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Subject: Re: [OMPI users] a question about [MPI]IO on systems without network filesystem
From: Rob Latham (robl_at_[hidden])
Date: 2010-10-19 14:46:51


On Thu, Sep 30, 2010 at 09:00:31AM -0400, Richard Treumann wrote:
> It is possible for MPI-IO to be implemented in a way that lets a single
> process or the set of process on a node act as the disk i/O agents for the
> entire job but someone else will need to tell you if OpenMPI can do this,
> I think OpenMPI built on the ROMIO MPI-IO implementation and based on my
> outdated knowledge of ROMIO, I would be a bit surprised if it has his
> option.

SURPRISE!!! ROMIO has been able to do this since about 2002 (It was
my first ROMIO project when I came to Argonne).

now, if you do independent i/o or you do i/o on comm_self, then ROMIO
can't really do anything for you.

But...
- if you use collective I/O
- and you set the "cb_config_list" to contain the machine name of the
  one node with a disk (or if everyone has a disk, pick one to be the
  master)
- and you set "romio_no_indep_rw" to "enable"

then two things will happen. first, ROMIO will enter "deferred open"
mode, meaning only the designated I/O aggregators will open the file.
second, your collective MPI_File_*_all calls will all go through the
one node you gave in the cb_config_list.

Try it and if it does/doesn't work, I'd like to hear.

==rob

-- 
Rob Latham
Mathematics and Computer Science Division
Argonne National Lab, IL USA