alt Hacker NewsA scrub only reads allocated space, so in your 10TB example, a scrub would only read whatever portion of that 10TB is actually occupied by data. It's also usually recommended to keep your usage below 80% of the total pool size to avoid performance issues, so the worst case in your scenario would be more like ~53% assuming you follow the 80% rule.
Is the 80% rule real or just passed down across decades like other “x% free” rules? Those waste enormous amounts of resources on modern systems and I kind of doubt ZFS actually needs a dozen terabytes or more of free space in order to not shit the bed. Just like Linux doesn’t actually need >100 GB of free memory to work properly.