Amelio Vazquez-Reina's questions

Let's say I have the following setup:

    ssh       ssh
A ------> B ------> C
    ^          ^
 using A's   using B's
   key         key

I'm trying to configure this in .ssh/config in the host A as follows:

Host C
    HostName C.com
    IdentityFile path_to_key_1
    ProxyCommand ssh -i path_to_key_2 B -W %h:%p

Which identify file and path go into path_to_key_1 and path_to_key_2 ?

For example, does path_to_key_1 refer to a path in A and path_to_key_2 to a path in B? Or are they both supposed to be paths in A?

I have read in various places (e.g. here and here) that NFS' I/O performance does not scale, while Lustre's does, and that Lustre can deliver better I/O rates in general.

There seem to be various architectural differences between the two, but I can't identify what exactly gives Lustre the bump in speed or scalability. I guess my question is somewhat conceptual, but what is the key difference with respect to NFS, or feature, that allows Lustre to scale and deliver faster I/O rates?