counts= (/ 0, 0, 0 /)

[dkokron]

I have multiple 2D and 3D variables I want to write to the same file. The data are distributed in memory by the third dimension. Each MPI rank can own one or more levels of a given variable. I want to write the data in parallel.

Lets assume we are writing a 3D variable. 3DVar(globalsizes(1), globalsizes(2), 4)

If(MemberOfIOComm) then
  nf90_open(trim(FileName), NF90_WRITE, ncid, comm=IOComm, info=MPI_INFO_NULL)
  nf90_inq_varid(ncid, trim(var_name), varid)
  nf90_var_par_access(ncid, varid, nf90_collective)   ! needed to prevent library finger wagging
  start = (/  1,  1,  mystart /)
  counts= (/ globalsizes(1), globalsizes(2),  k /)
  nf90_put_var(ncid, varid, 3DVar, start=start, count=counts)
endif

IOComm is a subset of MPI_Comm_World with a number of ranks that is larger than the third dimension of any variable being written. Lets say MPI_Comm_World is 16 ranks, IOComm has 8 ranks while the largest 3D has 4 levels and they are distributed in memory to two of the ranks in IOComm. mystart is 1 on the first rank and 3 on the other while k is 2 for both.

The above code gets deadlocked because NC4_put_vars() calls MPI_Allreduce() presumably on IOComm.

If I set counts= (/ 0, 0, 0 /) for the remaining 6 ranks in IOComm that don't contribute data to the file then I get
NetCDF: Index exceeds dimension bound

Does NetCDF support this scenario?
How?