MatLoad/VecLoad with user-specified partitioning

[SUN Chun]

Hello PETSc developers,

I want to use MatLoad to load matrix binary files from external storage.
Then solve it or do whatever in parallel. However, as soon as I load it
with MatLoad, I found that PETSc partition it automatically. I need to
specify the partition myself. However I can't figure out where to
properly use MatSetSizes: if I do MatSetSizes before MatLoad, it does
nothing and MatGetSizes still gives me the auto-partitioning result from
PETSc; if I do MatSetSizes after MatLoad, it simply crashes with error
message "cannot change/reset row sizes...".

Neither can I find a proper example...

Any comments would be great appreciated. Thank you very much.

Chun