN (or dedicated Express Route) to the extended network, the higher latency will prevent the TCP window from opening sufficiently to fill the available bandwidth.

Resolution

This is an expected performance outcome using SMB file transfer. However, other protocol for file transfer such as SFTP will not show such issue. To mitigate this issue, use another file transfer protocol for file transfer or use robocopy command on Windows with "Multi-Thread" option to achieve higher file transfer rate. 

An example as: robocopy C:\source \\server\dest /MT:32

Additional Information

Microsoft official KB: 

• Slow SMB files transfer speed
• robocopy