Seagate Exos X18 ST16000NM000J

[rvassar]

shots taken that have enormous sentimental value and I would never want to lose.

Since you made this statement, I have to add "RAID is not a backup." ZFS is one of the finest file systems available for data protection. But you still need a backup plan.

 

[ChrisRJ]

Hi ChrisRJ,
are you telling me to simply use the two mirrored disks with vdev data without any other vdev?

Yes

If yes, but in this case without 2 SSD disks for the vdev special I would have lower performance than using 2 disks for vdev data and 2 SSDs for vdev special
It's correct ?

Yes, to this as well. But what performance do you need? Premature performance optimization is never a good idea and from the high-level description you gave, there is nothing that indicates a hard need for an SSD-based metadata vdev.

Lastly, you are new to TrueNAS and ZFS and this would increase your overall risk.

I would start with 2 HDDs and check the performance. Then take it from there.

 

[Etorix]

are you telling me to simply use the two mirrored disks with vdev data without any other vdev?

I can't speak for Chris, but if data security is high on the list, I'd get two more hard drives to make it a 4-wide raidz2 (or one extra drive to make it a 3-way mirror). With 16 TB drives, there could be quite a lot of data at risk in case a single drive would fail—hence double redundancy.
And then an external backup. (And then an off-site backup…)

I hope the 4000 MHz RAM is running at 3200 MHz, not overclocked by the consumer motherboard.