ZFS raidz expansion is coming
github.com> The feature@raidz_expansion on-disk feature flag must be enabled to initiate an expansion, and it remains active for the life of the pool. In other words, pools with expanded RAIDZ vdevs can not be imported by older releases of the ZFS software.
Interesting - this indicates that the resulting vdev is on-disk different than a fresh vdev would be. Normally when you have a RAID expansion/level change, the format during the conversion is different, but when it is complete the on-disk is identical to it having always been that way.
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
Sounds like it, but I wonder if they'll be able to add a "final pass rewrite" that would scrub it clean.