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  1. What is the difference between an active and a passive cable?
  2. What is the difference between QSFP and QSFP+?
  3. Given the choice between copper and fiber, for a very short distance like 3 feet max, is one better than the other or approximately equal?
  4. The Mellanox card MHQH19B-XTR Mellanox ConnectX-2 VPI QSFP QDR InfiniBand 10GB Ethernet PCI-E HCA says it is ethernet and infiniband. I thought those are different technologies altogether?
  5. Does this card support verbs programming and is the driver supported by relatively modern OS like CentOS 6.x?

Finally, even if no one knows that question, what is the right cable for this card?

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  1. What is the difference between an active and a passive cable?

Passive cables only work to lengths of about 5m or under. Active cables boost the signal for increased distance. See this article

  1. What is the difference between QSFP and QSFP+?

I think the QSFP+ supports higher data rates so 56Gbit FDR Infiniband. Maybe someone can correct me if that's wrong? See this article

  1. Given the choice between copper and fiber, for a very short distance like 3 feet max, is one better than the other or approximately equal?

Fibre cables are more expensive, but physically thinner. The short copper cables are quite a lot cheaper.

  1. The Mellanox card MHQH19B-XTR Mellanox ConnectX-2 VPI QSFP QDR InfiniBand 10GB Ethernet PCI-E HCA says it is ethernet and infiniband. I thought those are different technologies altogether?

Mellanox VPI cards allow either technology. The signally is apparently identical or very similar. Since Mellanox make their own silicon they have been able to combine the technologies.

  1. Does this card support verbs programming and is the driver supported by relatively modern OS like CentOS 6.x?

Yes and yes, provided the appropriate kernel modules & libraries are loaded.

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