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:

root@OpenMediaVault:~# ifconfig
eth0      Link encap:Ethernet  HWaddr d0:50:99:7c:27:95
          inet addr:192.168.1.4  Bcast:192.168.1.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:370449 errors:0 dropped:7 overruns:0 frame:0
          TX packets:1552174 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:26510167 (25.2 MiB)  TX bytes:2309768401 (2.1 GiB)
          Interrupt:135 Base address:0x4000


lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:36 errors:0 dropped:0 overruns:0 frame:0
          TX packets:36 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:2644 (2.5 KiB)  TX bytes:2644 (2.5 KiB)

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?

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.

Driver as the issue sounds correct. Doing more reading about this, I'll probably just reset all my network to 1500, the continual hassle doesn't justify the performance gain.

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 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.

Yes, good point. A 10 Git connection is what I need. Just need to wait until those make their way into consumer products :).