[petsc-users] Clarification on MatMPIBAIJSetPreallocation (d_nnz and o_nnz)

Hello Barry, I am doing some preliminary work to add a coupled solver in my code. I am reading some documentation on MPIBAIJ as the matrix will be composed of blocks of size 3x3 in 2D and 4x4 in 3D for each cell in the domain. I can't quite understand how to set d_nnz and o_nnz. What is their size? Should I provide a number of non-zero entries for each block or should I do it line by line as in MATAIJ? Would you be able to provide me with a silly example? Thank you!