if (not FileSystemStorageExt::EXTENSION_ID.valid()) {
FileSystemStorageExt::EXTENSION_ID = simgrid::s4u::Storage::extension_create<FileSystemStorageExt>();
if (not FileSystemStorageExt::EXTENSION_ID.valid()) {
FileSystemStorageExt::EXTENSION_ID = simgrid::s4u::Storage::extension_create<FileSystemStorageExt>();