How to backup / partition before rooting?
You do not have to backup Entire /. Rooting phone only unlocks bootloader for custom recovery and involves installations of the su binary in /system partition and the overall partitions that can be affected due to bad root are: /system, /data, /cache. Rest of the partitions wont really be bothered.

An effective way to backup your everything is to use ADB Backup. It will backup ALL of your data except SMS (so backup them up manually with some app beforehand).

Hope this helps.