Trying to set up mergerfs to use with Radarr/Sonarr and rclone cache

Right now I am using rclone cache on my Whatbox server w/ a Plex Media Server (also installed on my Whatbox server). I'm looking to move my Radarr/Sonarr installation to my Whatbox as well, and I'm trying to figure out how to set up mergerfs to meet my needs. As I mentioned previously, I already have a mounted cache drive that Plex uses. Would I use mergerfs to combine that directory with a local directory to create the merged directory? If so, would I then change the settings in Plex to point to this new merged directory? Also, can I point Radarr/Sonarr to this directory as well, or is the merged directory read-only? If I can write to it, how does it know which of the two source directories the files should be actually written to? Sorry for the bombardment of questions, I'm just trying to wrap my head around this before I actually start trying to set it up.

mergerfs is similar to unionfs, but it has policies that can be used to meet pretty much any use case that you want.

He's got a few good sections here:

My particular use case relates to having a local directory that I always want to write to and my rclone mount, which is still read/write, but I never actually write anything there directly.

Everything points to the mergerfs directory, in my case which is /gmedia as that is a combination of /data/local and my /GD (rclone).

My policy says this:

category.action=all,category.create=ff

Which is for any write operation, use the first found item in the list.

My mount is comprised of "/data/local:/GD" so /data/local is always written to for anything.

At night, I do a rclone move from /data/local and copy up to my GD.

This gives me the benefit of hard linking since everything is on /data/local underneath so it never crosses a physical disk.

I'm definitely not the mergerfs expert, but for my use case, it's a very simple setup of two disks.

Ok, thanks. Do you know if pointing Plex to the merged drive instead of the mounted cache drive would cause any performance issues with Plex?

Nope, I've been doing that for quite some time. There was one minor item that popped up with a recent release that was fixed that I wrote up here:

https://forum.rclone.org/t/mergerfs-2-25-seems-to-not-work-with-rclone/10258/6

He's working on adding the fix to the next release of mergerfs and that should occur shortly.