[mpich-discuss] reading from a read only directory

[Geoffrey Irving]

I got the following error trying to slurp in a large file
(slice-17.pentago) with MPI_File_read_ordered:

rank 0: pentago/mpi/io.cpp:read_sections:397:
MPI_File_read_ordered(file,raw.data(),raw.size(),MPI_BYTE,MPI_STATUS_IGNO
ADIOI_CRAY_OPEN(102): Access denied to file all-1/.slice-17.pentago.shfp.670064

The directory all-1 was read only due to paranoia over accidentally
deleting expensively obtained data.  Looking at ADIOI_Shfp_fname in
the mpich source, it looks like this error is intentional: the shared
file pointer routines generate temporary files in the same directory
as the read-from file.  I couldn't find any attempts at recovery if
the file cannot be written in that place.  This behavior doesn't seem
to have changed since the beginning of the git repository.

Is my reading of the code correct: MPI_File_read_ordered can't be used
on a file in a read only directory?

I can see the motivation for this in the case of noncollective shared
routines: since the other processes aren't necessarily doing any MPI
at the moment, the only way to synchronize is through the file system.
 And because there might be all sorts of different filesystems in
operation, the easiest way to ensure that we're touching the right one
is use the same directory.  I can't imagine any reasonable use of the
noncollective shared routines, but maybe that's a different
discussion.  Is this an unfortunate leak between broken routines which
need questionable trickery and perfectly good routines like
MPI_File_read_ordered?

Thanks,
Geoffrey