Geoffrey,
"Is the particle IO in YT that calls h5py spawned by multiple processors or is it doing it serially?"
For your purposes, h5py is only used to *write* particle data to disk after the halos have been found (if you are saving them to disk, which you must do explicitly, of course). And in this case, it will open up one file using h5py per MPI task. I'm guessing that they're actually concerned about reading particle data, because that is more disk intensive. This is done with functions written in C that read the data, not h5py. Here each MPI task does its own reading of data, and may open up multiple files to retrieve the particle data it needs depending on the layouts of grids in the .cpuNNNN files. Does that help? -- Stephen Skory s@skory.us http://stephenskory.com/ 510.621.3687 (google voice)