I recently migrated my NAS from TrueNAS Core 13.0U6.3 to TrueNAS Scale 24.10 and after minor issues during the migration, everything seems to be working well.
I did notice that under TrueNAS Core, my available RAM was consumed by ZFS Cache (ARC) to the point where eventually there were only single digit GB free. Free RAM is wasted RAM so a lot of the time this meant my setup could saturate a 10Gb link so long as what I was accessing was in ARC.
In TrueNAS Scale, after accessing quite a lot of the data on my NAS the ZFS cache reached the same level as what it did in TrueNAS Core. I was testing to ensure I wasn’t limited to 50% or RAM for ARC but I see that issues has been fixed. No tweaking requird now.
However after a few weeks I’ve noticed the ZFS Cache value decrease to less than 10GB. Under TrueNAS Core this value never decreased unless I deliberately put memory pressure on the NAS, or rebooted it.
How come TrueNAS Scale reduces the size of the ZFS cache over time. Is this expected?
Doing a couple of large file transfers the cache climbs back up
