[Beowulf] Questions about upgrading InfiniBand

[Prentice Bisbal]

Beowulfers,

I'm planning on adding some upgrades to my existing cluster, which has
66 compute nodes pluss the head node. Networking consists of a Cisco
7012 IB switch with 6 out of 12 line cards installed, giving me a
capacity of 72 DDR ports, expandable to 144, and two 40-port ethernet
switches that have only six extra ports between them.

I'd like to add a Lustre filesystem (over InfiniBand)  to my cluster,
and then begin adding/replacing nodes in the cluster. Obviously, I'll
need to increase capacity of both my IB and ethernet networks. The
questions I have are about upgrading my InifiniBand.

1. It looks like QLogic is out of the InfiniBand business. Is Mellanox
the only game in town these days?

2. Due to the size of my cluster, it looks like buying a just a
core/enterprise IB switch with capacity for ~100 ports is the best
option (I don't expect my cluster to go much bigger than this in the
next 4-5 years).  Based on that criteria, it looks like the Mellanox
IS5100 is my only option. Am I over looking other options?

http://www.mellanox.com/content/pages.php?pg=products_dyn&product_family=71&menu_section=49

3. In my searching yesterday, I didn't find any FDR core/enterprise
switches with > 36 ports, other than the Mellanox SX6536. At 648 ports,
the SX6536is too big for my needs. I've got to be over looking other
products, right?

http://www.mellanox.com/content/pages.php?pg=products_dyn&product_family=122&menu_section=49

4. Adding an additional line card to my existing switch looks like it
will cost me only ~$5,000, and give me the additional capacity I'll need
for the next 1-2 years. I'm thinking it makes sense to do that, and wait
for affordable FDR switches to come out with the port count I'm looking
for instead of upgrading to QDR right now, and start buying hardware
with FDR HCAs in preparation for that.  Please feel free to
agree/disagree. This brings me to my next question...

5. FDR and QDR should be backwards compatible with my existing DDR
hardware, but how exactly does work? If I have, say an FDR switch with a
mixture of FDR, QDR, and DDR HCAs, will the whole fabric slow down to
the lowest-common denominator, or will the slow-down be based on the two
nodes involved in the communication only? When I googled for an answer,
all I found were marketing documents that guaranteed backwards
compatibility, but didn't go to this level of detail, I searched the
standard spec (v1.2.1), and didn't find an obvious answer to this question.

6. I see some Mellanox docs saying their FDR switches are compliant with
v1.3 of the standard, but the latest version available for download is
1.2.1. I take it the final version of 1.3 hasn't been ratified yet. Is
that correct?

-- 
Prentice