I’ve originally wanted to make a bug report in the TrueNAS itself. But it requires a Jira account. And having 2 accounts for the TrueNAS is a little bit too much. So I post it here. Well, actually it’s not a bug but rather a lack of UX.
I’ve changed HW (via transferring all drives), and all was ok, except network config (as expected). I’ve just assigned “new” NICs to my LAN and SAN bridges. My LAN bridge has a default 1500 MTU, and SAN has 9000. “New” NICs had 1500 (as expected), and I forgot to change it 
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My SAN network was unstable from time to time. Ping always worked. iperf3 had always shown (almost) zeroes. Dockerised PBS worked as expected. After I’d changed the NIC MTU to 9000, all went back to normal.
It was my fault. BUT! It would be very beneficial if there was an MTU discrepancy check (as there is with IPs). Proxmox, for one, does have this MTU check.
Steps to reproduce
- Create a bridge with MTU 9000.
- Add a bridge member with MTU 1500.
- Add some IP to the bridge.
- Save the configuration.
- Connect some client to the NIC. The client should have MTU 9000 as well.
- Mount SMB share or run iperf3 on the client.
- Pull the hair because of a very unstable network. Which works for some applications but sometimes can’t even list files on the SMB share. The debug session for this “heisenbug” gave me a hard time
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Expected result
Warning or error of MTU discrepancy during step 4.
Actual result
See steps 5-7.