Where is the overhead for ZFS RAIDZ2 on 4K sector disks going?
Solution 1:
The glib answer, because RAIDZ(2) sucks on 4k drives.
- Issue #548: Highly inefficient use of space observed when using raidz2 with ashift=12
- Issue #1807: zvol on RAIDZ2 takes up double the expected space #1807
It has to do with the parity calculations gobbing up a boatload of additional space, especially with smaller files. Try doing the same test against files that are under 50MB if you want to see some real inefficiencies.