[Beowulf] Mixing different MTU settings on the same LAN?

Douglas Eadline deadline at eadline.org
Mon May 18 11:57:59 EDT 2009


Carsten

Within a given network the MTU must be the same.
For instance a head node will normally have at least
two networks

1) LAN
2) private cluster network

The LAN is usually the standard MTU (1500) while the
private cluster network can be what ever the NICs support.
Not all NICs support Jumbo frames.

You can set the MTU with ifconfig, but it is usually best to
set these when nodes when they boot because logging in to a node
with a default MTU (1500) and changing it to say 6000
means the head node is still at 1500 while the node
is as 6000. Unless you have a dual network in
you cluster (one for compute and one for admin/NFS)
Then you can login over one network and change the other.

--
Doug







> Hi,
>
> maybe my bad feeling is just non-sense but when looking on the web for
> information, it seems that there is no definitive answer I have found so
> far. Our data network runs at MTU9000 but the last batch of test nodes
> we bought seem to have problems with Jumbo frame (Yukon2 based NICs) or
> cannot do Jumbo frames at all (nVidia).
>
> I've run some early tests and these *seem* to suggest that the IP-layer
> takes correctly care of this (e.g. tcpdump shows that the maximum header
> length it 1514 bytes when the link is in use).
>
> Since famous Wikipeda states "Limit varies by vendor. For correct
> interoperation whole ethernet network must have same MTU. Usually only
> seen in special purpose networks." on their page on
> http://en.wikipedia.org/wiki/Maximum_transmission_unit
>
> and others (netbsd-help page IIRC) suggests to separate these nodes from
> the main network via a router I'm a bit confused who is right.
>
> Anyone here knowing much more on networking and can give me thumbs up or
> down?
>
> Thanks
>
> Carsten
>
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--
Doug

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