[Just1689]

Could it be related to the following?

> After the expansion completes, old blocks remain with their old data-to-parity ratio (e.g. 5-wide RAIDZ2, has 3 data to 2 parity), but distributed among the larger set of disks. New blocks will be written with the new data-to-parity

[bombcar]

Sounds like it, but I wonder if they'll be able to add a "final pass rewrite" that would scrub it clean.