4 更に親切なことに

• 更に親切なことに、その対策を指示してもらう:-)

  "Yes, or to be exact, that is the disk size reported by the INT13;
  and as below you do get the same value from UEFI, the limit seems
  to be set by the RAID controller itself. In this case it means
  that the best way to address the issue is to create one smaller
  lun for boot disk (zroot pool) and larger for data. Or of course
  you can have separate FreeBSD ZFS partition for zroot, just make
  sure it will fit inside the first 2TB. 
  
  Of course there may be option for RAID firmware update, or
  configuration settings for lun, or use JBOD mode (if supported by
  the card). JBOD would be the best because in the current setup,
  the pool is vulnerable against silent data corruption (checksum
  errors) and has no way to recover (this is the reason why RAID
  setups are not preferred with zfs). "
  

  つまり、

  1. RAIDのJBODを使って全てのディスクを個別に論理ブロックとする
  2. それらをZFSのRAID (RAID-Z*)でまとめる⁽²⁾
  3. このとき、/bootは別プールとする