I work with Tom at Datto. He presented all of his design decisions at the OpenZFS developer conference here: http://open-zfs.org/wiki/OpenZFS_Developer_Summit_2016#Prese...
Slides: https://drive.google.com/file/d/0B5hUzsxe4cdmU3ZTRXNxa2JIaDQ...
Video: https://youtu.be/frnLiXclAMo