I’m doing a rebuild on a backup drive, where a strange thing happened. After extensive testing, narrowing down where the problem was, an “fsck.ext4 -a” couldn’t detect a subtle, depeer issue. To detect the issue was brutally difficult, because the drive also got clean bill of health from smartctl as well. I reformatted it to BTRFS, to afford me scrubbing. It was only during a scrub that I could see the deeper issue - a massive slowdown in scrub speed partway through the scrub. So to those haters who like to hate on BTRFS, there are legit occasions where the scrub is actually indispensable, compared to, say, EXT4.