discard Controls whether ext4 should issue discard/TRIM nodiscard(*) commands to the underlying block device when blocks are freed. This is useful for SSD devices and sparse/thinly-provisioned LUNs, but it is off by default until sufficient testing has been done.
Where does safety factor into all of this?
So until the maintainers of the filesystem want to enable a performance optimization by default, you need to be _really_ careful with it. If they even suggest it might be risky, then caveat emptor.