Spare 250GB SSD - add to cache pool or use UD for Docker/Appdata?

[DZMM]

I've got a spare 250GB SSD lying around.  It's not worth much, so I can't really be bothered to put it on eBay unless I've got other stuff to sell at the same time.  It'll also be replacing my smallest 2TB drive that is noisy and slow, and is slowing down parity checks etc  I can afford to remove the 2TB from my array as I'm creating a fair bit of headroom through moving stuff online that I can access effectively via an rclone mount.

 

In terms of using it, I can see two options:

 

1. Add it to my cache pool to get an extra 125GB storage - this will provide more headroom as my appdata (plex) increases and reduces the amount of times mover has to run, so less PC noise
2. Add it as an unassigned drive and move my docker image (20GB - about 75% full) and my appdata (70GB but will increase as I've gone back to using Plex and thumbnails are being created)

 

I already run my VMs on a nvme drive, so that's not a scenario.

 

What do people think?  Or, is there a 3rd option?

 

#1 Pros - less complexity, provides redundancy and I think it will speed up my cache pool as I believe files are accessed from each drive sequentially 

 

#2 Pros - I like the idea of isolating docker, so my cache pool is only doing writes - theoretically splitting the workload between a 500GB pool and a 250UD for docker/appdata will be better than running it all in a 625GB pool?????  I'm not worried about losing appdata on the UD, as I already back that up and docker images are disposable.