I have a downloads share that's configure to Prefer cache. The size is 250GB, what happens if I start downloading a 300GB torrent full of small files. Will it full the cache and spill over the main array and when the torrent is finished, only then the mover will move the rest of the files, or what?
Thanks

Chances you will the torrent failing with ‘out of space’ errors.    Once a drive has been selected for a file Unraid will never change its mind and will generate an error if the file does not fit.  
 

In your stated case it might make a difference if you have the torrent client set to allocate all the space for files from the outset or not and what you have set as the Minimum Free Space setting for the cache as to whether any part of the torrent will overflow to the main array.

Thanks, that makes sense. I'll set it up to pre-allocate, and set the Minimum Free Space to something like 20% of the drive that will cause new files to go to the array and in theory the cache should never fill so much that something would fail.