[Okta Webinar] Learn how to a build a cloud-first strategyRegister Now


Optimizing MTU for NFS

Posted on 2007-10-15
Medium Priority
Last Modified: 2012-08-14
I have been configuring a site to site VPN using a cisco router (with the help of some experst on EE)

I now have the vpn tunnel functioning and stable, the networks are also routing fine. I have had to lower the MTU settings to get the data trasfer to work.

My quiestion is that is there a way of optimising this especially for NFS transfer? or is it more trial and error.

I have been changing MTU and MSS values and the trasfer does work, however it appears to stall and then retry, so the amount of data jumps in big blocks. It seems as if the retry time gets larger and larger untill it hangs completely.

We have found that at different MTU settings the data only writes the file header and hangs, other times it stops at 800k?

I can ping the down the VPN with packet sizes of upto 1472. I am using a cisco 878 on an SDSL line, to a Netgear router. We are copying from a Redhat server to a Ready NAS device.
Question by:Paul-Brooks
  • 3
  • 3

Expert Comment

ID: 20081826
You may want to also set black hole detection with the following registry settings.
I have found this helps the situation.
There may be a router on the internet between your workstation and your network causing problems.



Author Comment

ID: 20081913
can this help? we are using redhat? to a linux based NAS box?
LVL 20

Expert Comment

ID: 20082344
How do I set the MTU for my network interface? ( RedHat )
Independent Software Vendors: We Want Your Opinion

We value your feedback.

Take our survey and automatically be enter to win anyone of the following:
Yeti Cooler, Amazon eGift Card, and Movie eGift Card!


Author Comment

ID: 20083745
this is not answering the question i am asking! why the file appears to write in blocks of data, and hangs at 800k?

Expert Comment

ID: 20085961
If your packets encounter a router that has it's mtu set very low your packets will become fragmented to the point the VPN no longer can reassemble them. Some VPN's handle it better than others.
You may not always encounter this router so when you do ping tests they frequently go un-detected.
Black hole detection works to discover these routers and drops the packet MTU size dynamically to get past this router.
Another thing you might try is replacing the network cards involved on the server/workstation.
I have seen this as the cause of the problem also.
VPN connections are very picky and having a file stop or fail at a particular size like 800k is a classic symptom of this problem.

Author Comment

ID: 20086048
what i mean is you have given my registry vaules for windows? i am wokring in a linux environment, it there a wat to either set this on the router (cisco 878) or linux server?

Accepted Solution

lausengdn earned 1500 total points
ID: 20086219
Redhat calls "BlackHole Discovery"  "Path MTU discovery" they say it's configured automatically.
see this info:

You said you were changing your MTU so you may know the following info already.

I would look at replacing the network cards involved.
If this problem is happening across many pc's the server network card is probably the problem.
If it's only happening on a test pc your using the network card in that pc might be the problem.

I have not had as much success with this problem configuring firewalls or routers as I have had configuring the MTU settings on workstations and servers themselves.

Featured Post

What does it mean to be "Always On"?

Is your cloud always on? With an Always On cloud you won't have to worry about downtime for maintenance or software application code updates, ensuring that your bottom line isn't affected.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

The Cisco RV042 router is a popular small network interfacing device that is often used as an internet gateway. Network administrators need to get at the management interface to make settings, change passwords, etc. This access is generally done usi…
Shadow IT is coming out of the shadows as more businesses are choosing cloud-based applications. It is now a multi-cloud world for most organizations. Simultaneously, most businesses have yet to consolidate with one cloud provider or define an offic…
After creating this article (http://www.experts-exchange.com/articles/23699/Setup-Mikrotik-routers-with-OSPF.html), I decided to make a video (no audio) to show you how to configure the routers and run some trace routes and pings between the 7 sites…
Windows 10 is mostly good. However the one thing that annoys me is how many clicks you have to do to dial a VPN connection. You have to go to settings from the start menu, (2 clicks), Network and Internet (1 click), Click VPN (another click) then fi…

873 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question