Double the RAM, double the ARC — probably worth it.

Yes, going from 8GB to 16GB will give ZFS’s ARC noticeably more room to breathe. 8GB is the bare minimum for FreeNAS even without heavy use — with ZFS, most of that RAM gets eaten by the ARC by default. Adding another 8GB essentially doubles your cache for recently accessed data.

Here’s the deal: ZFS ARC uses RAM to cache frequently read data so the disks don’t have to spin up as much. With 8GB total, the ARC gets maybe 5–6GB after overhead. With 16GB, it gets closer to 12GB. That extra space means more active files stay in memory, which translates to lower latency for repeated reads and less disk thrashing.

Will it be a night-and-day difference? That depends on your workload. If you’re serving media files to a couple of users or backing up a small office, you’ll notice snappier navigation and less drive chatter. If you’re doing heavy database transactions or many simultaneous random reads, the improvement is even bigger. If you’re just storing a few rarely-accessed files and don’t care about speed, you might not feel it much.

One more thing: FreeNAS/ZFS will use whatever RAM you give it. It’s not like a video game where more RAM does nothing — ARC scales proportionally. So 8GB to 16GB is one of the best bang-for-buck upgrades you can make on a low-RAM system.

Just make sure your motherboard supports the RAM type and that the new stick is compatible with your existing one. Mixing speeds or timings can cause instability. Ideally buy a matched pair.