RAID-Z wasted space - asize roundups to nparity +1

[Adam Nowacki]

I've just found something very weird in the ZFS code.

sys/cddl/contrib/opensolaris/uts/common/fs/zfs/vdev_raidz.c:504 in HEAD

Can someone explain the reason behind this line of code? What it does is 
align on-disk record size to a multiple of number of parity disks + 1 
... this really doesn't make any sense. So far as I can tell those extra 
sectors are just padding - completely unused.

For the array I'm using this results in 4.8% of wasted disk space - 
1.7TB. It's a 12x 3TB disk RAID-Z2.