ZFS bug or feature

After detach ada0 and attach him (reboot between that)

boot only one disk
Apr 17 13:33:52 x220 kernel: ada0 at ahcich2 bus 0 scbus0 target 0 lun 0
Apr 17 13:33:52 x220 kernel: ada0: <OCZ-NOCTI 2.15> ATA-8 SATA 2.x device
Apr 17 13:33:52 x220 kernel: ada0: Serial Number OCZ-J8928HU1G10KR9XM
Apr 17 13:33:52 x220 kernel: ada0: 300.000MB/s transfers (SATA 2.x,
UDMA6, PIO 8192bytes)
Apr 17 13:33:52 x220 kernel: ada0: Command Queueing enabled
Apr 17 13:33:52 x220 kernel: ada0: 114473MB (234441648 512 byte
sectors: 1H 63S/T 65535C)
Apr 17 13:33:52 x220 kernel: ada0: Previously was known as ad4

After attach ada0 im see this

# camcontrol devlist
<Corsair Neutron SSD M206>         at scbus0 target 0 lun 0 (ada0,pass0)
<OCZ-NOCTI 2.15>                   at scbus1 target 0 lun 0 (ada1,pass1)

# zpool status
  pool: x220pool
 state: ONLINE
status: Some supported features are not enabled on the pool. The pool can
        still be used, but some features are unavailable.
action: Enable all features using 'zpool upgrade'. Once this is done,
        the pool may no longer be accessible by software that does not support
        the features. See zpool-features(7) for details.
  scan: resilvered 42.4M in 0h0m with 0 errors on Thu Apr 17 13:48:42 2014
config:

        NAME                                     STATE     READ WRITE CKSUM
        x220pool                                 ONLINE       0     0     0
          mirror-0                               ONLINE       0     0     0
            diskid/DISK-OCZ-J8928HU1G10KR9XMs1a  ONLINE       0     0     0
            ada0s1a                              ONLINE       0     0     0

errors: No known data errors

# gpart show
=>       63  468862065  ada0  MBR  (224G)
         63  234441585     1  freebsd  (112G)
  234441648  234420480     2  freebsd  (112G)

=>        0  234441585  ada0s1  BSD  (112G)
          0  234441585       1  freebsd-zfs  (112G)

=>       63  234441585  diskid/DISK-OCZ-J8928HU1G10KR9XM  MBR  (112G)
         63  234441585                                 1  freebsd
[active]  (112G)

=>        0  234441585  diskid/DISK-OCZ-J8928HU1G10KR9XMs1  BSD  (112G)
          0  234441585                                   1  freebsd-zfs  (112G)

Apr 17 13:48:43 x220 kernel: ada0 at ahcich0 bus 0 scbus0 target 0 lun 0
Apr 17 13:48:43 x220 kernel: ada0: <Corsair Neutron SSD M206> ATA-8
SATA 3.x device
Apr 17 13:48:43 x220 kernel: ada0: Serial Number 1240792700001802000A
Apr 17 13:48:43 x220 kernel: ada0: 600.000MB/s transfers (SATA 3.x,
UDMA6, PIO 8192bytes)
Apr 17 13:48:43 x220 kernel: ada0: Command Queueing enabled
Apr 17 13:48:43 x220 kernel: ada0: 228936MB (468862128 512 byte
sectors: 16H 63S/T 16383C)
Apr 17 13:48:43 x220 kernel: ada0: Previously was known as ad4
Apr 17 13:48:43 x220 kernel: ada1 at ahcich2 bus 0 scbus1 target 0 lun 0
Apr 17 13:48:43 x220 kernel: ada1: <OCZ-NOCTI 2.15> ATA-8 SATA 2.x device
Apr 17 13:48:43 x220 kernel: ada1: Serial Number OCZ-J8928HU1G10KR9XM
Apr 17 13:48:43 x220 kernel: ada1: 300.000MB/s transfers (SATA 2.x,
UDMA6, PIO 8192bytes)
Apr 17 13:48:43 x220 kernel: ada1: Command Queueing enabled
Apr 17 13:48:43 x220 kernel: ada1: 114473MB (234441648 512 byte
sectors: 1H 63S/T 65535C)
Apr 17 13:48:43 x220 kernel: ada1: Previously was known as ad6

# zpool history -il | tail -6
2014-04-17.13:33:51 [txg:8004153] open pool version 5000; software
version 5000/5; uts  11.0-CURRENT 1100005 amd64 [on x220.local]
2014-04-17.13:48:36 [txg:8004321] open pool version 5000; software
version 5000/5; uts  11.0-CURRENT 1100005 amd64 [on x220.local]
2014-04-17.13:48:41 [txg:8004324] scan setup func=2 mintxg=8004151
maxtxg=8004320 [on x220.local]
2014-04-17.13:48:42 [txg:8004325] scan done complete=1 [on x220.local]
2014-04-17.13:53:53 zpool clear x220pool [user 0 (root) on x220.local]