From: Robert Minsk <rminsk@(email surpressed)>
Subject: Re: Jumbo Frames and Older Machines
   Date: Tue, 14 Sep 2010 22:21:37 -0400
Msg# 1960
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For NFS mounts you can use the 'tcp' option.  tcp figures out what the maximum
packet size it can use on a connection.  tcp sends larger and larger ICMP
packets with the 'do not fragment' flag until one comes back with an error.

http://en.wikipedia.org/wiki/Path_MTU_discovery

Unfortunatly udp can not determine what the maximum packet size it can use.
Rush uses udp in several places.  The only way to fix the udp problem is to
lower all you hosts MTU to the smallest of all the machines, in your case
1518.  There is really no way for rush to work around this as it is a problem
with mismatched MTUs and udp.

Daniel Browne wrote:
> [posted to rush.general]
> 
> The system rejects any mtu larger than 1518. We've had lots of other issues=
>  with it as well, so we've basically got no choice but to change back to st=
> andard.
> 
> 
> On Sep 8, 2010, at 7:17 PM, Greg Ercolano wrote:
> 
> [posted to rush.general]
> 
> 	Up the MTU using ifconfig(1) to match the other boxes if you can.
> 	I believe mismatched MTUs affects UDP packet assembly.
> 
> Daniel Browne wrote:
> > We're in the midst of some network upgrades and are having some difficulties
> > transitioning to using ethernet Jumbo frames. Several of our older machines
> > (PowerMac G5's) do not have hardware support for Jumbo frames. The switch
> > they are connected to allows me to configure frame mtu size on a per-port
> > basis, but this is not preventing packet drops enough to allow Rush to work
> > properly. Jobs are taking inordinately long (which may be due to NFS packet
> > loss caused by jumbo frames) and I cannot push updated hosts list files
> > to these machines.
> > 
> > If anyone can suggest a solution I'm open to it, though we overall are having
> > enough issues that we may need to abandon Jumbo frames altogether until
> > such time as the older equipment is decommissioned.
> >
> > If anyone can suggest a solution I'm open to it, though we overall are having
> > enough issues that we may need to abandon Jumbo frames altogether until
> > such time as the older equipment is decommissioned.
> 
> -- 
> Greg Ercolano, erco@(email surpressed)
> Seriss Corporation
> Rush Render Queue, http://seriss.com/rush/
> Tel: +1 626-576-0010 ext.23
> Fax: +1 626-576-0020
> Cel: +1 310-266-8906

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