10 GbE Network Tuning on Debian

Introduction 

 This document describes a small, focused kernel network tuning for Debian systems connected via 10 GbE, using a custom /etc/sysctl.d/10g.conf file and the standard sysctl mechanism to apply the settings. These values follow common recommendations for high‑bandwidth Linux hosts by increasing TCP buffer limits and using modern congestion control and queuing disciplines. 

 Purpose of 10g.conf 

 The goal of 10g.conf is to allow the TCP stack to efficiently fill a 10 Gb/s link for protocols like SMB and NFS while remaining conservative enough for general‑purpose servers. It does this by: 

 

 Increasing the maximum socket buffer sizes for receive and transmit. 

 Raising the autotuning ceiling for TCP read and write buffers. 

 Improving backlog and connection queue limits for busy hosts. 

 Enabling modern congestion control ( htcp ) and fair queuing ( fq ). 

 

 10g.conf Content 

 Create /etc/sysctl.d/10g.conf with the following content: 

 # /etc/sysctl.d/10g.conf

# Basic 10 GbE TCP tuning for Debian / Linux.

# Focus: higher throughput for SMB/NFS and other bulk transfers over 10G.

# Allow larger TCP socket buffers for high-bandwidth links

net.core.rmem_max = 33554432

net.core.wmem_max = 33554432

net.core.rmem_default = 262144

net.core.wmem_default = 262144

# TCP autotuning limits: min, default, max

net.ipv4.tcp_rmem = 4096 87380 33554432

net.ipv4.tcp_wmem = 4096 65536 33554432

# Optional but often helpful for busy 10GbE hosts

net.core.netdev_max_backlog = 250000

net.core.somaxconn = 4096

# Use modern congestion control and fair queueing

net.ipv4.tcp_congestion_control = htcp

net.core.default_qdisc = fq 

 Parameter Notes 

 

 net.core.rmem_max / wmem_max : Maximum per‑socket buffer size; 32 MB is typical for 10 GbE hosts and is widely recommended. 

 tcp_rmem / tcp_wmem : Trio of values (min, default, max) used by TCP autotuning; the higher max allows large windows on clean high‑latency or high‑bandwidth paths. 

 netdev_max_backlog : Maximum number of packets that can be queued when the kernel receives them faster than it can process; 250 000 is a common safe value on modern hardware. 

 tcp_congestion_control = htcp : Uses HighSpeed TCP Congestion Control, designed for fast long‑fat‑pipe links. 

 default_qdisc = fq : Fair Queuing with pacing, often recommended for servers with modern kernels. 

 

 Applying the Configuration 

 Immediate Application 

 After saving /etc/sysctl.d/10g.conf , apply the settings to the running kernel: 

 sudo sysctl --system 

 This command reloads all configuration files under /etc/sysctl.d , /run/sysctl.d and /usr/lib/sysctl.d , and applies all settings without requiring a reboot. 

 Note: The settings take effect immediately for new connections. Existing long‑lived TCP sessions may continue using their previous buffer sizes until they are re‑established. 

 Verification 

 You can verify that the values have been applied by querying a few key parameters: 

 sysctl net.core.rmem_max

sysctl net.core.wmem_max

sysctl net.ipv4.tcp_rmem

sysctl net.ipv4.tcp_wmem

sysctl net.ipv4.tcp_congestion_control

sysctl net.core.default_qdisc 

 The output should reflect the values specified in 10g.conf , confirming that the tuning is active. 

 Usage and Testing 

 Once the configuration is applied, you can re‑test SMB/NFS throughput (for example, from your Nextcloud host or other Debian clients) using representative workloads or tools like iperf3 , large file copies, or application‑level benchmarks. The tuning primarily benefits sustained transfers where the network path was previously constrained by default TCP buffer limits rather than disk or CPU.