Technical limits with Jellyfin ?

[TheDreadPirate]

As I'm thinking of transferring my Jellyfin install to a TrueNAS Scale server (using the built-in TrueCharts app), are you recommending against that? Even if I store config files etc. on a SSD within that system, that still would be ZFS...

Having your database on a ZFS file system is possible, it is very much not recommended.  Database READ performance is fine, but when you are writing to a database that is on a ZFS file system there are serious performance penalties.

And because your database is big, that adds to the performance penalty.

If possible, move your database to an EXT4 file system.

If the ZFS filesystem is a single SSD, and the file system block size is 4KB or 8KB, it would be acceptable.  The problems start to happen when a database is on a multi-disk ZFS array.  The block size is usually something larger like 128KB or more.

ZFS is a copy-on-write file system.  Meaning that instead of just modifying the block in place, it makes a copy of the entire block being modified.  So no matter how small of a change you are making you are, at minimum, writing a single block.  So if your are making A LOT of write transactions to the jellyfin database, which would happen during a library scan, there is a lot of unnecessary writing happening.  And this seriously reduces database performance.  Probably not noticeable for a small library with the database on a SSD.

But if the database is on a hard drive based ZFS array with, probably, larger block sizes, combined with a large library and, thus, large database, you are going to run into serious performance issues.  You are going to experience something called "write amplification".  If you want to write 50MB to your database, the file system forces you do write 1GB (numbers pulled out of my butt).

It sounds like the OP used a symlink to move the jellyfin data directory on their ZFS array storing their library instead of the root file system.  Which is usually EXT4 in Linux.