10. Parallel I/O

10.1. RAID

Task

Reconstruct the missing data from disk 0 in Fig. 10.1.1
Briefly describe the difference between RAID 5 and RAID 6.

10.2. MPI-IO

POSIX provides a model for widely portable file system, but the portability and optimization required for parallel I/O cannot be achieved with the POSIX interface. MPI-IO is an alternative low-level interface specifically designed for parallel file I/O.

MPI_File_open:

Before accessing a file in MPI it needs to be opened with MPI_File_open.

int MPI_File_open(MPI_Comm comm,
                  char *filename,
                  int amode,
                  MPI_Info info,
                  MPI_File * fh);

comm: Communicator of processes that will access file.
filename: name of the file.
amode: access mode of file (RDONLY, RDWR, WRONLY, …)
info: info object.
MPI_File: opened MPI file.

MPI_File_close:

Before Finalizing MPI all files should be closed with MPI_File_close.

int MPI_File_close(MPI_File * fh);

MPI_File_write_at:

Once a file is opened, it can be accessed with various MPI functions. MPI_File_write_at writes data to a file with an offset. Different offsets on different processes allow to write in parallel writing without interference.

int MPI_File_write_at(MPI_File fh,
                      MPI_Offset offset,
                      void *buf,
                      int count,
                      MPI_Datatype datatype,
                      MPI_Status * status);

fh: opened MPI file.
offset: the offset from the start of the file in bytes.
buf: pointer to array which is writen in the file.
count: number of elements written in the file.
datatype: datatype of array.
status: status object.

Task

Mandelbrot with MPI-IO

Adjust your implementation of the Mandelbrot-Set for MPI-IO
Use the same specifications (real_min, reak_max, maxIterations, resolution, …) as in Task 9.3
All processes should compute a part of the whole image.
Use MPI-IO to write the partial images of all processes to a Portable Pixel Map(.ppm).
- Write the color values as MPI_UINT8_T.
- Use the Magic Number P6 to encode that the image is stored in byte format (one byte per color component).
Be careful when using MPI-IO, start with only a few processes.