# Parallel File I/O¶

The Sidre IOManager class provides an interface to manage parallel I/O of hierarchical data managed by Sidre. It enables the writing of data from parallel runs and can be used for the purposes of restart or visualization.

## Introduction¶

The IOManager class provides parallel I/O services to Sidre. It relies on the fact that Sidre Group and View objects are capable of saving and loading themselves. The I/O operations provided by those two classes are inherently serial. So the IOManager class coordinates I/O operations of multiple Sidre objects that exist across the MPI ranks of a parallel run.

• The internal details of the I/O of individual Sidre objects are opaque to the IOManager class, which needs only to make calls to I/O methods in Group and View class methods.
• Sidre data can be written from M ranks to N files (M >= N), and the files can be read to restart a run on M ranks.
• When saving output, a root file is created that contains bookkeeping information that is used to coordinate a subsequent restart read.
• The calling code can also add extra data to the root file to provide metadata that gives necessary instructions to visualization tools.

## Parallel I/O using the IOManager class¶

A Sidre IOManager object is created with an MPI communicator and provides various overloads of write() and read() methods. These methods save a Group object in parallel to a set of files and read a Group object from existing files. The I/O manager can optionally use the SCR library for scalable I/O management (such as using burst buffers when available).

In typical usage, a run that calls read() on a certain set of files should be executed on the same number of MPI ranks as the run that created those files with a write() call. However, if using the sidre_hdf5 protocol, there are some usage patterns that do not have this limitation.

A read() call using the sidre_hdf5 protocol will work when called from a greater number of MPI ranks. If write() was performed on N ranks and read() is called while running on M ranks (M > N), then data will be read into ranks 0 to N-1, and all ranks higher than N-1 will receive no data.

If the read() method is called using the sidre_hdf5 protocol to read data that was created on a larger number of ranks, this will work only in the case that the data was written in a file-per-processor mode (M ranks to M files). In this case, the data in the group being filled with file input will look a bit different than in other usage patterns, since a group on one rank will end up with data from multiple ranks. An integer scalar view named reduced_input_ranks will be added to the group with the value being the number of ranks that wrote the files. The data from each output rank will be read into subgroups located at rank_{%07d}/sidre_input in the input group’s data hierarchy.

Warning

If the read() is called to read data that was created on a larger number of ranks than the current run with files produced in M-to-N mode (M > N), an error will occur. Support for this type of usage is intended to be added in the future.

In the following example, an IOManager object is created and used to write the contents of the “root” group in parallel.

First include needed headers.

#include "axom/config.hpp"  // for AXOM_USE_HDF5

#include "conduit_blueprint.hpp"

#include "conduit_relay.hpp"

#ifdef AXOM_USE_HDF5
#include "conduit_relay_io_hdf5.hpp"
#endif

#include "axom/sidre/core/sidre.hpp"
#include "axom/sidre/spio/IOManager.hpp"
#include "axom/fmt.hpp"

#include "mpi.h"


Then use IOManager to save in parallel.

  /*
* Contents of the DataStore written to files with IOManager.
*/
int num_files = num_output;
axom::sidre::IOManager writer(MPI_COMM_WORLD);

const std::string file_name = "out_spio_parallel_write_read";

writer.write(root, num_files, file_name, PROTOCOL, "tree_%07d");

std::string root_name = file_name + ROOT_EXT;


  /*
* Create another DataStore that holds nothing but the root group.
*/
DataStore* ds2 = new DataStore();

/*
* Read from the files that were written above.
*/



## IOManager class use¶

An IOManager object is constructed with an MPI communicator and does I/O operations on all ranks associated with that communicator.

The core functionality of the IOManager class is contained in its write() and read() methods.

void write(sidre::DataGroup * group,
int num_files,
const std::string& file_string,
const std::string& protocol);


The write() method is called on each rank and passed a group pointer for its local data. The calling code specifies the number of output files, and the I/O manager organizes the output so that each file receives data from a roughly equal number of ranks. The files containing the data from the group will have names of the format file_string/file_string_*******.suffix, with a 7-digit integer value identifying the files from 0 to num_files-1, and the suffix indicating the file format according to the protocol argument. Additionally the write() method will produce a root file with the name file_string.root that holds bookkeeping data about other files and can also receive extra user-specified data.

void read(sidre::DataGroup * group,
const std::string& root_file);


The read() methods is called in parallel with the root file as an argument. It must be called on a run with the same processor count as the run that called the write() method. The first argument is a pointer to a group that contains no child groups and owns no views. The second argument is the name of the root file; the information in the root file is used to identify the files that each processor will read to load data into the argument group.

The write() and read() methods above are sufficient to do a restart save/load when the data in the group hierarchy is completely owned by the Sidre data structures. If Sidre is used to manage data that is externally allocated (i.e., the hiearchy contains external views), the loading procedure requires additional steps to restore data in the same externally-allocated state.

First the read() method is called, and the full hierarchy structure of the group is loaded, but no data is allocated for views identified as external. Then, the calling code can examine the group and allocate data for the external views, as needed. The View class method setExternalDataPtr() is used to associate the pointer with the view. Once this is done, the I/O manager method loadExternalData() can be used to load the data from the file into the user-allocated arrays.

Below is a code example for loading external data. We assume that this code somehow has knowledge that the root group contains a single external view at the location “fields/external_array” describing an array of doubles. See the Sidre API Documentation for details about Group and View class methods to query the Sidre data structures for this type of information when the code does not have a priori knowledge.

// Construct a DataStore with an empty root group.
DataStore * ds = new DataStore();
DataGroup * root = ds->getRoot();

// Read from file into the root group.  The full Sidre hierarchy is built,
// but the external view is created without allocating a data buffer.

// Get a pointer to the external view.
DataView * external_view = root->getView("fields/external_array");

// Allocate storage for the array and associate it with the view.
double * external_array = new double[external_view->getNumElements()];
external_view->setExternalDataPtr(external_array);

// Load the data values from file into the external view.


## User-specified data in the root file¶

The root file is automatically created to provide the IOManager object with bookkeeping information that is used when reading data, but it can also be used to store additional data that may be useful to the calling code or is needed to allow other tools to interact with the data in the output files, such as for visualization. For example, a Conduit Mesh Blueprint index can be stored in a DataGroup written to the root file to provide metadata about the mesh layout and data fields that can be visualized from the output files.

void writeGroupToRootFile(sidre::DataGroup * group,
const std::string& file_name);

void writeGroupToRootFileAtPath(sidre::DataGroup * group,
const std::string& file_name,
const std::string& group_path);

void writeViewToRootFileAtPath(sidre::DataView * view,
const std::string& file_name,
const std::string& group_path);


The above methods are used to write this extra data to the root file. The first simply writes data from the given group to the top of the root file, while the latter two methods write their Sidre objects to a path that must already exist in the root file.