The volume of data generated by scientific applications grows and the pressure on the I/O system of HPC systems also increases. For this reason, an I/O behavior model is proposed for scientific MPI (Message Passing Interface) parallel applications. The goal is to analyze the applications’ impact on the I/O system. Analyzing the MPI parallel applications at POSIX-IO level allows observing how the application’s data are treated at that level. In this research work, the following is presented: the I/O behavior model definition at POSIX-IO level (PIOM-PX model definition), the methodology applied to extract this model and the PIOM-PX-Trace-Tool. As PIOM-PX is based on the I/O phase concept, it can identify the more significant phases. Phases that have more influence than others in the I/O system and they could provoke a bottleneck or a poor performance. Analysis based on I/O phases allows identifying, delimiting, and trying to reduce each phase’s impact on the I/O system. PIOM-PX is part of proposed model PIOM. PIOM integrates the I/O behavior model at POSIX-IO level (PIOMPX) and the I/O behavior model at MPI-IO level (PIOM-MP, formerly known as PAS2P-IO). The model provides the information necessary to replicate an application’s behavior in different systems using synthetic programmables programs. PIOM-PX-Trace-Tool allows interception of POSIX-IO instructions used during the application execution. The experiments carried out are executed in several standar HPC systems and the Cloud platform, where it is able to test the utility of the proposed model PIOM.