I also had two mirrored drives fail simultaneously in my zpool a few days ago. There was nothing on them so I wasn't worried. WD Reds in my case;
Using matched drives seems to be a very bad idea for mirrors. I'll probably replace them with two different brands.
I also have a matched pair of HGST SAS Helium drives in the same backplane so hopefully I can catch those before they fail too if they're going to go at once, I _do_ have data on those.
Buying two identical drives has high chances of them being from a single batch, which makes them physically almost identical. It’s a pretty well-known raid-related fact, but some people aren’t aware of it or don’t take it seriously.
If they're bought together, like mine were, and they have close serials, they've be almost identical; if you then run them in a ZFS mirror like I was, they'll receive identical "load" as well.
Since mine had ~43000 hours, they didn't fail prematurely, they just aged out, and since they appear to have been built pretty well, they both aged out at the same time. Annoying for a ZFS mirror, but indicates good quality control in my opinion.
If they're ~identical construction and being mirrored so that they have the same write/read pattern history, it could trigger the same failure mode simultaneously.
Why bad? What's considered a good/bad lifetime for these? Mine had ~43000 power on hours, I don't know if that's good or bad for a WD Red (CMR) drive, but they weren't particularly heavily loaded, and their temps were good, so I'm fairly happy with how long they lasted (though longer would have been nice).
Using matched drives seems to be a very bad idea for mirrors. I'll probably replace them with two different brands.
I also have a matched pair of HGST SAS Helium drives in the same backplane so hopefully I can catch those before they fail too if they're going to go at once, I _do_ have data on those.