Technical limits with Jellyfin ?

[TheDreadPirate]

Thanks for clarification about "issue" with ZFS but I would have 2 additional questions:
-> is It still a problem on a ZFS array that have high speed (mine read/writes at around 2.5g/s which should compensate the problem no ?
-> the issue is only for write operations so nearly only when you scan libraries right ? out of it amount of data written in db is ridiculous no ?

Throughput isn't the problem.  It is I/O performance.  As you add disks to a ZFS array, or any kind of RAID array, your SEQUENTIAL throughput increases but your random I/O performance does not increase nearly as much.  You are physically limited by how fast the read/write head in the hard drive can move around.  Random I/O performance is very important for database operations.  Read or write.

In general, we recommend that the jellyfin databases reside on an SSD.  You issues are compounded by having a large database on a hard drive based ZFS array.  And since you have the entire /var/lib/jellyfin directory on the same array, your database is competing for resources with the jellyfin cache and your reading your media to send to users.