# dmesg
(snip)
xbd0: 40960MB <Virtual Block Device> at device/vbd/768 on xenbusb_front0
xbd0: attaching as ada0
(snip)
GEOM: ada0: the secondary GPT header is not in the last LBA.
(snip)
# zpool set autoexpand=on zroot
# gpart show
=>      40  41942960  ada0  GPT  (40G) [CORRUPT]
        40      1024     1  freebsd-boot  (512K)
      1064       984        - free -  (492K)
      2048   4194304     2  freebsd-swap  (2.0G)
   4196352  37744640     3  freebsd-zfs  (18G)
  41940992      2008        - free -  (1.0M)

# gpart recover ada0
ada0 recovered
# gpart show
=>      40  83886000  ada0  GPT  (40G)
        40      1024     1  freebsd-boot  (512K)
      1064       984        - free -  (492K)
      2048   4194304     2  freebsd-swap  (2.0G)
   4196352  37744640     3  freebsd-zfs  (18G)
  41940992  41945048        - free -  (20G)

# gpart resize -i 3 ada0
ada0p3 resized
# gpart show ada0
=>      40  83886000  ada0  GPT  (40G)
        40      1024     1  freebsd-boot  (512K)
      1064       984        - free -  (492K)
      2048   4194304     2  freebsd-swap  (2.0G)
   4196352  79689688     3  freebsd-zfs  (38G)
# zpool list
NAME    SIZE  ALLOC   FREE  CKPOINT  EXPANDSZ   FRAG    CAP  DEDUP    HEALTH  ALTROOT
zroot  37.5G   708M  36.8G        -         -     0%     1%  1.00x    ONLINE  -
# zpool status
  pool: zroot
 state: ONLINE
config:

        NAME        STATE     READ WRITE CKSUM
        zroot       ONLINE       0     0     0
          ada0p3    ONLINE       0     0     0

errors: No known data errors
# zpool set autoexpand=off zroot
# zpool get autoexpand zroot
NAME   PROPERTY    VALUE   SOURCE
zroot  autoexpand  off     default
#