What is the maximum length of the cable can be for infiniband(RDMA)?

For example. Here have said, that it can be up to 10 M for a four-channel copper cable. And using the Connects Cables using fiber, which fully electrically compatible with standard InfiniBand 4x and Ethernet 10GBaseCX4, up to 100 M:

http://electronicdesign.com/communications/system-extends-infiniband-cable-reach-100-m

Solution

The article that you linked to is from 2007. Six years is infinity in this area. So the answer depends on many parameters.

Which HCA vendor?
Which cable vendor?
Copper or optical?
Passive or active?
What speed are you willing to achieve in your IB setup?

For instance, you can get a 300m active cable that supports speeds up to QDR (40 Gb/s). Or you can get a 0.5m passive copper cable that supports up speeds up to FDR10.

In general, the longer the cable you need and the faster the speed you want, you'll have to jump to "fancier" technology.

You can get a passive copper cable that supports FDR10 with length up to 5m. Connect same pair of FDR10-capable HCAs with 7m cable of the same family - and HCAs will auto-negotiate max speed at QDR.