openPMD supports writing to and reading from ADIOS2
For this, the installed copy of openPMD must have been built with support for the ADIOS2 backend.
To build openPMD with support for ADIOS2, use the CMake option
For further information, check out the installation guide,
build dependencies and the build options.
ADIOS2 has several engines for alternative file formats and other kinds of backends, yet natively writes to
The openPMD API uses the BP4 engine as the default file engine and the SST engine for streaming support.
We currently leverage the default ADIOS2 transport parameters, i.e.
POSIX on Unix systems and
FStream on Windows.
ADIOS2 is optimized towards organizing the process of reading/writing data into IO steps.
In order to activate steps, it is imperative to use the Streaming API (which can be used for either file-based or streaming-based workflows).
With ADIOS2 release 2.6.0 containing a bug (fixed in development versions, see PR #2348) that disallows random-accessing steps in file-based engines, step-based processing must currently be opted in to via use of the JSON parameter
adios2.engine.usesteps = true when using a file-based engine such as BP3 or BP4.
With these ADIOS2 releases, files written in such a way may only be read using the streaming API.
Upon reading a file, the ADIOS2 backend will automatically recognize whether it has been written with or without steps, ignoring the JSON option mentioned above.
Steps are mandatory for streaming-based engines and trying to switch them off will result in a runtime error.
ADIOS2 will in general dump data to disk/transport only upon closing a file/engine or a step.
If not using steps, users are hence strongly encouraged to use file-based iteration layout (by creating a Series with a filename pattern such as
simData_%06T.bp) and enforce dumping to disk by
Iteration::close()-ing an iteration after writing to it.
Otherwise, out-of-memory errors are likely to occur.
The ADIOS2 SST engine for streaming can be picked by specifying the ending
.sst instead of
The following environment variables control ADIOS2 I/O behavior at runtime. Fine-tuning these is especially useful when running at large scale.
Turns on/off profiling information right after a run.
Online creation of the adios journal file (
Number of files to be created, 0 indicates maximum number possible.
Please refer to the ADIOS2 documentation for details on I/O tuning.
In case the ADIOS2 backend was not compiled but only the deprecated ADIOS1 backend, the default of
OPENPMD_BP_BACKEND will fall back to
Be advised that ADIOS1 only supports
.bp files up to the internal version BP3, while ADIOS2 supports BP3, BP4 and later formats.
Notice that the ADIOS2 backend is alternatively configurable via JSON parameters.
Due to performance considerations, the ADIOS2 backend configures ADIOS2 not to compute any dataset statistics (Min/Max) by default.
Statistics may be activated by setting the JSON parameter
adios2.engine.parameters.StatsLevel = "1".
Best Practice at Large Scale¶
A good practice at scale is to disable the online creation of the metadata file.
After writing the data, run
bpmeta on the (to-be-created) filename to generate the metadata file offline (repeat per iteration for file-based encoding).
This metadata file is needed for reading, while the actual heavy data resides in
<metadata filename>.dir/ directories.
Note that such a tool is not yet available for ADIOS2, but the
bpmeta utility provided by ADIOS1 is capable of processing files written by ADIOS2.
Further options depend heavily on filesystem type, specific file striping, network infrastructure and available RAM on the aggregator nodes. A good number for substreams is usually the number of contributing nodes divided by four.
For fine-tuning at extreme scale or for exotic systems, please refer to the ADIOS2 manual and talk to your filesystem admins and the ADIOS2 authors. Be aware that extreme-scale I/O is a research topic after all.
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