All I can find is stuff about SSDs, or the SSH Deamon, or people saying not to use them for the array or parity (which, as I understand it, will cause problems) but none of this answers my simple question.

 

Will using a pair of 4TB SSHDs as a cache pool cause any problems or will it just work?  Didn't want to just try it because this Unraid box IS the backup and losing any of the data would be catastrophic.

 

Thanks in advance

why are you skeptical? they're just normal hard disk platter drives with bigger flash memory caches. on an sshd you get very marginal gains in read and writes because after you use it, it learns what you frequent and moves that into the cache(bigger on these drives than on a standard hdd), so the platters don't have to spin up and and seek for your data. It just loads from the flash memory. but as a cache drive in unraid that's the purpose of the entire cache pool.  it assumes everything there is going to be a frequently modified data block or a temporary spot for dumped data that moves it to the array when ever you have scheduled it to. so ssds are the preferred, for speed. dockers/vms and all temporary data is stored on them. they benefit extra from the speed of ssds. 

tl;dr yes it will just work. but wont offer much speed advantage over the array. which speed is the intended purpose of the cache pool. But it will set up directories in a better fashion and allow you to create proper paths for your dockers and create a smooth transition for the future if you did ever replace with ssds.