Slow 2,5 GBit Network connection to my Synology NAS

Issues & Assistance
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5

Vertify Raw Network Speed(critical first test)

Install iperf3 on both sides if needed (sudo pacman -S iperf3 on CachyOS).

  • On NAS (server): iperf3 -s

  • On CachyOS (client): iperf3 -c <NAS_IP> -P 4 -t 30 (use parallel streams).
    Expected: 2.3+ Gbit/s bidirectional if the link is healthy.
    If iperf is good (280+ MB/s) but SMB is slow → protocol/disk issue.
    If iperf is also low → network/driver problem

Tune Network Buffers:

For your Intel I226 interface (check name with ip link or nmcli, enpXsY)

sudo ethtool -G <interface> tx 4096 rx 4096 rx-jumbo 4096

Make it persistent (e.g., via a NetworkManager dispatcher script or systemd service). Also try:

sudo ethtool -K <interface> tso on gso on gro on

SMB Tuning on CachyOS

Edit your mount in /etc/fstab (or use smb.conf if hosting). Add options like:
vers=3.1.1,cache=strict,rsize=1048576,wsize=1048576,actimeo=0
Or try mounting with mount -t cifs … -o … manually.
Disable client-side caching temporarily or experiment with noperm / different vers (3.0 vs 3.1.1).
Check dmesg | grep -i smb or Samba logs for errors.

NAS-SIDE CHECKS:

  • Confirm the USB adapter is on a USB 3.x port and not overheating.

  • In Synology DSM: Enable SMB3, check for any “oplocks” or caching settings. Try disabling “Opportunistic Locking” or test with NFS instead of SMB for comparison.

  • Update DSM firmware and the USB adapter’s driver/firmware if available.

Other Tweaks that may help:

  • Increase ring buffers and disable some offloads if needed: ethtool -C adaptive-tx off adaptive-rx off rx-usecs 0.

  • Check for power saving: ethtool -s wol d or kernel params.

  • CachyOS-specific: Ensure you’re on a recent kernel (CachyOS has optimized kernels—try the LTS one if on bleeding-edge). Rebuild modules if using custom kernel.

  • Test direct cable (pc to NAS, no switch) to rule out the Sodala Switch.

Boot a live USB of another distro with the same Intel i226 and test speeds and Monitor CPU usage,interrupts (cat /proc/interrupts), and temps during transfer.

If iperf3 shows full 2.5 Gbit but SMB stays slow, the bottleneck is likely Samba + CachyOS or Synology’s SMB handling (common complaint with USB adapters on DSM). Many users see better results with tuned mounts or switching to NFSv4. Thats all I can think of atm.