Why My 10 Gigabit Ethernet Was Running at 300 Megabits
The surprising culprit behind network performance issues: LSO V2 setting
1Unexpected Slowdown in 10GbE Network
File transfer speeds stalled at 100-200 Mbps on a system with UniFi UNAS Pro 8 storage and 10GbE NIC. Initially suspected the NAS and mechanical disks. Six 16TB drives in RAID 6 configuration and NVMe SSD cache were tested. However, stopping Immich photo service and changing cache settings didn't resolve the issue.
2Isolating the Problem Source
iperf3 tests bypassing the NAS showed inconsistent performance: 133 Mbps receive, 1.33 Gbps transmit between two computers. This proved the issue was independent of storage and pointed to NIC/Windows interaction. Asymmetric performance indicated configuration errors rather than hardware faults.
3Discovering Packet Drops
Intel E610-XT2 adapter statistics revealed nearly one million 'ReceivedDiscardedPackets'. During a 10-second iperf3 test, this counter increased by 268. Increasing receive buffers from default 512 to maximum 4096 eliminated packet drops and boosted receive speed from 1.33 Gbps to 5.15 Gbps. However, transmit speed remained stuck at 313 Mbps.
4The Critical Impact of LSO V2
Disabling Large Send Offload V2 (IPv4) increased transmit speed from 313 Mbps to 7.03 Gbps - a 22x improvement. This revealed LSO was causing severe performance issues in this specific hardware combination (Windows + Intel E610). Normally a CPU-saving feature, LSO can negatively impact certain configurations.
5Real-World Test Results
After fixes, sustained SMB write speeds to RAID 6 NAS reached 350 MB/s (2.8 Gbps) - within expected range for six mechanical drives. Measurements demonstrated how NIC settings directly affect storage performance. Proper tuning unlocked the system's full potential.
6Importance of NIC Optimization
Modern 10GbE NICs may not deliver optimal performance with default settings across all use cases. Testing advanced NIC features becomes critical, especially in cross-platform communications (Windows-Linux). Buffer sizes and offload features require hardware-specific tuning for best results.
7Troubleshooting Methodology
This case highlights systematic troubleshooting importance. Starting with storage tests, the process narrowed down to network layer using iperf3. After identifying NIC settings as culprit, each parameter was tested methodically. This approach minimized trial-and-error time that could span hours.
8Beware of Universal Solutions
While disabling LSO V2 brought 22x improvement in this specific case, it's not a universal fix. Microsoft documents how offload features may reduce performance in some configurations. Network issues require measurement before targeted interventions. One-size-fits-all solutions might degrade performance further.