[20130606] As described in FreeBSD 8.4-RELEASE Release Notes, FreeBSD ZFS subsystem has been updated to support feature flags for ZFS pools. However, the default version number of a newly created ZFS pool is still 28.

This is because FreeBSD 9.0 and 9.1 do not support the feature flags. This means ZFS pools with feature flag support cannot be used on FreeBSD 9.0 and 9.1. An 8.X system with v28 ZFS pools can be upgraded to 9.X with no problem. Note that zfs(8) send and receive commands do not work between pools with different versions. Once a ZFS pool is upgraded from v28, there is no way to upgrade the system to FreeBSD 9.0 and 9.1. FreeBSD 9.2 and later will support ZFS pools with feature flags.

To create a ZFS pool with feature flag support, use the zpool(8) create command and then the zpool(8) upgrade command.