MTU only on one side (host) in ISCSI setup - Consequence?

[altcapwn]

Hey everyone,

Long story short, I had to configure a last minute SAN using TrueNas.
The setup is pretty straight forward, and the details are in my signature.
The important part is that it's connected using ISCSI over 10GB SFP+ fibers.

The thing is that it was late, I was tired and I forgot to set the MTU of one of the interface in TRUENAS to 9000.
All interface in the ESXI are set at MTU 9000.

I really don't see any performance difference, but was wondering if it's worth changing the MTU if the SAN stays for long-time usage (waiting for the new SAN to arrive).
There might be unforseen consequences that I'm not aware of that may affect stability?

I'm not sure if the MTU sets itself at 9000 if the host is the initiator for the connection?

Thanks for your wisdom.

 

[altcapwn]

Hi everyone,

so while I was waiting for some VM to transfer, I did my own research and found this article : https://rickardnobel.se/troubleshoot-jumbo-frames-with-vmkping/

I've test a ping from the host to the TrueNas ISCSI IP using : vmkping -s 8972 -d IP and, unfortunately, the ping did not work, which means the communication does not work using Jumbo Frame. A standard vmkping work.

In other words, it's important to specify the MTU end-to-end.

Hope this can be useful to anyone.

Now does someone know if changing the MTU in TrueNAS restart the interface ?
I guess I'll wait the next maintenance window, when the VM's will be turned off, to change the MTU.

 

[Jessep]

Not enough information, direct connect ESXi host to SAN? or through storage switch?

 

[altcapwn]

Not enough information, direct connect ESXi host to SAN? or through storage switch?

Sorry about that, it's directly connected to the host.
vmk, vswitch and everything inside ESXi is setuped to 9000 mtu. TrueNAS interface (only 1 of the 2 ix port) has been left bland.

Tho, performance wise, even if it runs at 1500 MTU, it does not change anything, it even seems better?
(ix1 is the one left at 1500 mtu)

 

[mav@]

During TCP connection establishment sides exchange Maximum Segment Size they support. If one of sides does not support Jumbo Frames, they should negotiate usual < 1500 value, reducing efficiency. Some more complicated it may become if both sides support Jumbo Frames, but something in between does not, but in correctly configured network that should fall back too.

All that said, after you fix network MTUs you at very least need to reestablish the connections, or I guess may be even reboot to clear the results of previous Path MTU Discovery.