HDF5 is a clunky file format and dependency. There's a whole host of usual complaints, many of which have already been mentioned: http://cyrille.rossant.net/mov

HDF5 is a clunky file format and dependency. There's a whole host of usual complaints, many of which have already been mentioned: http://cyrille.rossant.net/moving-away-hdf5/ http://cyrille.rossant.net/moving-away-hdf5/ My biggest personal annoyance is that HDF5 isn't thread safe^, so it only supports parallel reading and writing via multiple processes. This makes parallel computing a pain. This is especially annoying when using HDF5's built-in compression, which hogs a lot of CPU. Inter-process communication is slower than reading from SSDs, so that isn't a great alternative: http://matthewrocklin.com/blog/work/2015/12/29/data-bandwidth http://matthewrocklin.com/blog/work/2015/12/29/data-bandwidt... There's a lot to be said for file formats that you can simply memory map, and that's exactly what Feather/Arrow are. Building out-of-core workflows on top of should be a joy. Wes -- does the Python library for Feather already release the GIL? ^ you can use and/or compile HDF5 with a global lock, but the underlying library still isn't thread safe.