[Lustre-discuss] Multiple IB ports

[Peter Kjellström]

On Tuesday, March 22, 2011 06:15:35 am Atul Vidwansa wrote:
> Hi Brian,
> 
> With one 4x QDR IB port, you can achieve 2 GB/Sec on single client,
> multi-threaded workload provided that you have right storage (with enough
> bandwidth) at other end.  We have tested this multiple times at DDN.
> 
> I have seen sites that do IB-bonding across 2 ports but mostly in failover
> configuration. To get 10GB/Sec to a single node requires aggregating 5 QDR
> IB ports. You will need to confirm from your IB vendor (Mellanox? ), OS
> vendor (SGI/RedHat/Novell) and Lustre vendor whether they support
> aggregating so many links.  I think the challenge you will have is to find
> a Lustre client node that has enough x8 PCIe slots to sustain 3 dual-port
> Infiniband adapters at full rate

Just adding a small detail, a single port of QDR consumes all of the HCAs pci 
bandwidth so you would need 5 x8 IB HCAs for a total of 40 lanes of pci-
express. This will of course change with the introduction of future pci-
express generations...

/Peter

> (think multiple such nodes in a typical
> Lustre filesystem, not so economical). Other alternative is to find a
> server that can support 8X or 12X QDR IB port on the motherboard to get
> more bandwidth.
> 
> With a typical Lustre client memory of 24-64GB and memory to CPU bandwidth
> of 10GB/Sec (with standard DDR3-1333MHz  DIMMS), it is not possible to fit
> dataset larger than 2/3rd  of memory. If you still want to achieve
> 10GB/Sec of bandwidth between storage and memory, there are clever
> alternatives. You will have to stage your data into memory beforehand and
> keep memory pages locked and continue feeding data as these pages are
> consumed. It is lot harder than it seems on the paper.
> 
> Cheers,
> -Atul
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