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📚 Making MTU 9000 Persistent on XCP-ng (OVS)

📚 Guide Overview

Title: Making MTU 9000 Persistent on XCP-ng (OVS)

Author: MSLS Partners LLC (or your team name)

Last Updated: April 2025

Description: This guide explains how to configure MTU 9000 for Open vSwitch (OVS) inside XCP-ng environments. It ensures that jumbo frames remain active and survive reboots, greatly improving network performance for storage and virtual machines.

Difficulty Level: Intermediate 🛠️

Estimated Time: 15–20 minutes

Making

📖 Table of Contents

📖 Purpose

This guide explains how to correctly configure MTU 9000 on Open vSwitch (OVS) inside XCP-ng to ensure jumbo frames survive across reboots.

📖 Requirements

  • XCP-ng 8.x or newer
  • Open vSwitch in use (default in XCP-ng)
  • SSH access to the hypervisor
  • Root privileges

📖 Background

  • XCP-ng uses Open vSwitch (OVS) to manage VM networking.
  • Setting MTU at the Linux NIC level is not enough; OVS controls the real MTU behavior.
  • XCP-ng resets network bridges at boot, so MTU settings must be reapplied automatically.

📖 Steps

🛠 Step 1: Create the MTU Fix Bash Script

sudo nano /usr/local/bin/fix-ovs-mtu.sh

Paste this:

#!/bin/bash
sleep 20
ovs-vsctl set interface eth4 mtu_request=9000
ovs-vsctl set interface xenbr4 mtu_request=9000
echo "$(date) - MTU 9000 applied to eth4 and xenbr4" >> /var/log/fix-ovs-mtu.log

Make it executable:

sudo chmod +x /usr/local/bin/fix-ovs-mtu.sh

🛠 Step 2: Create the systemd Service

sudo nano /etc/systemd/system/fix-ovs-mtu.service

Paste this:

[Unit]
Description=Fix OVS Interfaces MTU to 9000 After Boot
After=network-online.target openvswitch-switch.service

[Service]
ExecStart=/usr/local/bin/fix-ovs-mtu.sh
Type=oneshot
RemainAfterExit=yes

[Install]
WantedBy=multi-user.target

🛠 Step 3: Enable the Service

sudo systemctl daemon-reload
sudo systemctl enable fix-ovs-mtu.service
sudo systemctl start fix-ovs-mtu.service
sudo systemctl status fix-ovs-mtu.service

🛠 Step 4: Validate

  • Check service status:
sudo systemctl status fix-ovs-mtu.service
  • Check MTU on physical NIC and bridge:settings:
  • Verify inside OVS:
ovs-vsctl list interface eth4 | grep mtu
ovs-vsctl list interface xenbr4 | grep mtu

📖 Expected Results

  • eth4 MTU = 9000
  • xenbr4 MTU = 9000
  • VMs' VIFs inherit MTU 9000
  • Jumbo frames supported end-to-end
Performance improvement (storage, VM transfers)

📖 Troubleshooting

Symptom Likely Cause Solution
MTU resets to 1500 after reboot Systemd service not enabled Check service status
Ping fragmentation on large packets Switch ports not configured for MTU 9000 Enable jumbo frames on switch ports
iperf3 speed low MTU not properly set at all layers Double-check NIC, bridge, VM, and switch

📖 Bonus: Quick MTU Validation Script

#!/bin/bash
echo "Checking eth4:"
ip link show eth4 | grep mtu
echo "Checking xenbr4:"
ip link show xenbr4 | grep mtu
echo "Checking VMs VIFs:"
ip link | grep vif | grep mtu

📖 Final Notes

  • Always verify MTU end-to-end:from Storagestorage → Switch → Hypervisor →to VM.
  • Setting mtu_request ininside OVS and using a smallsystemd fix ensures full persistence.
Better performance, lower CPU usage for 10GbE network operations.

✅ Quick Steps Checklist

    Create the MTU fix script at /usr/local/bin/fix-ovs-mtu.sh Make the script executable Create the systemd service guaranteesat success across reboots./etc/systemd/system/fix-ovs-mtu.service JumboReload framessystemd dramaticallyEnable improveand 10GbEstart performance,the especiallyservice Verify MTU settings on eth4 and xenbr4 after boot Validate VMs inherit MTU 9000 Test with storage traffic like iSCSIiperf3 and NFS.ping -M do -s 8972

    🎯

    End of Tutorial

    Created and maintained by MSLS Partners LLC 📚

    Empowering IT with clear documentation and professional best practices.

    Version 1.0 | Last Reviewed: April 2025

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