I am putting a TrueNAS system in my garage. Its a 4 bay system and I am putting in quite old used 8TB SAS disk in it. It will be hot, it will be cold, it will be dusty, and the system will only have 8GB of RAM total. Performance is not a concern, this is only for replicated backups
RAIDZ2 or Striped Mirrors? I’m thinking RAIDZ2 so ANY second drive can fail. What do you think? Networking will just be dual 1Gb.
Definitely a +1 for RAIDZ2 
I have 2 setups like that, not quite in that condition but far from ideal. Just had a SATA cable fail so far.
Yeah the main benefit of mirrors would be performance, which you already said is not an issue. So Then next question if it comes time to upgrade would buying 4 more drives be an issue for you? It sounds like the answer would be no since you would be buying a whole new case to fit the extra drives anyway. So I think that both of the pros of using mirrors over Raid Z2 would not apply in your case.
All of this hardware is existing/free, and I have a giant stack of 8TB SAS disks I got for free. I only actually need around 3TB of space, so a 4 disk RAIDZ2 with 8TB disks should be plenty for a long long time
But, if I decide I do need more space I will just slowly replace them one by one with larger disks, and expand at the end
I’ve got a 24 bay server with 3x 7-disk raidz1 w. a mix of 2tb sas and sata disks - the performance isn’t great at all and I’m scared of the redundancy issues. Got one hot spare in an extra bay which has already been replaced once.
I’d kill for a “giant stack” of 8tb HDDs - just six mirrored would replace 21, can I buy some of you?!