> My main storage setup is pretty simple. It a ZFS pool with four 10TB hard drives in a RAIDZ2 data vdev with an additional 256GB SDD as a cache vdev. That means two hard drives can die without me loosing that data. That gives me ~19TB of usable storage, which I’m currently using less than 10% of. Leaving plenty of room to grow.
I would question this when buying a new system and not having a bunch of disks laying around... having a RAID-Z2 with four 10GB disks offers the same space as a RAID1 with two 20GB disks. Since you don't need the space NOW, you could even go RAID1 with two 10TB disks and grow it by replacing it with two 20TB as soon as you need more. This in my opinion would be more cost effective, since you only need to replace 2 disks instead of 4 to grow. This would take less time and since prices per TB are probably getting lower over time, it could also save you a ton of money. I would also say that the ability of losing 2 disks won't save you from having a backup somewhere...