Irregularities when MTU changed from 1500

  • I tried changing the MTU to 9000 to increase the transmission efficiency on my gigabit home network. The change was reflected in the network tab (but strangely not in the dashboard). I checked with ifconfig and it wasn't taking effect.


    ifconfig from when I had the MTU set to 9000 through the OMV GUI:


    At that point I noticed things needing internet access were failing. Executing apt-get failed as did other like commands. Everything started working again when I put the MTU back at 1500 and rebooted.


    Any ideas on what was going on?

  • It's unrealistic to expect jumbo frames to work on the internet as most commercial internet service providers do not support their use.

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  • If IPV4 routers couldn't handle the mismatch between MTUs then nobody would ever be able to use MTUs that are different. Routers are capable of performing fragmentation of packets to cut them down to size so they fit into the smaller MTU-size tunnels (even if this isn't optimal).

  • The Windows 8/10 machines on my network are all set to an MTU of 9000 and it gives them a throughput boost when transferring large files. They all communicate just fine with the internet through my Linksys router/firewall.


    It's just OMV/Debian that seems to be having problems.

  • If anyone is interested on a Braswell based Celeron system with SSD drive (running the latest OMV 2.1), Gigabit switch I get:
    - 95 MB/s with 1500 MTU
    - 105 MB/s with 9000 MTU.


    However, maintenance for this is on the high side with compatibility issues (like this instance) and on the Windows side of things with Windows spontaneously changing itself back to 1500 when drivers update.

  • That's about a 10% improvement. How much variation do you see in day to day use with either setting?

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  • That simple test is all the data I've collected. It certainly is an improvement, but I'll probably keep it at 1500 for less hassle.


    Obviously the Gigabit connection is the bottleneck at 1 Gbps transfer rate when my SSDs are going to be maxing out the SATA3 connection (6 Gbps). I think if I really care about it I'll buy a board with dual-NICs and do link aggregation. However, I'll probably not bother as 95 MB/s is still very fast and I like that my system only uses 10 Watts.

    • Official Post

    Are you using the backports 3.16 kernel? I have a Celeron J1800 with an Intel i210 NIC and it can saturate gigabit (114 MB/s) over rsync (syncing movie files between my two servers). Never needed jumbo frames.

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  • I think if I really care about it I'll buy a board with dual-NICs and do link aggregation.


    Don't expect too much of link-aggregation. It uses a statically loadsharing depending for example on source-mac address. This scheduler is often configurable.
    That is very usefull in one (or many) to many communications.
    But in a p2p communication your traffic will always use only one of your links.

  • Maybe with Kaby/Canon Lake there could be some movement about this... now that faster SSD connections are on the market GBE is becoming more and more the bottleneck...


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  • Faced a similar problem. So far, it is only managed by hands with mac registration in the netplan configuration file.

    Q: if you leave netplan configuration via GUI, will it work? In particular, changing the MTU. (OMV v5.4.2-1)

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