Here's a quick debugging note on how to get the offsets of the pool's history log:

> ::spa
ADDR                 STATE NAME                                                
fffffffec529d5c0    ACTIVE d
> fffffffec529d5c0::print spa_t spa_history
spa_history = 0xd
> ::dbufs -n mos -o 0xd -b bonus
0xffffffff39948990
> 0xffffffff39948990::print dmu_buf_t db_data | ::print spa_history_phys_t
{
    sh_pool_create_len = 0x10c
    sh_phys_max_off = 0x2000000
    sh_bof = 0x10c
    sh_eof = 0x30c
    sh_records_lost = 0
}
>