[Users] strategies for bridging IB networks

[Chris Hunter]

Hi,
We have been using IB in our datacenter for quite some time. As a result 
we have built up a collection of non-homogeneous IB networks. Going 
forward we are trying to merge various isolated networks together.

To give an example, we have 3 separate IB networks, each is fat-tree 
topology:

S. storage network with a single 36port QDR IB switch connected to 20 
storage servers

A. non-blocking client network with 2 director class QDR switches 
connected to QDR bladechassis switches

B. blocking (4-1 oversubscribed) client network with 36port FDR 
top-of-rack switches connected to a director class FDR switch

Given practical limits (cable lengths, port counts) we *don't* want to 
re-cable everything to build a single fat-tree network.

So our question, what are strategies for connecting together fat-tree 
networks together (without re-cabling)?

Ideally we want to minimize congestion, use predictable routing (ie. 
through the director switches), etc.

We are considering:
- partitions/virtual fabrics; not clear this will help routing but 
should help congestion
- connect IB networks at the switches; this will create an "unbalanced 
tree" but hopefully routing will be sane


Any feedback appreciated,
chris hunter
chris.hunter at yale.edu