postgres on a non-journaling filesystem

hello,

I'm Maayan, I'm in a DBA team that uses postgresql.

I saw in the documentation on wals:

In the tip box that, it's better not to use a  journaling filesystem. and I wanted to ask how it works?

can't we get corruption that we can't recover from?

I mean what if postgres in the middle of a write to a wal and there is a crash, and it didn't finish. 

I'm assuming it will detect it when we will start postgres and write that it was rolled back, am I right?

and how does it work in the data level? if some of the 8k block is written but not all of it, and then there is a crash, how postgres deals with it?

Thanks in advance