I went with Mirrors when I created my pool because of this post and my initial problems with using RAIDZ when I first started using ZFS (inability to add more devices to vdev, must upgrade all drives to realize space increase, etc..). Just recently I got a 256 GB Samsung 960 Evo NVMe drive for my system so IOPS are no longer an issue for me since this is for home use and will easily handle everything I throw at it. Since I'm using FreeNAS 10 (yes I no it's not stable, save me the warnings lol) I have my NVMe drive in it's own pool and have it dedicated for VMs and Docker containers, mass storage is handled by my Storage pool.
My Storage pool, which consists of 6x 4 TB and 4x 1 TB drives with an SLOG, houses mostly multimedia (480P/720P/1080P/2160P, ranging from 256 MB to 20 GB per file), a dataset which is shared over SMB for my Steam Library, and a download directory which is used for long term seeding of torrents and as temporary space for Usenet downloads (they get copied to the multimedia dataset on the same drive).
My question is, would I be better off converting this back to RAIDZ2 in order to realize a bit more space (using a raidz calculator and adding in parity and reserved space equals about 1.5 TB gained without the use of the 4x 1 TB drives) or would it generally hurt performance? I currently have about 2 TB free out of 12.8 TB.
I wouldn't be doing this any time soon since I don't really have any place to backup ~9 TB and I would have to get a few drives to place those 4x 1 TB drives, I'm just curious.