Note that the listed probabilities of surviving N-disk failures are not correct, though after calculation, though the differences may not be that important.

Listed survival probability of N-disk failure in 8-drive/4-vdev mirror.

1: 1; 2: 0.857; 3: 0.667; 4: 0.400; 5: 0; 6: N/A; 7: N/A; 8: N/A

Proper survival probability:

1: 1; 2: 0.857; 3: 0.571; 4: 0.229; 5: 0; 6: 0; 7: 0; 8: 0

Comparatively, survival probability for 8-drive RAIDz4 with equal amount of usable space:

1: 1; 2: 1; 3: 1; 4: 1; 5: 0; 6: 0; 7: 0; 8: 0

Personally, I'd use multi-vdev mirror pools only for data that is either backed-up or data I can afford to lose completely.

That talks more about RAID5/Z1 vs mirroring.

You still need 6/Z2 if you want to have reasonable fault tolerance, unless you want to waste ungodly amount of space.

My reasoning for using mirrored vdevs was more about the ease of expanding the storage than the redundancy.

We just used RAID6... expanding by more drives is easy while keeping data waste low and the "we want to put bigger drives now" case isn't all that problematic considering that what you save you waste on low efficiency of RAID1 setup.

We did had a big case so we just ran 2xRAID6 setup and that one time where it was needed we just replaced to bigger drives one by one, while using the removed ones as spares for other machines. But that's benefit of scale bigger than "a NAS server".

Yeah mine is a home NAS, not a professional deployment. At first I was a bit disappointed at losing half the storage due to using mirrors but honestly it’s not a big deal considering the prices I managed to find on the drives. And I also wanted to expand at my own pace. The drive I recently used to replace the dead 4TB drive cost me 110€ and it’s a 10TB drive.

I got 2 so I’ll also replace the sibling of the dead 4TB one.