Warning: group-based iteration encoding in ADIOS2

src/IO/ADIOS/ADIOS2IOHandler.cpp

@@ -499,6 +499,16 @@ ADIOS2IOHandlerImpl::flush(internal::ParsedFlushParams &flushParams)
     return res;
 }
 
+static constexpr char const *warningADIOS2NoGroupbasedEncoding = &R"(
+[Warning] Use of group-based encoding in ADIOS2 is discouraged as it can lead
+to drastic performance issues, especially when **not** used in conjunction with
+IO steps.
+We advise you to pick either file-based encoding or variable-based
+iteration encoding for use with ADIOS2.
+For more details, refer to
+https://openpmd-api.readthedocs.io/en/latest/usage/concepts.html#iteration-and-series)"
+                                                                     [1];
+
 void ADIOS2IOHandlerImpl::createFile(
     Writable *writable, Parameter<Operation::CREATE_FILE> const &parameters)
 {
@@ -508,6 +518,14 @@ void ADIOS2IOHandlerImpl::createFile(
 
     if (!writable->written)
     {
+
+        if (!printedWarningsAlready.noGroupBased &&
+            parameters.encoding == IterationEncoding::groupBased)
+        {
+            std::cerr << warningADIOS2NoGroupbasedEncoding << std::endl;
+            printedWarningsAlready.noGroupBased = true;
+        }
+
         std::string name = parameters.name + fileSuffix();
 
         auto res_pair = getPossiblyExisting(name);

src/Iteration.cpp

@@ -206,6 +206,7 @@ void Iteration::flushFileBased(
         /* create file */
         Parameter<Operation::CREATE_FILE> fCreate;
         fCreate.name = filename;
+        fCreate.encoding = s.iterationEncoding();
         IOHandler()->enqueue(IOTask(&s.writable(), fCreate));
 
         /* create basePath */

src/Series.cpp

@@ -1195,6 +1195,7 @@ void Series::readOneIterationFileBased(std::string const &filePath)
     Parameter<Operation::READ_ATT> aRead;
 
     fOpen.name = filePath;
+    fOpen.encoding = iterationEncoding();
     IOHandler()->enqueue(IOTask(this, fOpen));
     IOHandler()->flush(internal::defaultFlushParams);
     series.iterations.parent() = getWritable(this);