Sequentiality of the transfers
Now once we setup the transfers, the first part of the transfer ops()() is done. Although it depends on the backend, normally it reads the file and opens the fd of the output backend. So in the case of gekkofs (or adhocfs) it starts a client instance.
It would be better to move the initialization on the pending ops loop, instead of the mpi message receive loop.
This can be easily solved initializing index to -1 (because no pending operation will start with a -1, and if the operation finishes we just delete the op)