StableBit DrivePool F720
When I move the Plex Media Server's database to the pool, the images no longer appear in the Plex UI. Why does this happen?
The reason for this is that Plex uses Hard Links to "cross link" all their local media cache images. That means they can have the files located in multiple locations, but only one copy of the actual file. It is a rather efficient method of storing the files. The problem with this method is that hard links only work on the same volume. You can't have hard links to files on other volumes. Ever. You can use symbolic links to do this, but that's not Plex uses.
Now, you're asking yourself "Well, StableBit DrivePool creates one large volume. So that shouldn't be an issue. Right?"
The problem is that the pool drive is a virtual drive. There isn't an actual file system. We just pass on the requests to the actual disks in the pool. Reparse points changes how that's handled a bit, but it's still essentially true. More importantly, while the pool appears to have a file system, it really doesn't. Hard links work inside NTFS, they depend on NTFS metadata and are a fundamental property of how NTFS works.
This all means that hard links do not work on the pool at all.
Since Plex uses hard links to link all of it's image metadata together, rather than using a database to do this, that means that it cannot properly store the image metadata one the pool.